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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.

MadHatter
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xczzhh
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1 Answers1

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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.

Dmitri Chubarov
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  • 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 02 '12 at 12:41
  • Yes that is the correct setup. – Dmitri Chubarov Jun 02 '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 02 '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 02 '12 at 15:26
  • Neighbors in OSPF, EIGRP or IS-IS send updates via local subnet multicast and then bundle and propagate these updates to other networks to which they are connected. This is flooding - a given LSA (for example) is seen- and forwarded by all of the OSPF speaking routers out of all of their (non-passive) OSPF-speaking interfaces. – rnxrx Jun 02 '12 at 15:33
  • @rnxrx thank you, but do multicast messages go over routers? or they are just limited to a broadcasting domain or...? – xczzhh Jun 02 '12 at 15:36
  • The multicasts are specifically to the local subnet only. Each router recalculates and compiles the updates before transmitting out its other interfaces. – rnxrx Jun 02 '12 at 15:44
  • @rnxrx thank yo again. According to the wikipedia entry of IP multicast, _Multicast uses network infrastructure efficiently by requiring the source to send a packet only once, even if it needs to be delivered to a large number of receivers. The **nodes** in the network take care of **replicating** the packet to reach multiple receivers only when necessary._ It seems not clear whether the replicated messages are the same multicast messages? – xczzhh Jun 03 '12 at 00:31
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    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 03 '12 at 01:16