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New Member

EIGRP Ajacenies are Unidirectional. Why?

As EIGRP only looks at incoming hello packets (EIGRP type 5 packets) to keep a peer neighbor relationship up on a particular side of a routed link., this can be a prime issue for "black hole" routing issues.

OSPF adjacencies are bi-directional. Makes more sense.

Why was EIGRP designed this way?


Re: EIGRP Ajacenies are Unidirectional. Why?

Two routers running EIGPR become neighbors when they see each other's hello packets on a common network. So the issue of black holes does not rise.

The basic problem with sending only routing updates is that you may not know when a path through a neighboring router is no longer available. You can't time out routes, expecting to receive a new routing table from your neighbors. EIGRP relies on neighbor relationships to reliably propagate routing table changes throughout the network; two routers become neighbors when they see each other's hello packets on a common network.

EIGRP sends hello packets every 5 seconds on high bandwidth links and every 60 seconds on low bandwidth multipoint links.

Refer this whitepaper for details:

New Member

Re: EIGRP Ajacenies are Unidirectional. Why?

Hello, Thx for that.

The problem arises when you have some form of cct with a TX and RX signal, and the link fails in one direction, ie the TX leg of a FR cct or other serial cct fails.

EIGRP will stay up in one direction thus causing "black holes" as the route over this link still appears to be valid in the routing table. UDLD has been developed for LANs for this type of problem.

What happens is that you will find that you have to configure your L2 keepalive protocol to co-inside with your EIGRP timers if you require fast convergence.

I see this as a problem :((

Many thx for taking time to read my post :)