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Route This Way: (E)IGRP




IGRP information is...scant. It's made by Cisco, and they have since moved on to EIGRP.



  • EIGRP talks to its neighbors it knows via the neighbor table. Neighbors are directly connected.
  • The topology table stores routes it learns from its directly connected neighbors 
  • Hellos are sent every few seconds (Depending on Network capability) to make sure the neighbor is still there. A hold time is around 15 seconds. The router will drop the connection if the neighbor doesn't respond.
  • Routers get updates via multicast address 224.0.0.10





 New commands -

 🧠 no auto-summary - Back in RIP-land, I advertised the networks using the networks and not the individual IP addresses. ("Summarize at the boundary to the classful network.") You may not wish to do that in EIGRP.

🥏  router eigrp [AS#]
The AS is Autonomous System number. Essentially, "How many routers are under the same networking entity?" Imagine a Botnet, but legal, and helping the internet to run.

🐲 Wildcard masks
With RIP, we could put in the first part (Network, maybe the host) of an IP address and be fine. The router understood it. Now, we have to wildcard-inverse-mask.

Subnet mask of 255.255.255.0 (/24) would be 0.0.0.255. Imagine it as "What's left in this IP segment."

(/30) (255.255.255.252) Would be 0.0.0.3 (I think. The 1 and 2 bits at the end remain.)


Router A (I renamed them part of the way through this to better keep track)


Note the subnet wildcard bits.

Onto Router B - and the adjacency is made!


And Router C


show ip route


Look under 'variably subnetted' - see the [90/2681856]?

That is the administrative distance. Lower is better. 90 is EIGRP, 0 (or 1) would be static.

show ip eigrp neighbors


One on the left, and one on the right.

The bits I didn't remember I used resources for, one of which is this. The topology is all mine.
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