EIGRP over GRE tunnel

Unanswered Question
Oct 19th, 2007

Have A customer of would like to implement EIGRP over GRE, would routers drop the Multicast routing updates.

The Multicast has a TTL of 1 but the GRE tunnel shows no hop.

thank you

I have this problem too.
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Edison Ortiz Fri, 10/19/2007 - 09:49

When creating the GRE tunnel, make sure they are part of the same IP subnet.

Example,

RouterA

interface tunnel1

ip address 10.0.0.1 255.255.255.252

router eigrp 1

network 10.0.0.1 0.0.0.0

RouterB

interface tunnel2

ip address 10.0.0.2 255.255.255.252

router eigrp 1

network 10.0.0.2 0.0.0.0

it should work w/o problems (need to verify reachability beforehand).

Richard Burts Fri, 10/19/2007 - 09:59

I have run EIGRP over GRE tunnels many times. It works. The TTL is not an issue. The EIGRP multicast packet with TTL of 1 is encapsulated in an IP unicast header for transmission to the tunnel destination. It is the TTL of the GRE header that is decremented as the packet is forwarded through intermediate networks toward the tunnel destination. At the tunnel destination when the EIGRP packet is evaluated its TTL is valid.

HTH

Rick

Edison Ortiz Fri, 10/19/2007 - 10:27

TTL on a tunnel is not an issue as unicast/multicast traffic is treated as being on the same segment.

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