Cisco – Are neighbors formed in EIGRP and OSPF always directly connected

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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.

Best Answer

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.

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Firewall – Border firewalls and routing — static vs dynamic routing

Put external routers into HSRP/VRRP pair. Get an AS. Start doing BGP with your ISPs.

Then, everywhere inside your environment, the paths will be static. You just need to have failover, and that's what's achieved with VRRP/HSRP and firewalls in A/A or A/S configuration.

Alternatives are messier and/or less robust:

Dynamic throughout is an unnecessary hassle.
Static outbound on the firewalls would make it difficult/inflexible/impossible to use both uplinks simultaneously.
Inbound redundancy will be a world of pain without kludges like PePLink/Ecessa/EdgeXOS.

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You're running a discontinuous classful network 172.16.0.0 / 16 in two network portions. In this particular case, auto summarization will make your routing table inconsistent by summarizing the network above and advertising it to the other side and vice-versa.

Try adding no auto-summary to your eigrp settings on ALL routers so that the final configuration looks like:

    router eigrp 1
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     network x.x.x.x

Best Answer

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