Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I always thought that neighbors formed in EIGRP were not necessarily directly connected because the only requirement for two routers to be neighbors is that they share the same Autonomous System and K-values, but it seems that I was wrong. I have looked up several books, they do not seem to give a clear answer.

And I am even more confused with OSPF...

Please, give me some light here. Thank you.

share|improve this question
up vote 2 down vote accepted

Neighbors are the routers that are directly accessible through directly connected interfaces.

EIGRP and IGRP are used for routing within an autonomous system. Inter-AS dynamic routing is set up using the BGP protocol.

OSPF is another inter-AS dynamic routing protocol.

Since an autonomous system is formed by a connected set of routers it is true that routers that participate in EIGRP or OSPF are always connected.

Neighbors are the routers that are directly accessible through directly connected interfaces. That does not necessary mean a physical wire connecting the two routers. That only means that there is no EIGRP router in between.

share|improve this answer
Thank you so much for the answer. Do you mean that neighbors are only formed between directly connected EIGRP routers/OSPF routers(the same rule applies to OSPF?)? – xczzhh Jun 2 '12 at 12:41
Yes that is the correct setup. – Dmitri Chubarov Jun 2 '12 at 12:48
Thank you again. But, I've learnt from books that neighbor routers in OSPF send LSA to each other to communicate. As LSA messages transmit in the network by means of flooding, meaning they go across routers after routers(assuming that they are all OSPF routers), I am not so sure what this flooding means, isn't it enough by just using multicasting or broadcasting? – xczzhh Jun 2 '12 at 15:08
OSPF LSAs are send with multicast. Quote RFC2328: "OSPF also provides for the authentication of routing updates, and utilizes IP multicast when sending/receiving the updates" – Dmitri Chubarov Jun 2 '12 at 15:26
Keep reading the wikipedia entry. There are certain multicast addresses that are explicitly not to be forwarded beyond the local subnet by any router. These include the addresses used by EIGRP and OSPF. Routers can reach one another efficiently (i.e. without having to form a full mesh of unicast connections ) but each router still has to repackage the routing information to multicast (..or unicast) these updates out another interface. – rnxrx Jun 3 '12 at 1:16

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.