EIGRP Metrics
EIGRP uses metrics in the same way as IGRP. Each route in the route table has an associated metric. EIGRP uses a composite metric much like IGRP, except that it is modified by a multiplier of 256. Recall from Chapter 10, "Distance Vector Protocols: Interior Gateway Routing Protocol (EIGRP)," that bandwidth, delay, load, reliability, and MTU are the submetrics. Like IGRP, EIGRP chooses a route based primarily on bandwidth and delay, or the composite metric with the lowest numerical value. When EIGRP calculates this metric for a route, it calls it the feasible distance to the route. EIGRP calculates a feasible distance to all routes in the network. The following list is a detailed description of the five EIGRP submetrics:
• Bandwidth—Bandwidth is expressed in units of kilobits. It must be statically configured to accurately represent the interfaces that EIGRP is running on. For example, the default bandwidth of a 56-kbps interface and a T1 interface is 1544 kbps. To accurately adjust the bandwidth, use the bandwidth kbps interface subcommand. Table 11-1 highlights some common bandwidth values.
• Delay—Delay is expressed in microseconds. It, too, must be statically configured to accurately represent the interface that EIGRP is running on. The delay on an interface can be adjusted with the delay time_in_microseconds interface subcommand. Common delay values are represented in Table 11-1.
• Reliability—Reliability is a dynamic number in the range of 1 to 255, where 255 is a 100 percent reliable link and 1 is an unreliable link.
• Load—Load is the number in the range of 1 to 255 that shows the output load of an interface. This value is dynamic and can be viewed using the show interfaces command. A value of 1 indicates a minimally loaded link, whereas 255 indicates a 100 percent loaded link.
• MTU—The maximum transmission unit (MTU) is the recorded smallest MTU value in the path, usually 1500.
NOTE
Whenever you are influencing routing decisions in IGRP or EIGRP, use the metric of delay over bandwidth. Changing bandwidth can affect other routing protocols, such as OSPF. Changing delay affects only IGRP and EIGRP.
Table 11-1 highlights the common metrics used.
Table 11-1 Common IGRP and EIGRP Metrics
Table 11-1 highlights the common metrics used.
Table 11-1 Common IGRP and EIGRP Metrics
|
Medium |
Bandwidth |
Delay |
|
100-Mbps ATM |
100,000 kbps |
100 |ls |
|
Gigabit Ethernet |
100,000 kbps |
100 |ls |
|
Fast Ethernet |
100,000 kbps |
100 |ls |
|
FDDI |
100,000 kbps |
100 |s |
|
HSSI |
45,045 kbps |
20,000 | s |
|
16-Mbps Token Ring |
16,000 kbps |
630 | s |
|
10-Mbps Ethernet |
10,000 kbps |
1000 |ls |
|
T1 |
1544 kbps |
20,000 | s |
|
DS-0 |
64 kbps |
20,000 | s |
|
56-kbps media |
56 kbps |
20,000 | s |
EIGRP uses a composite metric (CM) that is derived from the five submetrics. When EIGRP computes the composite metric, it uses a formula that involves five constants or "k" values. The constant values have default value such as the following:
By setting k2, k4, and k5 to 0, it essentially nullifies the submetrics of load, reliability, and MTU. This is precisely why you should first use delay and then bandwidth when trying to influence which routes EIGRP prefers. The formula EIGRP uses to calculate the composite metric is as follows:
CM = 256 x ([k1 x BWmim + (k2 x BWmim) / (256-LOAD) + k3 x DELAYsum] x X)
where the following is true:
BWmim = 10' / bandwidth_of_slowest_link DELAYsum = I (delays_along_the_path)
X = k5 / (reliability + k4) if and only if k1<>1, if k1 = 1 then X = 1
With the k values set at the default value you have k1 = k3 = 1 k2 = k4 = k5 = 0
NOTE The router calculation of the composite metric will always differ slightly from the result when it is performed by longhand. This is because of the way the router handles floatingpoint mathematics; there will be slight rounding discrepancies.
Using the default values of constants, k1 = k3 = 1 and k2 = k4 = k5 =0, the formula quickly breaks down to this:
Substituting the constants, you have the following:
CM = 256 x ([1 x BWmim + (0 * BWmm) / (256-LOAD) + 1 x DELAYsum] x 1) CM = 256 x ([BWmim + (0) / (256-LOAD) + DELAYsum] x 1) CM = 256 x (BWmim + DELAYsum)
NOTE For reference, the metric is computed the same way for IGRP, except the result of bandwidth and delay is not multiplied by 256, and the DELAYsum variable is divided by 10. CM = (k1 x BWmin + [k2 x BWmin] / [256-LOAD] + [k3 x DELAYsum] x X) where the following is true:
BWmin = 10' / bandwidth_of_slowest_link
DELAYsum = S(delays_along_the_path) / 10
X = k5 / (reliability + k4) if and only if k1<>1, if k1=1 then X=1
k1=k3=1
k2=k4=k5=0
With k values set at the default value, you have: CM = BWmin + DELAYsum
To demonstrate composite metric calculation, refer to Figure 11-1. In this example, EIGRP calculates a composite metric on the alpha router to 172.16.1.0/24, which resides on the charlie router.
Assuming that the bandwidth statements been set by an astute engineer, the lowest bandwidth on the path between alpha and charlie routers would be 56. Therefore, you have
The delay is the summation of the delays on the outbound interfaces only. The summation ends with the delay on the interface in which the final subnet resides. From alpha to bravo, the delay is 20000; from bravo to charlie, it is 1000; this includes the final interface on charlie, which has a delay of 1000. Therefore, you have
The composite metric now yields the following:
Figure 11-1 EIGRP Routing Updates
EIGRP 65001
alpha
56 kbps Delay-2000^S Bandwidth=56
56 kbps Delay-20000^S Bandwidth=56
bravo
10 Mbps Delay=1000^S Bandwidth=10000
10 Mbps Delay=1000^S Bandwidth=10000
10 Mbps Delay=1000^S Bandwidth=10000
charlie
10 Mbps Delay-1000^S Bandwidth=10000
The submetrics and the composite metric can be confirmed by performing the show ip route 172.16.1.0 command on the alpha router, as in Example 11-1. Remember, because of rounding errors, the metric does not match exactly. Example 11-1 show ip route Command Output Highlighting the EIGRP Metrics alpha#show ip route 172.16.1.0
Routing entry for 172.16.1.0/24
Known via "eigrp 65001", distance 90, metric 46277376, type internal Redistributing via eigrp 65001
Last update from 172.16.3.1 on Serial7, 00:50:53 ago Routing Descriptor Blocks:
* 172.16.3.1, from 172.16.3.1, 00:50:53 ago, via Serial7 Route metric is 46277376, traffic share count is 1
Total delay is 22000 microseconds, minimum bandwidth is 56 Kbit Reliability 255/255, minimum MTU 1500 bytes Loading 1/255, Hops 2
alpha#
When using metrics to influence routing decisions, use the delay xx interface command. Be sure to include a delay at each side of the interface if you want symmetrical routing—that is, packets will take the same route back to the source. By default, EIGRP will perform equal-cost load balancing over routes. For example, if you perform a show ip route command and see two routes to a destination reported, EIGRP will load-balance over those routes.
To demonstrate the use of the delay metric, we have added another Ethernet segment between the bravo and charlie routers and a loopback interface, 172.16.128.1/24, on the charlie router, as illustrated in Figure 11-2.
Figure 11-2 EIGRP Load Sharing
EIGRP 65001
alpha
56 kbps Delay = 20000|iS Bandwidth = 56
56 kbps Delay = 20000| S Bandwidth = 56
bravo
E5-IP-172.16.16.2/24
10 Mbps Delay =1000| S Bandwidth = 10000
charlie
Loopback 20 IP-172.16.128.1/24
If you perform a show ip route command on the bravo router, as shown in Example 11-2, you see two routes to the 172.16.128.0/24 network. The show ip eigrp topology command also lists the routes and the composite metric to them.
Example 11-2 Two Routes Reported to 172.16.128.0/24
bravo#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks D 172.16.128.0/24 [90/409600] via 172.16.2.1, 00:23:50, Ethernet4
[90/409600] via 172.16.16.1, 00:23:50, Ethernet5 C 172.16.16.0/24 is directly connected, Ethernet5
C 172.16.2.0/24 is directly connected, Ethernet4
C 172.16.3.0/30 is directly connected, Serial1
bravo#
If you want EIGRP to prefer one path to the other, add the delay command on each side of the interface. It is important to note that changing the delay of a link will affect only the routing protocol, not the actual throughput of the link.
Continuing with the example, set the delay of the link so that the primary link to 172.16.128.0 will be through 172.16.16.1. This can be accomplished by adding a delay of 1000 to the e4 interface of the bravo router and under the e0/1 interface of the charlie router. Example 11-3 demonstrates the configuration of delay on the bravo router.
Example 11-3 Addition of the delay Command bravo#conf t
Enter configuration commands, one per line. End with CNTL/Z. bravo(config)#int e4 bravo(config-if)#delay 1000 bravo(config-if)#~Z
Example 11-4 shows the route table of the bravo router after the delay was added to the bravo and charlie routers.
Example 11-4 One Route to the 172.16.128.0/24 Route bravo#show ip route
172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
D 172.16.128.0/24 [90/409600] via 172.16.16.1, 00:00:11, Ethernet5
C 172.16.16.0/24 is directly connected, Ethernet5
C 172.16.2.0/24 is directly connected, Ethernet4
C 172.16.3.0/30 is directly connected, Serial1
bravo#
Keep in mind that although the second route is removed from the routing table, EIGRP still knows of the route and will keep it as a feasible successor.
The k values also can be manipulated to influence routing decisions. This can be accomplished with the metric weights tos k1 k2 k3 k4 k5 command. Manipulating these values directly impacts how EIGRP derives the composite metric for all routes. Change the metric weights only when working with Cisco to solve specific problems.
Continue reading here: Show ip eigrp neighbors Command
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