Greg explores how and why subnets are exchanged in the topology exchange process of EIGRP. Learn about the function of split-horizon and when it's useful to disable it.
- [Instructor] Once EIGRP forms neighbors, the exchange of topology information can begin. Topology primarily exists of prefix, prefix length, and metric information. This topology information comes from one of a few sources, either connected subnets that are covered by a network command, connected subnets on interfaces that have the neighbor command configured, or any prefixes that have been redistributed into the EIGRP process.
On Router 2, I've adjusted the bandwidth to 100 and the delay value to 400, which should make this link fairly undesirable. Connecting to Router 1 and examining the topology table shows the adjustments. Issuing a show ip eigrp topology 192.168.1.0/28 should show two possible routes for this subnet. I'm going to scroll up a little bit to show the metric value for both.
The top entry, which is Router 3, shows standard bandwidth and accumulated delay values. Scrolling back down, we see the bottom entry, which is Router 2 whose settings were adjusted, shows the lowest bandwidth on the link as well as the much higher total delay. Issuing a show ip route verifies that Router 3 is the chosen path. Seems logical to me. I'll now connect to Router 3 and show the topology table for 192.168.0.0/30, show ip eigrp topology 192.168.0.0/30.
This is the subnet that connects Router 1 to Router 2. Which path will be preferred? An administrator wouldn't be blamed for thinking that the path to Router 1 would always be preferred, since Router 2 had its link settings adjusted and should appear as a poor choice, right? Wrong. Looking at the topology information, bandwidth and delay are identical. Bandwidth and delay are calculated on outgoing interfaces in the path. Since bandwidth and delay settings are identical on all outgoing interfaces from Router 3's perspective, they are equal cost paths.
Something else to discuss when talking about topology is EIGRP's use of split horizon. In EIGRP, split horizon will prevent a router from advertising a prefix to a neighbor who is listed as best path for that same prefix. Referring to our previous example's diagram, I'll examine Router 1's topology table. It seems like there should be two routes for 192.168.1.0/28, which is the subnet between Router 2 and 3, but since Router 2's topology table sees Router 1 as best path, it won't advertise the 192.168.1.0/28 prefix to Router 1.
Thus the topology table on Router 1 sees no prefix coming from Router 2. There are some special use cases where split horizon can and should be disabled on an interface. The main case an administrator is likely to run into is in a frame relay network utilizing a single subinterface. It all boils down to the non-broadcast multi-access behavior of these links. Generally, it's several remote sites connecting to a hub site, though the remotes can't communicate with each other, so there's no full mesh over the connection.
Looking at this example, Remote 1 will send topology info on 172.16.0.0/24 to the hub site over its single frame relay interface. The nature of frame relay is that it doesn't allow full mesh connectivity, so Remote 1 and Remote 2 can't communicate, so all information must loop through the hub site. Unfortunately, due to split horizon, the topology info sent from Remote 1 won't be allowed to be retransmitted out of the preferred path interface it was received on.
So Remote 2 has no knowledge of 172.16.0.0/24. In this example, the command no ip split-horizon can be added to serial 0/0/1.10. After this, the topology info from the remotes can be shared. While talking about frame relay, a small design consideration is how much EIGRP traffic is produced. By default, EIGRP is allowed to use 50% of the interface bandwidth.
If this happens to be a legacy link, like a T1, this could theoretically be problematic. Cisco developed the interface ip bandwidth-percent subcommand specifically to deal with this. The percent value is what percent of the overall bandwidth EIGRP is allowed to consume. Any messages that exceed this threshold are queued in memory. I'd suggest reviewing the first half of this chapter again. The concept of outgoing cost can be slightly confusing.
- Configuring a basic Enhanced Interior Gateway Routing Protocol (EIGRP) network
- Static neighbors or K-values
- Topology exchange
- Limiting query scope
- EIGRP default routing
- EIGRP for IPv6
- Open Shortest Path First (OSPF) fundamentals