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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.
- 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.
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
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
- 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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