Routing Traffic Transport and Database Synchronization

The EIGRP routing protocol and autonomous system number is enabled by way of the global configuration command router eigrp autonomous-system-number . As mentioned, an AS is a logical grouping of routers that fall under a common administration. Here is the syntax for starting an EIGRP routing process on a router at the global configuration prompt:

 RouterA(config)# router eigrp autonomous-system-number 

The autonomous-system-number parameter is any number between 1 and 65,535 (16-bit). Make sure that you configure all the EIGRP routers in your network with the same AS number. Listing 7.5 will properly configure a serial interface on RouterBBR with an IP address of 10.1.1.1 and a 24-bit mask, as well as a bandwidth value of 128 Kbps. The EIGRP router process is enabled and defined with AS 100 as well.

Listing 7.5 A Sample EIGRP Serial Interface Configuration

 RouterBBR(config)# interface s0 RouterBBR(config-if)# ip address 10.1.1.1 255.255.255.0 RouterBBR(config-if)# bandwidth 128 <Output Omitted> RouterBBR(config)# router eigrp 100 RouterBBR(config-router)# network 10.0.0.0 RouterBBR(config-router)# network 172.168.0.0 255.255.0.0 

The network network-number router configuration command is used on RouterBBR to identify the networks on which this EIGRP router will advertise routes.

We also used a bandwidth value of 128 for our serial interface in Listing 7.5. Realize that an EIGRP routing process will use up to 50% of the bandwidth of any interface on which it is configured. When configuring EIGRP on serial interfaces, you issue the bandwidth kilobits command while the router is still in interface configuration mode. Remember that the speed (bandwidth) of the link must be expressed in kilobits. This is an important step because EIGRP uses the bandwidth value when it computes the cost of the route.

If no bandwidth is designated, EIGRP assumes that the bandwidth on the serial link is 1.544Mbps (T1) and makes its routing decisions based on this default value.

Because EIGRP supposes that serial links operate at T1 speeds, it may be important to manually configure the bandwidth of the connection to prevent the link from being saturated with routing updates. EIGRP can be configured to use a set percentage of declared bandwidth on either an interface or subinterface by manually overriding the default. Setting the allocation percentage too high could result in a loss of EIGRP packets, whereas configuring the value too low increases convergence time and drastically reduces network performance. The command bandwidth 256 , for example, allocates up to 50% (128Kbps) of the specified bandwidth.

If your present routing policy demands that you declare a low bandwidth value on an interface, this percentage of utilization can be adjusted to a value higher than 100%. To modify the default settings, you must first place the router in interface configuration mode by using the interface serial interface-number command. Then you can adjust the bandwidth on an interface or subinterface with the following syntax:

 RouterA(config-if)#  ip bandwidth-percent eigrp   as-number percent  

For example, the following entries configure EIGRP to utilize 200% of the configured bandwidth on the first subinterface connected to AS number 10. Here we have set the percent parameter to a value larger than 100% to overcome an unusually low bandwidth configured for the routing policy:

 RouterA(config-if)# interface serial 0.1 RouterA(config-if)# ip bandwidth-percent eigrp 10 200 

The interface number should be typed as a whole number that designates the interface that should be configured, or as two numbers that are separated by a period that corresponds to interface.subinterface .

In another example, we use the code from Listing 7.5 to set both the bandwidth and the bandwidth-percent parameter at the same time for a fractional T1:

 RouterA(config)# interface s0 RouterA(config-if)# ip address 10.1.1.1 255.255.255.0 RouterA(config-if)# bandwidth 128 RouterA(config-if)# ip bandwidth-percent eigrp 100 200 

In the preceding sample, we allow EIGRP to utilize the CIR for a fractional T1 of 256Kbps on the interface (200% of the configured 128Kbps bandwidth).

EIGRP can distribute traffic across four equal-cost paths, by default, to increase utilization and improve the effective network bandwidth. The maximum-paths router configuration command is implemented to increase the number of equal-cost routes to the same destination network. Using this command, you can ask for up to six equal routes to be stored in the EIGRP topology table. The default number of equal-cost paths that can be stored by EIGRP is four; the maximum-paths command increases that number to six.

The router configuration command variance multiplier enables you to load balance between two or more redundant routes with different metrics by assigning a multiplier value to a route's current feasible distance. This allows for the use of other routes to the same network but with higher metrics than the feasible successor route.

A separate variance table does not exist.

The multiplier keyword is an integer between 1 and 128 with a default value of 1. A value of 1 indicates that equal-cost load balancing will be performed by the router. When the multiplier value is increased, the router multiplies the value by the metric of the successor route for which multiple paths exist. Next, the router compares the metric of the redundant routes with the modified metric of the successor route. Any redundant routes that have a metric value less than the modified metric of the successor route are included in the load-balancing process and will be used by the router. The amount of traffic that the router will send over the unequal -cost paths is proportional to the degree by which the route's metric varies from the successor route's modified metric.

In Figure 7.2, RouterA declares RouterB as the successor to RouterD, instead of RouterC, because it has the lowest feasible distance value of 20 (10+10) as opposed to 50 (20+30). After you configure RouterA with a variance multiplier value of 2, RouterC may be used for unequal-cost load balancing if certain conditions are met. Figure 7.2 provides an example of when the two conditions are not met. Notice that the advertised distance value on RouterC to RouterD is 30. Because RouterC's advertised distance value of 30 is greater than RouterA's feasible distance of 20, it will not be considered for load balancing. More importantly, RouterC will not be considered because the feasible distance through RouterC (50) is more than twice (variance of 2) the feasible distance of the successor RouterB (20).

Figure 7.2. Using the variance keyword in EIGRP operations.

Figure 7.3 illustrates an example of when EIGRP load balances with a variance multiplier of 2.

Figure 7.3. An EIGRP router load balancing using the variance keyword.

In this example, RouterC may be used for unequal-cost load balancing because two conditions are met. First, the advertised distance on RouterC to RouterD is 15. Because RouterC's advertised distance value of 15 is less than RouterA's feasible distance of 20, it will be considered for load balancing. More importantly, RouterC will be considered because the feasible distance through RouterC (30) is less than twice (variance of 2) the feasible distance (20) of the successor RouterB. This is a simple example. If you had four to six multiple paths, you could experiment with different variance multipliers to include or exclude certain unequal cost paths.