www.networkbulls.com
Best Institute for CCNA CCNP CCSP CCIP CCIE Training in India
M-44, Old Dlf, Sector-14 Gurgaon, Haryana, India
Call: +91-9654672192
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
- 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.
- 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.
- 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.
- 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.
| 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.
| 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.
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.
No comments:
Post a Comment