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Cisco has developed EIGRP support for IPv6 networks to route IPv6 prefixes. EIGRP for IPv6 is configured and managed separately from EIGRP for IPv4; no network statements are used. EIGRP for IPv6 retains all the same characteristics (network discovery, DUAL, modules) and functions as EIGRP for IPv4. The major themes with EIGRP for IPv6 are as follows:

• Implements the protocol-independent modules.
• Does EIGRP neighbor discovery and recovery.
• Uses reliable transport.
• Implements the DUAL algorithm for a loop-free topology.
• Uses the same metrics as EIGRP for IPv4 networks.
• Has the same timers as EIGRP for IPv4.
• Uses same concepts of feasible successors and feasible distance as EIGRP for IPv4.
• Uses the same packet types as EIGRP for IPv4.
• Managed and configured separately from EIGRP for IPv4.
• Requires a router ID before it can start running.
• Configured on interfaces. No network statements are used.

The difference is the use of IPv6 prefixes and the use of IPv6 multicast group FF02::A for EIGRP updates. Because EIGRP for IPv6 uses the same characteristics and functions as EIGRP for IPv4 covered in the previous section on EIGRP, they are not repeated here.

EIGRP for IPv6 Design

Use EIGRP for IPv6 in large geographic IPv6 networks. EIGRP's diameter can scale up to 255 hops, but this network diameter is not recommended. EIGRP authentication can be used instead of IPv6 authentication.

EIGRP for IPv6 can be used in the site-to-site WAN and IPsec VPNs. In the enterprise campus, EIGRP can be used in data centers, server distribution, building distribution, and the network core.

EIGRP's DUAL algorithm provides for fast convergence and routing loop prevention. EIGRP does not broadcast its routing table periodically, so there is no large network overhead. The only constraint is that EIGRP for IPv6 is restricted to Cisco routers.

EIGRP for IPv6 Summary

The characteristics of EIGRP for IPv6 are as follows:

• Uses the same characteristics and functions as EIGRP for IPv4.
• Hybrid routing protocol (a distance-vector protocol that has link-state protocol characteristics).
• Uses Next Header protocol 88.
• Routes IPv6 prefixes.
• Default composite metric uses bandwidth and delay.
• You can factor load and reliability into the metric.
• Sends partial route updates only when there are changes.
• Supports EIGRP MD5 authentication.
• Uses DUAL for loop prevention and fast convergence.
• By default, equal-cost load balancing. Unequal-cost load balancing with the variance command.
• Administrative distance is 90 for EIGRP internal routes, 170 for EIGRP external routes, and 5 for EIGRP summary routes.
• Uses IPv6 multicast FF02::A for EIGRP updates.
• High scalability; used in large networks.

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Cisco Systems released EIGRP in the early 1990s as an evolution of IGRP toward a more scalable routing protocol for large internetworks. EIGRP is a classless protocol that permits the use of VLSMs and that supports CIDR for the scalable allocation of IP addresses. EIGRP does not send routing updates periodically, as does IGRP. EIGRP allows for authentication with MD5. EIGRP autosummarizes networks at network borders and can load-balance over unequal-cost paths. Packets using EIGRP use IP 88. Only Cisco routers can use EIGRP.

EIGRP is an advanced distance-vector protocol that implements some characteristics similar to those of link-state protocols. Some Cisco documentation refers to EIGRP as a hybrid protocol. EIGRP advertises its routing table to its neighbors as distance-vector protocols do, but it uses hellos and forms neighbor relationships as link-state protocols do. EIGRP sends partial updates when a metric or the topology changes on the network. It does not send full routing-table updates in periodic fashion as do distance-vector protocols. EIGRP uses DUAL to determine loop-free paths to destinations. This section discusses DUAL.

By default, EIGRP load-balances traffic if several paths have equal cost to the destination. EIGRP performs unequal-cost load balancing if you configure it with the variance n command. EIGRP includes routes that are equal to or less than n times the minimum metric route to a destination. As in RIP and IGRP, EIGRP also summarizes IP networks at network boundaries.

EIGRP internal routes have an administrative distance of 90. EIGRP summary routes have an administrative distance of 5, and EIGRP external routes (from redistribution) have an administrative distance of 170.

EIGRP Components

EIGRP has four components that characterize it:

• Protocol-dependent modules
• Neighbor discovery and recovery
• Reliable Transport Protocol (RTP)
• DUAL

You should know the role of the EIGRP components, which are described in the following sections.

Protocol-Dependent Modules

EIGRP uses different modules that independently support IP, Internetwork Packet Exchange (IPX), and AppleTalk routed protocols. These modules are the logical interface between DUAL and routing protocols such as IPX RIP, AppleTalk Routing Table Maintenance Protocol (RTMP), and IGRP. The EIGRP module sends and receives packets but passes received information to DUAL, which makes routing decisions.

EIGRP automatically redistributes with IGRP if you configure both protocols with the same autonomous system number. When configured to support IPX, EIGRP communicates with the IPX RIP and forwards the route information to DUAL to select the best paths. AppleTalk EIGRP automatically redistributes routes with AppleTalk RTMP to support AppleTalk networks. AppleTalk is not a CCDA objective and is not covered in this book.

Neighbor Discovery and Recovery

EIGRP discovers and maintains information about its neighbors. It multicasts hello packets (224.0.0.10) every 5 seconds on most interfaces. The router builds a table with EIGRP neighbor information. The holdtime to maintain a neighbor is 3 times the hello time: 15 seconds. If the router does not receive a hello in 15 seconds, it removes the neighbor from the table. EIGRP multicasts hellos every 60 seconds on multipoint WAN interfaces (X.25, Frame Relay, ATM) with speeds less than a T-1 (1.544 Mbps), inclusive. The neighbor holdtime is 180 seconds on these types of interfaces. To summarize, hello/holdtime timers are 5/15 seconds for high-speed links and 60/180 seconds for low-speed links.

Example 10-4 shows an EIGRP neighbor database. The table lists the neighbor's IP address, the interface to reach it, the neighbor holdtime timer, and the uptime.

Example 10-4. EIGRP Neighbor Database

Router# show ip eigrp neighbor IP-EIGRP neighbors for process 100 H Address Interface Hold Uptime SRTT RTO Q Seq Type (sec) (ms) Cnt Num 1 172.17.1.1 Se0 11 00:11:27 16 200 0 2 0 172.17.2.1 Et0 12 00:16:11 22 200 0 3

RTP

EIGRP uses RTP to manage EIGRP packets. RTP ensures the reliable delivery of route updates and also uses sequence numbers to ensure ordered delivery. It sends update packets using multicast address 224.0.0.10. It acknowledges updates using unicast hello packets with no data.

DUAL

EIGRP implements DUAL to select paths and guarantee freedom from routing loops. J.J. Garcia Luna-Aceves developed DUAL. It is mathematically proven to result in a loop-free topology, providing no need for periodic updates or route-holddown mechanisms that make convergence slower.

DUAL selects a best path and a second-best path to reach a destination. The best path selected by DUAL is the successor, and the second-best path (if available) is the feasible successor. The feasible distance is the lowest calculated metric of a path to reach the destination. The topology table in Example 10-5 shows the feasible distance. The example also shows two paths (Ethernet 0 and Ethernet 1) to reach 172.16.4.0/30. Because the paths have different metrics, DUAL chooses only one successor.

Example 10-5. Feasible Distance as Shown in the EIGRP Topology Table

Router8# show ip eigrp topology IP-EIGRP Topology Table for AS(100)/ID(172.16.3.1) Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply, r - reply Status, s - sia Status P 172.16.4.0/30, 1 successors, FD is 2195456 via 172.16.1.1 (2195456/2169856), Ethernet0 via 172.16.5.1 (2376193/2348271), Ethernet1 P 172.16.1.0/24, 1 successors, FD is 281600 via Connected, Ethernet0

The route entries in Example 10-5 are marked with a P for the passive state. A destination is in passive state when the router is not performing any recomputations for the entry. If the successor goes down and the route entry has feasible successors, the router does not need to perform any recomputations and does not go into active state.

DUAL places the route entry for a destination into active state if the successor goes down and there are no feasible successors. EIGRP routers send query packets to neighboring routers to find a feasible successor to the destination. A neighboring router can send a reply packet that indicates it has a feasible successor or a query packet. The query packet indicates that the neighboring router does not have a feasible successor and will participate in the recomputation. A route does not return to passive state until it has received a reply packet from each neighboring router. If the router does not receive all the replies before the "active-time" timer expires, DUAL declares the route as stuck in active (SIA). The default active timer is 3 minutes.

EIGRP Timers

EIGRP sets updates only when necessary and sends them only to neighboring routers. There is no periodic update timer.

EIGRP uses hello packets to learn of neighboring routers. On high-speed networks, the default hello packet interval is 5 seconds. On multipoint networks with link speeds of T1 and slower, hello packets are unicast every 60 seconds.

The holdtime to maintain a neighbor adjacency is 3 times the hello time: 15 seconds. If a router does not receive a hello within the holdtime, it removes the neighbor from the table. Hellos are multicast every 60 seconds on multipoint WAN interfaces (X.25, Frame Relay, ATM) with speeds less than 1.544 Mbps, inclusive. The neighbor holdtime is 180 seconds on these types of interfaces. To summarize, hello/holdtime timers are 5/15 seconds for high-speed links and 60/180 seconds for multipoint WAN links less than 1.544 Mbps, inclusive.

Note

EIGRP does not send updates using a broadcast address; instead, it sends them to the multicast address 224.0.0.10 (all EIGRP routers).

EIGRP Metrics

EIGRP uses the same composite metric as IGRP, but the BW term is multiplied by 256 for finer granularity. The composite metric is based on bandwidth, delay, load, and reliability. MTU is not an attribute for calculating the composite metric.

EIGRP calculates the composite metric with the following formula:

EIGRP_metric = {k1 * BW + [(k2 * BW)/(256 – load)] + k3 * delay} * {k5/(reliability + k4)}

In this formula, BW is the lowest interface bandwidth in the path, and delay is the sum of all outbound interface delays in the path. The router dynamically measures reliability and load. It expresses 100 percent reliability as 255/255. It expresses load as a fraction of 255. An interface with no load is represented as 1/255.

Bandwidth is the inverse minimum bandwidth (in kbps) of the path in bits per second scaled by a factor of 256 * 10⁷. The formula for bandwidth is

(256 * 10⁷)/BW_min

The delay is the sum of the outgoing interface delays (in microseconds) to the destination. A delay of all 1s (that is, a delay of hexadecimal FFFFFFFF) indicates that the network is unreachable. The formula for delay is

[sum of delays] * 256

Reliability is a value between 1 and 255. Cisco IOS routers display reliability as a fraction of 255. That is, 255/255 is 100 percent reliability, or a perfectly stable link; a value of 229/255 represents a 90 percent reliable link.

Load is a value between 1 and 255. A load of 255/255 indicates a completely saturated link. A load of 127/255 represents a 50 percent saturated link.

By default, k1 = k3 = 1 and k2 = k4 = k5 = 0. EIGRP's default composite metric, adjusted for scaling factors, is

EIGRP_metric = 256 * { [10⁷/BW_min] + [sum_of_delays] }

BW_min is in kbps, and sum_of_delays is in 10s of microseconds. The bandwidth and delay for an Ethernet interface are 10 Mbps and 1 ms, respectively.

The calculated EIGRP BW metric is

256 * 10⁷/BW = 256 * 10⁷/10,000
= 256 * 10,000
= 256,000

The calculated EIGRP delay metric is

256 * sum of delay = 256 * 1 ms
= 256 * 100 * 10 microseconds
= 25,600 (in 10s of microseconds)

Table 10-3 shows some default values for bandwidth and delay.

Table 10-3. Default EIGRP Values for Bandwidth and Delay

Media Type	Delay	Bandwidth
Satellite	5120 (2 seconds)	5120 (500 Mbps)
Ethernet	25,600 (1 ms)	256,000 (10 Mbps)
T-1 (1.544 Mbps)	512,000 (20,000 ms)	1,657,856
64 kbps	512,000	40,000,000
56 kbps	512,000	45,714,176

As with IGRP, you use the metric weights subcommand to change EIGRP metric computation. You can change the k values in the EIGRP composite metric formula to select which EIGRP metrics to use. The command to change the k values is the metric weights tos k1 k2 k3 k4 k5 subcommand under router eigrp n. The tos value is always 0. You set the other arguments to 1 or 0 to alter the composite metric. For example, if you want the EIGRP composite metric to use all the parameters, the command is as follows:

router eigrp n
 metric weights 0 1 1 1 1 1

EIGRP Packet Types

EIGRP uses five packet types:

• Hello— EIGRP uses hello packets in the discovery of neighbors. They are multicast to 224.0.0.10. By default, EIGRP sends hello packets every 5 seconds (60 seconds on WAN links with 1.544 Mbps speeds or less).
• Acknowledgment— An acknowledgment packet acknowledges the receipt of an update packet. It is a hello packet with no data. EIGRP sends acknowledgment packets to the unicast address of the sender of the update packet.
• Update— Update packets contain routing information for destinations. EIGRP unicasts update packets to newly discovered neighbors; otherwise, it multicasts update packets to 224.0.0.10 when a link or metric changes. Update packets are acknowledged to ensure reliable transmission.
• Query— EIGRP sends query packets to find feasible successors to a destination. Query packets are always multicast unless they are sent as a response; then they are unicast back to the originator.
• Reply— EIGRP sends reply packets to respond to query packets. Reply packets provide a feasible successor to the sender of the query. Reply packets are unicast to the sender of the query packet.

EIGRP Design

When designing a network with EIGRP, remember that it supports VLSMs, CIDR, and network summarization. EIGRP allows for the summarization of routes in a hierarchical network. EIGRP is not limited to 16 hops as RIP is; therefore, the network diameter can exceed this limit. In fact, the EIGRP diameter can be 225 hops. The default diameter is 100. EIGRP can be used in the site-to-site WAN and IPsec VPNs. In the enterprise campus, EIGRP can be used in data centers, server distribution, building distribution, and the network core.

EIGRP does not broadcast its routing table periodically, so there is no large network overhead. You can use EIGRP for large networks; it is a potential routing protocol for the core of a large network. EIGRP further provides for route authentication.

As shown in Figure 10-8, when you use EIGRP, all segments can have different subnet masks.

EIGRP Summary

The characteristics of EIGRP follow:

• Hybrid routing protocol (a distance-vector protocol that has link-state protocol characteristics).
• Uses IP protocol number 88.
• Classless protocol (supports VLSMs).
• Default composite metric uses bandwidth and delay.
• You can factor load and reliability into the metric.
• Sends partial route updates only when there are changes.
• Supports MD5 authentication.
• Uses DUAL for loop prevention and fast convergence.
• By default, equal-cost load balancing. Unequal-cost load balancing with the variance command.
• Administrative distance is 90 for EIGRP internal routes, 170 for EIGRP external routes, and 5 for EIGRP summary routes.
• High scalability; used in large networks.
• Does not require a hierarchical physical topology.

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Cisco Systems developed IGRP to overcome the limitations of RIPv1. IGRP is a distance-vector routing protocol that considers a composite metric that, by default, uses bandwidth and delay as parameters instead of hop count. IGRP is not limited to RIP's 15-hop limit. IGRP has a maximum hop limit of 100 by default and can be configured to support a network diameter of 255.

Note

IGRP is no longer a CCDA test topic. EIGRP is the enhanced version of IGRP. However, reading this section will provide a good foundation for learning EIGRP in the section that follows.

With IGRP, routers usually select paths with a larger minimum-link bandwidth over paths with a smaller hop count. Links do not have a hop count. They are exactly one hop.

IGRP is a classful protocol and cannot implement VLSM or CIDR. IGRP summarizes at network boundaries. As in RIP, IGRP implements split horizon with poison reverse, triggered updates, and holddown timers for stability and loop prevention. Another benefit of IGRP is that it can load-balance over unequal-cost links. As a routing protocol developed by Cisco, IGRP is available only on Cisco routers.

By default, IGRP load-balances traffic if several paths have equal cost to the destination. IGRP does unequal-cost load balancing if configured with the variance command.

IGRP Timers

IGRP sends its routing table to its neighbors every 90 seconds. IGRP's default update period of 90 seconds is a benefit compared to RIP, which can consume excessive bandwidth when sending updates every 30 seconds. IGRP uses an invalid timer to mark a route as invalid after 270 seconds (3 times the update timer). As with RIP, IGRP uses a flush timer to remove a route from the routing table; the default flush timer is set to 630 seconds (7 times the update period and more than 10 minutes).

If a network goes down or the metric for the network increases, the route is placed in holddown. The router accepts no new changes for the route until the holddown timer expires. This setup prevents routing loops in the network. The default holddown timer is 280 seconds (3 times the update timer plus 10 seconds). Table 10-2 summarizes the default settings for IGRP timers.

Table 10-2. IGRP Timers

IGRP Timer	Default Time
Update	90 seconds
Invalid	270 seconds
Holddown	280 seconds
Flush	630 seconds

IGRP Metrics

IGRP uses a composite metric based on bandwidth, delay, load, and reliability. Chapter 9 discusses these metrics. By default, IGRP uses bandwidth and delay to calculate the composite metric, as follows:

IGRP_metric = {k1 * BW + [(k2 * BW)/(256 – load)] + k3 * delay} * {k5/(reliability + k4)}

In this formula, BW is the lowest interface bandwidth in the path, and delay is the sum of all outbound interface delays in the path. The router dynamically measures reliability and load. The values of reliability and load used in the metric computation range from 1 to 255. Cisco IOS routers display 100 percent reliability as 255/255. They also display load as a fraction of 255. They display an interface with no load as 1/255. By default, k1 and k3 are set to 1, and k2, k4, and k5 are set to 0. With the default values, the metric becomes

IGRP_metric = {1 * BW + [(0 * BW)/(256 – load)] + 1 * delay} * {0/(reliability + 0)}

IGRP_metric = BW + delay

The BW is 10,000,000 divided by the smallest of all the bandwidths (in kbps) from outgoing interfaces to the destination. To find delay, add all the delays (in microseconds) from the outgoing interfaces to the destination, and divide this number by 10. (The delay is in 10s of microseconds.)

Example 10-3 shows the output interfaces of two routers. For a source host to reach network 172.16.2.0, a path takes the serial link and then the Ethernet interface. The bandwidths are 10,000 and 1544; the slowest bandwidth is 1544. The sum of delays is 20000 + 1000 = 21000.

Example 10-3. show interface

RouterA> show interface serial 0 Serial0 is up, line protocol is up Hardware is HD64570 Internet address is 172.16.4.1/24 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 RouterB> show interface ethernet 0 Ethernet0 is up, line protocol is up Hardware is Lance, address is 0010.7b80.bad5 (bia 0010.7b80.bad5) Internet address is 172.16.2.1/24 MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, reliability 255/255, txload 1/255, rxload 1/255

The IGRP metric is calculated as follows:

IGRP_metric = (10,000,000/1544) + (20000 + 1000)/10

IGRP_metric = 6476 + 2100 = 8576

You can change the default metrics using the metric weight tos k1 k2 k3 k4 k5 subcommand under router igrp. Cisco once intended to implement the tos field as a specialized service in IGRP. However, it was never implemented, so the value of tos is always 0. The k arguments are the k values used to build the composite metric. For example, if you want to use all metrics, the command is as follows:

router igrp n
 metric weight 0 1 1 1 1 1

IGRP Design

IGRP should not be used in the design of new networks because it does not support VLSMs. The IP addressing scheme with IGRP requires the same subnet mask for the entire IP network, a flat IP network. IGRP does not support CIDR and network summarization within the major network boundary. IGRP is not limited to a maximum of 15 hops as RIP is; therefore, the network diameter can be larger than that of networks using RIP. IGRP also broadcasts its routing table every 90 seconds, which produces less network overhead than RIP. IGRP is limited to Cisco-only networks.

Drawbacks of IGRP are that it lacks VLSM support and that it broadcasts its entire table every 90 seconds. Its slow convergence makes it too slow for time-sensitive applications. EIGRP is recommended over IGRP.

As shown in Figure 10-7, when you use IGRP, all segments must have the same subnet mask.

IGRP Summary

The characteristics of IGRP follow:

• Distance-vector protocol.
• Uses IP protocol number 9.
• Classful protocol (no support for CIDR).
• No support for VLSMs.
• Composite metric using bandwidth and delay by default.
• You can include load and reliability in the metric.
• Route updates are sent every 90 seconds.
• 104 routes per IGRP message.
• Hop count is limited to 100 by default and is configurable up to 255.
• No support for authentication.
• Implements split horizon with poison reverse.
• Implements triggered updates.
• By default, equal-cost load balancing. Unequal-cost load balancing with the variance command.
• Administrative distance is 100.
• Previously used in large networks; now replaced by EIGRP.

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RIPng (RIP next generation) is the version of RIP that can be used in IPv6 networks. It is described in RFC 2080. Most of the RIP mechanisms from RIPv2 remain the same. RIPng still has a 15-hop limit, counting to infinity, and split horizon with poison reverse. A hop count of 16 still indicates an unreachable route.

Instead of using UDP port 520 as in RIPv2, RIPng uses UDP port 521. RIPng supports IPv6 addresses and prefixes. RIPng uses multicast group FF02::9 for RIPng updates to all RIPng routers.

RIPng Timers

RIPng timers are similar to RIPv2. Periodic updates are sent every 30 seconds. The default invalid timeout for routes to expire is 180 seconds, the default holddown timer is 180 seconds, and the default garbage-collection timer is 120 seconds.

Authentication

RIPng does not implement authentication methods in its protocol as RIPv2 does. RIPng relies on built-in IPv6 authentication functions.

RIPng Message Format

Figure 10-5 shows the RIPng routing message. Each route table entry (RTE) consists of the IPv6 prefix, route tag, prefix length, and metric.

The following describes each field:

• Command— Indicates whether the packet is a request or response message. This field is set to 1 for a request and to 2 for a response.
• Version— Set to 1, the first version of RIPng.
• IPv6 prefix— The destination 128-bit IPv6 prefix.
• Route tag— As with RIPv2, this is a method that distinguishes internal routes (learned by RIP) from external routes (learned by external protocols). Tagged during redistribution.
• Prefix length— Indicates the significant part of the prefix.
• Metric— This 8-bit field contains the router hop metric.

RIPv2 has a Next Hop field for each of its route entries. An RTE with a metric of 0xFF indicates the next-hop address to reduce the number of route entries in RIPng. It groups all RTEs after it to summarize all destinations to that particular next-hop address. Figure 10-6 shows the format of the special RTE indicating the next-hop entry.

RIPng Design

RIPng has low scalability. As with RIPv2, it is limited to 15 hops; therefore, the network diameter cannot exceed this limit. RIPng also broadcasts its routing table every 30 seconds, which causes network overhead. RIPng can be used only in small networks.

RIPng Summary

The characteristics of RIPng are as follows:

• Distance-vector protocol for IPv6 networks only.
• Uses UDP port 521.
• Metric is router hop count.
• Maximum hop count is 15; infinite (unreachable) routes have a metric of 16.
• Periodic route updates are sent every 30 seconds to multicast address FF02::9.
• Uses IPv6 functions for authentication.
• Implements split horizon with poison reverse.
• Implements triggered updates.
• Prefix length included in route entry.
• Administrative distance for RIPv2 is 120.
• Not scalable. Used in small networks.

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RIPv2 was first described in RFC 1388 and RFC 1723 (1994); the current RFC is 2453, written in November 1998. Although current environments use advanced routing protocols such as OSPF and EIGRP, some networks still use RIP. The need to use VLSMs and other requirements prompted the definition of RIPv2.

RIPv2 improves on RIPv1 with the ability to use VLSM, with support for route authentication, and with multicasting of route updates. RIPv2 supports CIDR. It still sends updates every 30 seconds and retains the 15-hop limit; it also uses triggered updates. RIPv2 still uses UDP port 520; the RIP process is responsible for checking the version number. It retains the loop-prevention strategies of poison reverse and counting to infinity. On Cisco routers, RIPv2 has the same administrative distance as RIPv1, which is 120. Finally, RIPv2 uses the IP address 224.0.0.9 when multicasting route updates to other RIP routers. As in RIPv1, RIPv2 by default summarizes IP networks at network boundaries. You can disable autosummarization if required.

You can use RIPv2 in small networks where VLSM is required. It also works at the edge of larger networks.

Authentication

Authentication can prevent communication with any RIP routers that are not intended to be part of the network, such as UNIX stations running routed. Only RIP updates with the authentication password are accepted. RFC 1723 defines simple plain-text authentication for RIPv2.

MD5 Authentication

In addition to plain-text passwords, the Cisco implementation provides the ability to use Message Digest 5 (MD5) authentication, which is defined in RFC 1321. Its algorithm takes as input a message of arbitrary length and produces as output a 128-bit fingerprint or message digest of the input, making it much more secure than plain-text passwords.

RIPv2 Forwarding Information Base

RIPv2 maintains a routing table database as in Version 1. The difference is that it also keeps the subnet mask information. The following list repeats the table information of RIPv1:

• IP address— The IP address of the destination host or network, with subnet mask
• Gateway— The first gateway along the path to the destination
• Interface— The physical network that must be used to reach the destination
• Metric— A number indicating the number of hops to the destination
• Timer— The amount of time since the route entry was last updated

RIPv2 Message Format

The RIPv2 message format takes advantage of the unused fields in the RIPv1 message format by adding subnet masks and other information. Figure 10-3 shows the RIPv2 message format.

The following describes each field:

• Command— Indicates whether the packet is a request or response message. The request message asks that a router send all or a part of its routing table. Response messages contain route entries. The router sends the response periodically or as a reply to a request.
• Version— Specifies the RIP version used. It is set to 2 for RIPv2 and to 1 for RIPv1.
• AFI— Specifies the address family used. RIP is designed to carry routing information for several different protocols. Each entry has an AFI to indicate the type of address specified. The AFI for IP is 2. The AFI is set to 0xFFF for the first entry to indicate that the remainder of the entry contains authentication information.
• Route tag— Provides a method for distinguishing between internal routes (learned by RIP) and external routes (learned from other protocols). You can add this optional attribute during the redistribution of routing protocols.
• IP address— Specifies the IP address (network) of the destination.
• Subnet mask— Contains the subnet mask for the destination. If this field is 0, no subnet mask has been specified for the entry.
• Next hop— Indicates the IP address of the next hop where packets are sent to reach the destination.
• Metric— Indicates how many router hops to reach the destination. The metric is between 1 and 15 for a valid route or 16 for an unreachable or infinite route.

Again, as in Version 1, the router permits up to 25 occurrences of the last five 32-bit words (20 bytes) for up to 25 routes per RIP message. If the AFI specifies an authenticated message, the router can specify only 24 routing table entries. The updates are sent to the multicast address of 224.0.0.9.

RIPv2 Timers

RIPv2 timers are the same as in Version 1. They send periodic updates every 30 seconds. The default invalid timer is 180 seconds, the holddown timer is 180 seconds, and the flush timer is 240 seconds. You can write this list as 30/180/180/240, representing the U/I/H/F timers.

RIPv2 Design

Things to remember in designing a network with RIPv2 include that it supports VLSM within networks and CIDR for network summarization across adjacent networks. RIPv2 allows for the summarization of routes in a hierarchical network. RIPv2 is still limited to 16 hops; therefore, the network diameter cannot exceed this limit. RIPv2 multicasts its routing table every 30 seconds to the multicast IP address 224.0.0.9. RIPv2 is usually limited to accessing networks where it can interoperate with servers running routed or with non-Cisco routers. RIPv2 also appears at the edge of larger internetworks. RIPv2 further provides for route authentication.

As shown in Figure 10-4, when you use RIPv2, all segments can have different subnet masks.

RIPv2 Summary

The characteristics of RIPv2 follow:

• Distance-vector protocol.
• Uses UDP port 520.
• Classless protocol (support for CIDR).
• Supports VLSMs.
• Metric is router hop count.
• Low scalability: maximum hop count is 15; infinite (unreachable) routes have a metric of 16.
• Periodic route updates are sent every 30 seconds to multicast address 224.0.0.9.
• 25 routes per RIP message (24 if you use authentication).
• Supports authentication.
• Implements split horizon with poison reverse.
• Implements triggered updates.
• Subnet mask included in route entry.
• Administrative distance for RIPv2 is 120.
• Not scalable. Used in small, flat networks or at the edge of larger networks.

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RFC 1058 from June 1988 defines RIPv1. RIP is a distance-vector routing protocol that uses router hop count as the metric. RIPv1 is a classful routing protocol that does not support VLSMs or classless interdomain routing (CIDR). RIPv1 is no longer a topic on the CCDA test. But reading this section will help you understand the evolution of this routing protocol and help you compare it to the later versions.

There is no method for authenticating route updates with RIPv1. A RIP router sends a copy of its routing table to its neighbors every 30 seconds. RIP uses split horizon with poison reverse; therefore, route updates are sent out an interface with an infinite metric for routes learned (received) from the same interface.

The RIP standard was based on the popular routed program used in UNIX systems since the 1980s. The Cisco implementation of RIP adds support for load balancing. RIP load-balances traffic if several paths have the same metric (equal-cost load balancing) to a destination. Also, RIP sends triggered updates when a route's metric changes. Triggered updates can help the network converge faster rather than wait for the periodic update. RIP has an administrative distance of 120. Chapter 9, "Routing Protocol Selection Criteria," covers administrative distance.

RIPv1 summarizes to IP network values at network boundaries. A network boundary occurs at a router that has one or more interfaces that do not participate in the specified IP network. The IP address assigned to the interface determines participation. IP class determines the network value. For example, an IP network that uses 24-bit subnetworks from 180.100.50.0/24 to 180.100.120.0/24 is summarized to 180.100.0.0/16 at a network boundary.

RIPv1 Forwarding Information Base

The RIPv1 protocol keeps the following information about each destination:

• IP address— IP address of the destination host or network
• Gateway— The first gateway along the path to the destination
• Interface— The physical network that must be used to reach the destination
• Metric— The number of hops to the destination
• Timer— The amount of time since the entry was last updated

The database is updated with the route updates received from neighboring routers. As shown in Example 10-1, the show ip rip database command shows a router's RIP private database.

Example 10-1. show ip rip database Command

router9# show ip rip database 172.16.0.0/16 auto-summary 172.16.1.0/24 directly connected, Ethernet0 172.16.2.0/24 [1] via 172.16.4.2, 00:00:06, Serial0 172.16.3.0/24 [1] via 172.16.1.2, 00:00:02, Ethernet0 172.16.4.0/24 directly connected, Serial0

RIPv1 Message Format

The RIPv1 message format is described in RFC 1058 and is shown in Figure 10-1. The RIP messages are encapsulated using User Datagram Protocol (UDP). RIP uses the well-known UDP port 520.

The following describes each field:

• Command— Describes the packet's purpose. The RFC describes five commands, two of which are obsolete and one of which is reserved. The two used commands are

  - Request— Requests all or part of the responding router's routing table.

  - Response— Contains all or part of the sender's routing table. This message might be a response to a request, or it might be an update message generated by the sender.



• Version— Set to a value of 1 for RIPv1.


• Address Family Identifier (AFI)— Set to a value of 2 for IP.


• IP address— The destination route. It might be a network address, subnet, or host route. Special route 0.0.0.0 is used for the default route.


• Metric— A field that is 32 bits in length. It contains a value between 1 and 15 inclusive, specifying the current metric for the destination. The metric is set to 16 to indicate that a destination is unreachable.

Because RIP has a maximum hop count, it implements counting to infinity. For RIP, infinity is 16 hops. Notice that the RIP message has no subnet masks accompanying each route. Five 32-bit words are repeated for each route entry: AFI (16 bits); unused, which is 0 (16 bits); IP address; two more 32-bit unused fields; and the 32-bit metric. Five 32-bit words equals 20 bytes for each route entry. Up to 25 routes are allowed in each RIP message. The maximum datagram size is limited to 512 bytes, not including the IP header. Calculating 25 routes by 20 bytes each, plus the RIP header (4 bytes), plus an 8-byte UDP header, you get 512 bytes.

RIPv1 Timers

The Cisco implementation of RIPv1 uses four timers:

• Update
• Invalid
• Flush
• Holddown

RIPv1 sends its full routing table out all configured interfaces. The table is sent periodically as a broadcast (255.255.255.255) to all hosts.

Update Timer

The update timer specifies the frequency of the periodic broadcasts. By default, the update timer is set to 30 seconds. Each route has a timeout value associated with it. The timeout gets reset every time the router receives a routing update containing the route.

Invalid Timer

When the timeout value expires, the route is marked as unreachable because it is marked invalid. The router marks the route invalid by setting the metric to 16. The route is retained in the routing table. By default, the invalid timer is 180 seconds, or six update periods (30 * 6 = 180).

Flush Timer

A route entry marked as invalid is retained in the routing table until the flush timer expires. By default, the flush timer is 240 seconds, which is 60 seconds longer than the invalid timer.

Holddown Timer

Cisco implements an additional timer for RIP, the holddown timer. The holddown timer stabilizes routes by setting an allowed time for which routing information about different paths is suppressed. After the metric for a route entry changes, the router accepts no updates for the route until the holddown timer expires. By default, the holddown timer is 180 seconds.

The output of the show ip protocol command, as shown in Example 10-2, shows the timers for RIP, unchanged from the defaults.

Example 10-2. RIP Timers Verified with show ip protocol

Code View: router9> show ip protocol Routing Protocol is "rip" Sending updates every 30 seconds, next due in 3 seconds Invalid after 180 seconds, hold down 180, flushed after 240 Outgoing update filter list for all interfaces is Incoming update filter list for all interfaces is Redistributing: rip Default version control: send version 1, receive any version Interface Send Recv Triggered RIP Key-chain Ethernet0 1 1 2 Serial0 1 1 2 Automatic network summarization is in effect Routing for Networks: 172.16.0.0 Routing Information Sources: Gateway Distance Last Update 172.16.4.2 120 00:00:00 172.16.1.2 120 00:00:07 Distance: (default is 120)

RIPv1 Design

New networks should not be designed using RIPv1. It does not support VLSMs and CIDR. The IP addressing scheme with RIPv1 requires the same subnet mask for the entire IP network, a flat IP network. As shown in Figure 10-2, when you use RIPv1, all segments must have the same subnet mask.

RIPv1 has low scalability. It is limited to 15 hops; therefore, the network diameter cannot exceed this limit. RIPv1 also broadcasts its routing table every 30 seconds. RIP's slow convergence time prevents it from being used as an IGP when time-sensitive data, such as voice and video, is being transmitted across the network. RIPv1 is usually limited to access networks where it can interoperate with servers running routed or with non-Cisco routers.

RIPv1 Summary

The characteristics of RIPv1 follow:

• Distance-vector protocol.
• Uses UDP port 520.
• Classful protocol (no support for VLSM or CIDR).
• Metric is router hop count.
• Low scalability: maximum hop count is 15; unreachable routes have a metric of 16.
• Periodic route updates broadcast every 30 seconds.
• 25 routes per RIPv1 message.
• Implements split horizon with poison reverse.
• Implements triggered updates.
• No support for authentication.
• Administrative distance for RIP is 120.
• Used in small, flat networks or at the edge of larger networks.

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On-demand routing (ODR) is a mechanism for reducing the overhead with routing. Only Cisco routers can use ODR. With ODR, there is no need to configure dynamic routing protocols or static routes at a hub router. ODR eliminates the need to manage static route configuration at the hub router.

Figure 9-8 shows a hub-and-spoke network where you can configure ODR. The stub router is the spoke router in the hub-and-spoke network. The stub network consists of small LAN segments connected to the stub router and a WAN connection to the hub. Because all outgoing traffic travels via the WAN, no external routing information is necessary.

ODR simplifies the configuration of IP with stub networks in which the hub routers dynamically maintain routes to the stub networks. With ODR, the stub router advertises the IP prefixes of its connected networks to the hub router. It does so without requiring the configuration of an IP routing protocol at the stub routers.

ODR uses Cisco Discovery Protocol (CDP) for communication between hub and stub routers. CDP must be enabled for ODR to work. CDP updates every 60 seconds. Because ODR route prefixes are carried in CDP messages, a change is not reported until the CDP message is sent.

The hub router receives the prefix routes from its stub routers. You can configure the hub router to redistribute these prefixes into a dynamic routing protocol to propagate those routes to the rest of the internetwork. Stub routers are configured with a static default route to the hub router.

The benefits of ODR are as follows:

• Less routing overhead than dynamic routing protocols
• No configuration or management of static routes on the hub router
• Reduced circuit utilization

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Routing protocols use a metric to determine best routes to a destination. Some routing protocols use a combination of metrics to build a composite metric for best path selection. This section describes metrics and also covers routing loop-prevention techniques. You must understand each metric for the CCDA.

Some routing metric parameters are

• Hop count
• Bandwidth
• Cost
• Load
• Delay
• Reliability
• Maximum transmission unit (MTU)

Hop Count

The hop count parameter counts the number of links between routers the packet must traverse to reach a destination. The RIP routing protocol uses hop count as the metric for route selection. If all links were the same bandwidth, this metric would work well. The problem with routing protocols that use only this metric is that the shortest hop count is not always the most appropriate path. For example, between two paths to a destination network—one with two 56-kbps links and another with four T1 links—the router chooses the first path because of the lower number of hops (see Figure 9-3). However, this is not necessarily the best path. You would prefer to transfer a 20-MB file via the T1 links instead of the 56-kbps links.

Bandwidth

The bandwidth parameter uses the interface bandwidth to determine a best path to a destination network. When bandwidth is the metric, the router prefers the path with the highest bandwidth to a destination. For example, a Fast Ethernet (100 Mbps) is preferred over a DS-3 (45 Mbps). As shown in Figure 9-3, a router using bandwidth to determine a path would select Path 2 because of the larger bandwidth, 1.5 Mbps over 56 kbps.

If a routing protocol uses only bandwidth as the metric and the path has several different speeds, the protocol can use the lowest speed in the path to determine the bandwidth for the path. EIGRP and IGRP use the minimum path bandwidth, inverted and scaled, as one part of the metric calculation. In Figure 9-4, Path 1 has two segments, with 256 kbps and 512 kbps of bandwidth. Because the smaller speed is 256 kbps, this speed is used as Path 1's bandwidth. The smallest bandwidth in Path 2 is 384 kbps. When the router has to choose between Path 1 and Path 2, it selects Path 2 because 384 kbps is larger than 256 kbps.

Cost

Cost is the name of the metric used by OSPF and IS-IS. In OSPF on a Cisco router, a link's default cost is derived from the interface's bandwidth.

Cisco's implementation of IS-IS assigns a default cost of 10 to all interfaces.

The formula to calculate cost in OSPF is

10⁸/BW

where BW is the interface's default or configured bandwidth.

For 10-Mbps Ethernet, cost is calculated as follows:

BW = 10 Mbps = 10 * 10⁶ = 10,000,000 = 10⁷
cost (Ethernet) = 10⁸ / 10⁷ = 10

The sum of all the costs to reach a destination is the metric for that route. The lowest cost is the preferred path.

Figure 9-5 shows an example of how the path costs are calculated. The path cost is the sum of all costs in the path. The cost for Path 1 is 350 + 180 = 530. The cost for Path 2 is 15 + 50 + 100 + 50 = 215.

Because the cost of Path 2 is less than that of Path 1, Path 2 is selected as the best route to the destination.

Load

The load parameter refers to the degree to which the interface link is busy. The router keeps track of interface utilization; routing protocols can use this metric when calculating the best route. Load is one of the five parameters included in the definition of the IGRP and EIGRP metric. By default, it is not used to calculate the composite metric. If you have 512-kbps and 256-kbps links to reach a destination, but the 512-kbps circuit is 99 percent busy and the 256-kbps is only 5 percent busy, the 256 kbps link is the preferred path. On Cisco routers, the percentage of load is shown as a fraction over 255. Utilization at 100 percent is shown as 255/255, and utilization at 0 percent is shown as 0/255. Example 9-1 shows the load of a serial interface at 5/255 (1.9 percent).

Example 9-1. Interface Load

router3>show interface serial 1 Serial1 is up, line protocol is up Hardware is PQUICC Serial Internet address is 10.100.1.1/24 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 5/255

Delay

The delay parameter refers to how long it takes to move a packet to the destination. Delay depends on many factors, such as link bandwidth, utilization, port queues, and physical distance traveled. Total delay is one of the five parameters included in the definition of the IGRP and EIGRP composite metric. By default, it is used to calculate the composite metric. You can configure an interface's delay with the delay tens-of-microseconds command, where tens-of-microseconds specifies the delay in tens of microseconds for an interface or network segment. As shown in Example 9-2, the interface's delay is 20,000 microseconds.

Example 9-2. Interface Delay

router3>show interface serial 1 Serial1 is up, line protocol is up Hardware is PQUICC Serial Internet address is 10.100.1.1/24 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255

Reliability

The reliability parameter is the dependability of a network link. Some WAN links tend to go up and down throughout the day. These links get a small reliability rating. Reliability is measured by factors such as a link's expected received keepalives and the number of packet drops and interface resets. If the ratio is high, the line is reliable. The best rating is 255/255, which is 100 percent reliability. Reliability is one of the five parameters included in the definition of the IGRP and EIGRP metric. By default, it is not used to calculate the composite metric. As shown in Example 9-3, you can verify an interface's reliability using the show interface command.

Example 9-3. Interface Reliability

router4#show interface serial 0 Serial0 is up, line protocol is up Hardware is PQUICC Serial MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255

Maximum Transmission Unit (MTU)

The MTU parameter is simply the maximum size of bytes a unit can have on an interface. If the outgoing packet is larger than the MTU, the IP protocol might need to fragment it. If a packet larger than the MTU has the "do not fragment" flag set, the packet is dropped. As shown in Example 9-4, you can verify an interface's MTU using the show interface command.

Example 9-4. Interface MTU

router4#show interface serial 0 Serial0 is up, line protocol is up Hardware is PQUICC Serial MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255

Routing Loop-Prevention Schemes

Some routing protocols employ schemes to prevent the creation of routing loops in the network. These schemes are

• Split horizon
• Split horizon with poison reverse
• Counting to infinity

Split Horizon

Split horizon is a technique used by distance-vector routing protocols to prevent routing loops. Routes that are learned from a neighboring router are not sent back to that neighboring router, thus suppressing the route. If the neighbor is already closer to the destination, it already has a better path.

In Figure 9-6, Routers 1, 2, and 3 learn about Networks A, B, C, and D. Router 2 learns about Network A from Router 1 and also has Networks B and C in its routing table. Router 3 advertises Network D to Router 2. Now, Router 2 knows about all networks. Router 2 sends its routing table to Router 3 without the route for Network D because it learned that route from Router 3.

Split Horizon with Poison Reverse

Split horizon with poison reverse is a route update sent out an interface with an infinite metric for routes learned (received) from the same interface. Poison reverse simply indicates that the learned route is unreachable. It is more reliable than split horizon alone. Examine Figure 9-7. Instead of suppressing the route for Network D, Router 2 sends that route in the routing table marked as unreachable. In RIP, the poison-reverse route is marked with a metric of 16 (infinite) to prevent that path from being used.

Counting to Infinity

Some routing protocols keep track of router hops as the packet travels through the network. In large networks where a routing loop might be present because of a network outage, routers might forward a packet without its reaching its destination.

Counting to infinity is a loop-prevention technique in which the router discards a packet when it reaches a maximum limit. It assumes that the network diameter is smaller than the maximum allowed hops. The router uses the Time-to-Live (TTL) field to count to infinity. The TTL starts at a set number and is decremented at each router hop. When the TTL equals 0, the packet is discarded. For IGRP and EIGRP, the TTL of routing updates is 100 by default.

Triggered Updates

Another loop-prevention and fast-convergence technique used by routing protocols is triggered updates. When a router interface changes state (up or down), the router is required to send an update message, even if it is not time for the periodic update message. Immediate notification about a network outage is key to maintaining valid routing entries within all routers in the network. Some distance-vector protocols, including RIP, specify a small time delay to avoid having triggered updates generate excessive network traffic. The time delay is variable for each router.

Summarization

Another characteristic of routing protocols is the ability to summarize routes. Protocols that support CIDR can perform summarization outside of IP class boundaries. By summarizing, the routing protocol can reduce the size of the routing table, and fewer routing updates on the network occur

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This section discusses the different types and characteristics of routing protocols.

Characteristics of routing-protocol design are

• Distance-vector, link-state, or hybrid— How routes are learned
• Interior or exterior— For use in private networks or the public Internet
• Classless (classless interdomain routing [CIDR] support) or classful— CIDR enables aggregation of network advertisements (supernetting) between routers
• Fixed-length or variable-length subnet masks (VLSM)— Conserve addresses within a network
• Flat or potentially hierarchical— Addresses scalability in large internetworks
• IPv4 or IPv6— Newer routing protocols are used for IPv6 networks

This section also covers the default administrative distance assigned to routes learned from each routing protocol or from static assignment. Routes are categorized as statically (manually) configured or dynamically learned from a routing protocol. The following sections cover all these characteristics.

Static Versus Dynamic Route Assignment

Static routes are manually configured on a router. They do not react to network outages. The one exception is when the static route specifies the outbound interface: If the interface goes down, the static route is removed from the routing table. Because static routes are unidirectional, they must be configured for each outgoing interface the router will use. The size of today's networks makes it impossible to manually configure and maintain all the routes in all the routers in a timely manner. Human configuration can involve many mistakes, which is why routing protocols exist. They use algorithms to advertise and learn about changes in the network topology.

The main benefit of static routing is that a router generates no routing protocol overhead. Because no routing protocol is enabled, no bandwidth is consumed by route advertisements between network devices. Another benefit of static routing protocols is that they are easier to configure and troubleshoot than dynamic routing protocols. Static routing is recommended for hub-and-spoke topologies with a low-speed remote connection. A default static route is configured at each remote site because the hub is the only route used to reach all other sites. Static routers are also used at network boundaries (Internet or partners) where routing information is not exchanged. These static routes are then redistributed into the internal dynamic routing protocol used.

Figure 9-1 shows a hub-and-spoke WAN where static routes are defined in the remote WAN routers because no routing protocols are configured. This setup eliminates routing protocol traffic on the low-bandwidth WAN circuits.

Routing protocols dynamically determine the best route to a destination. When the network topology changes, the routing protocol adjusts the routes without administrative intervention. Routing protocols use a metric to determine the best path toward a destination network. Some use a single measured value such as hop count. Others compute a metric value using one or more parameters. Routing metrics are discussed later in this chapter. The following is a list of dynamic routing protocols:

• RIPv1
• RIPv2
• IGRP
• EIGRP
• OSPF
• IS-IS
• RIPng
• OSPFv3
• EIGRP for IPv6
• Border Gateway Protocol (BGP)

Interior Versus Exterior Routing Protocols

Routing protocols can be categorized as interior gateway protocols (IGP) or exterior gateway protocols (EGP). IGPs are meant for routing within an organization's administrative domain—in other words, the organization's internal network. EGPs are routing protocols used to communicate with exterior domains. Figure 9-2 shows where an internetwork uses IGPs and EGPs with multiple autonomous administrative domains. BGP exchanges routing information between the internal network and an ISP. IGPs appear in the internal private network.

One of the first EGPs was called exactly that—Exterior Gateway Protocol. Today, BGP is the de facto (and the only available) exterior gateway protocol.

Potential IGPs for an IPv4 network are

• RIPv2
• OSPF
• IS-IS
• EIGRP

Potential IGPs for an IPv6 network are

• RIPng
• OSPFv3
• EIGRP for IPv6

RIPv1 is no longer recommended because RIPv2 is the most recent version of RIP. IGRP is an earlier version of EIGRP. IGRP is no longer a CCDA exam topic.

Distance-Vector Routing Protocols

The first IGP routing protocols introduced were distance-vector routing protocols. They used the Bellman-Ford algorithm to build the routing tables. With distance-vector routing protocols, routes are advertised as vectors of distance and direction. The distance metric is usually router hop count. The direction is the next-hop router (IP address) toward which to forward the packet. For RIP, the maximum number of hops is 15, which can be a serious limitation, especially in large nonhierarchical internetworks.

Distance-vector algorithms call for each router to send its entire routing table to only its immediate neighbors. The table is sent periodically (30 seconds for RIP and 90 seconds for IGRP). In the period between advertisements, each router builds a new table to send to its neighbors at the end of the period. Because each router relies on its neighbors for route information, it is commonly said that distance-vector protocols "route by rumor."

Having to wait half a minute for a new routing table with new routes is too long for today's networks. This is why distance-vector routing protocols have slow convergence.

RIPv2 and IGRP can send triggered updates—full routing table updates sent before the update timer has expired. A router can receive a routing table with 500 routes with only one route change, which creates serious overhead on the network—another drawback. Furthermore, RFC 2091 updates RIP with triggered extensions to allow triggered updates with only route changes. Cisco routers support this on fixed point-to-point interfaces.

The following is a list of IP distance-vector routing protocols:

• RIPv1 and RIPv2
• IGRP
• EIGRP (which could be considered a hybrid)
• RIPng

EIGRP

EIGRP is a hybrid routing protocol. It is a distance-vector protocol that implements some link-state routing protocol characteristics. Although EIGRP uses distance-vector metrics, it sends partial updates and maintains neighbor state information just as link-state protocols do. EIGRP does not send periodic updates as other distance-vector routing protocols do. The important thing to consider for the test is that EIGRP could be presented as a hybrid protocol. EIGRP metrics and mechanisms are discussed in Chapter 10, "RIP and EIGRP Characteristics and Design."

Link-State Routing Protocols

Link-state routing protocols address some of the limitations of distance-vector protocols. When running a link-state routing protocol, routers originate information about themselves (IP addresses), their connected links (the number and types of links), and the state of those links (up or down). The information is flooded to all routers in the network as changes in the link state occur. Each router makes a copy of the information received and forwards it without change. Each router independently calculates the best paths to each destination network, using a shortest path tree with itself as the root, and maintains a map of the network.

After the initial exchange of information, link-state updates are not sent unless a change in the topology occurs. Routers do send small Hello messages between neighbors to maintain neighbor relationships. If no updates have been sent, the routing table is refreshed after 30 minutes.

The following is a list of link-state routing protocols (including non-IP routing protocols):

• OSPF
• IS-IS
• OSPFv3
• IPX NetWare Link-Services Protocol (NLSP)

OSPF and IS-IS are covered in Chapter 11, "OSPF and IS-IS."

Distance-Vector Routing Protocols Versus Link-State Protocols

When choosing a routing protocol, consider that distance-vector routing protocols use more network bandwidth than link-state protocols. Distance-vector protocols generate more bandwidth overhead because of the large periodic routing updates. Link-state routing protocols do not generate significant routing update overhead but do use more router CPU and memory resources than distance-vector protocols. Generally, WAN bandwidth is a more expensive resource than router CPU and memory in modern devices.

Table 9-2 compares distance-vector to link-state routing protocols.

Table 9-2. Distance-Vector Versus Link-State Routing Protocols

Characteristic	Distance-Vector	Link-State
Scalability	Limited	Good
Convergence	Slow	Fast
Routing overhead	More traffic	Less traffic
Implementation	Easy	More complex
Protocols	RIPv1, RIPv2, IGRP, RIPng	OSPF, IS-IS, OSPFv3

EIGRP is a distance-vector protocol with link-state characteristics (hybrid) that give it high scalability, fast convergence, less routing overhead, and relatively easy configuration.

Hierarchical Versus Flat Routing Protocols

Some routing protocols require a network topology that must have a backbone network defined. This network contains some, or all, of the routers in the internetwork. When the internetwork is defined hierarchically, the backbone consists of only some devices. Backbone routers service and coordinate the routes and traffic to or from routers not in the local internetwork. The supported hierarchy is relatively shallow. Two levels of hierarchy are generally sufficient to provide scalability. Selected routers forward routes into the backbone. OSPF and IS-IS are hierarchical routing protocols.

Flat routing protocols do not allow a hierarchical network organization. They propagate all routing information throughout the network without dividing or summarizing large networks into smaller areas. Carefully designing network addressing to naturally support aggregation within routing-protocol advertisements can provide many of the benefits offered by hierarchical routing protocols. Every router is a peer of every other router in flat routing protocols; no router has a special role in the internetwork. RIPv1, IGRP, and RIPv2 are flat routing protocols. By default, EIGRP is a flat routing protocol, but it can be configured with manual summarization to support hierarchical designs.

Classless Versus Classful Routing Protocols

Routing protocols can be classified based on their support of VLSM and CIDR. Classful routing protocols do not advertise subnet masks in their routing updates; therefore, the configured subnet mask for the IP network must be the same throughout the entire internetwork. Furthermore, the subnets must, for all practical purposes, be contiguous within the larger internetwork. For example, if you use a classful routing protocol for network 130.170.0.0, you must use the chosen mask (such as 255.255.255.0) on all router interfaces using the 130.170.0.0 network. You must configure serial links with only two hosts and LANs with tens or hundreds of devices with the same mask of 255.255.255.0. The big disadvantage of classful routing protocols is that the network designer cannot take advantage of address summarization across networks (CIDR) or allocation of smaller or larger subnets within an IP network (VLSM). For example, with a classful routing protocol that uses a default mask of /25 for the entire network, you cannot assign a /30 subnet to a serial point-to-point circuit. Classful routing protocols are

• RIPv1
• IGRP

Classless routing protocols advertise the subnet mask with each route. You can configure subnetworks of a given IP network number with different subnet masks (VLSM). You can configure large LANs with a smaller subnet mask and configure serial links with a larger subnet mask, thereby conserving IP address space. Classless routing protocols also allow flexible route summarization and supernetting (CIDR). You create supernets by aggregating classful IP networks. For example, 200.100.100.0/23 is a supernet of 200.100.100.0/24 and 200.100.101.0/24. Classless routing protocols are

• RIPv2
• OSPF
• EIGRP
• IS-IS
• RIPng
• OSPFv3
• EIGRP for IPv6
• BGP

IPv4 Versus IPv6 Routing Protocols

With the increasing use of the IPv6 protocol, the CCDA must be prepared to design networks using IPv6 routing protocols. As IPv6 was defined, routing protocols needed to be updated to support the new IP address structure. None of the IPv4 routing protocols support IPv6 networks, and none of the IPv6 routing protocols are backward-compatible with IPv4 networks. But both protocols can coexist on the same network, each with their own routing protocol. Devices with dual stacks recognize which protocol is being used by the IP version field in the IP header.

RIPng is the IPv6-compatible RIP routing protocol. EIGRP for IPv6 is the new version of EIGRP that supports IPv6 networks. OSPFv3 was developed for IPv6, and OSPFv2 remains for IPv4. Internet drafts were written to provide IPv6 routing using IS-IS. Multiprotocol Extensions for BGP provide IPv6 support for BGP. Table 9-3 summarizes IPv4 versus IPv6 routing protocols.

Table 9-3. IPv4 and IPv6 Routing Protocols

IPv4 Routing Protocols	IPv6 Routing Protocols
RIPv1, RIPv2	RIPng
EIGRP	EIGRP for IPv6
OSPFv2	OSPFv3
IS-IS	IS-IS for IPv6
BGP	Multiprotocol Extensions for BGP

Administrative Distance

On Cisco routers running more than one routing protocol, it is possible for two different routing protocols to have a route to the same destination. Cisco routers assign each routing protocol an administrative distance. When multiple routes exist for a destination, the router selects the longest match. For example, if to reach a destination of 170.20.10.1 OSPF has a route prefix of 170.20.10.0/24 and EIGRP has a route prefix of 170.20.0.0/16, the OSPF route is preferred because the /24 prefix is longer than the /16 prefix. It is more specific.

In the event that two or more routing protocols offer the same route (with same prefix length) for inclusion in the routing table, the Cisco IOS router selects the route with the lowest administrative distance.

The administrative distance is a rating of the trustworthiness of a routing information source. Table 9-4 shows the default administrative distance for configured (static) or learned routes. In the table, you can see that static routes are trusted over dynamically learned routes. Within IGP routing protocols, EIGRP internal routes are trusted over OSPF, IS-IS, and RIP routes.

Table 9-4. Default Administrative Distances for IP Routes

IP Route	Administrative Distance
Connected interface	0
Static route directed to a connected interface	0
Static route directed to an IP address	1
EIGRP summary route	5
External BGP route	20
Internal EIGRP route	90
IGRP route	100
OSPF route	110
IS-IS route	115
RIP route	120
EGP route	140
External EIGRP route	170
Internal BGP route	200
Route of unknown origin	255

The administrative distance establishes the precedence used among routing algorithms. Suppose a router has an EIGRP route to network 172.20.10.0/24 with the best path out Ethernet 0 and an OSPF route for the same network out Ethernet 1. Because EIGRP has an administrative distance of 90 and OSPF has an administrative distance of 110, the router enters the EIGRP route in the routing table and sends packets with destinations of 172.20.10.0/24 out Ethernet 0.

Static routes have a default administrative distance of 1. There is one exception. If the static route points to a connected interface, it inherits the administrative distance of connected interfaces, which is 0. You can configure static routes with a different distance by appending the distance value to the end of the command.