EIGRP enhancements

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary IGP. So if you have several vendors inside your corporate LAN like HP or Juniper then it’s probably not your choice. However, EIGRP has several enhancements that make it even faster in convergence time in comparison to OSPF.

One of the main drawbacks of OSPF is that it consumes considerable amount of memory to maintain LSDB and CPU power to run Dijkstra on it. EIGRP doesn’t do that. Routers with EIGRP enabled on their interfaces exchange only partial information with their neighbors, as OSPF does. But EIGRP routers don’t maintain the whole topology. On that matters they behave more like RIP. Each router holds information about networks and next hop routers to reach them. But unlike RIP, for each network EIGRP finds primary and secondary (if possible) routes. So that in case of link failure router could immediately switch to the backup route. In EIGRP terminology main route is called successor route and alternative route is feasible successor route.

Also, EIGRP has more sophisticated metric calculation. It considers not only bandwidth, but also delay. The formula is:

metric  = (10^7 / least-bandwidth + cumulative-delay) * 256

Here least-bandwidth is the slowest link speed in kbps along the path and cumulative-delay is sum of all delays from the network to the router in tens of microseconds.

To understand how EIGRP preventsloops there is a need for another two terms. Feasible Distance (FD) is a metric of the best route to reach a subnet, as calculated on a router. And Reported Distance (RD) is a metric as calculated on a neighboring router and then reported and learned in an EIGRP update. The trick here is that route can be a feasible successor route only if its RD is less than FD. It guarantees that this route doesn’t go through this router. Because otherwise it would obviously be greater than FD.

Again, EIGRP is better IGP from all perspectives. The only barrier that restricts its proliferation is proprietary nature of the protocol.

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