From: Jian Gu (guxiaojian@gmail.com)
Date: Fri Dec 29 2006 - 22:44:30 ART
When a GRE packet comes in, the router will check both source and
destination of GRE header to decide which interface the packet is destined
to, if two tunnels have exactly the same tunnel source and destination, the
router will have no way to tell which tunnel interface the packet should go
to. When a packet is delievered to the wrong tunnel interface, routing
protocol will complain "not on common subnet" because the source IP address
of the tunnels are different.
This configuration should not be a problem if no routing protocol is running
on top the tunnels.
On 12/29/06, Swan, Jay <jswan@sugf.com> wrote:
>
> I believe you cannot have EIGRP running over two GRE tunnels that
> terminate on the same loopback interfaces without this happening. I
> actually tried to do this in an odd real world scenario a few years ago,
> and that was the answer that came back from the TAC escalation team. We
> had to create separate loopback interfaces to terminate each tunnel.
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto: nobody@groupstudy.com] On Behalf Of
> John Matus
> Sent: Friday, December 29, 2006 9:47 AM
> To: prashant shukla
> Cc: ccielab@groupstudy.com; nobody@groupstudy.com
> Subject: Re: EIGRP_over 2-Tunnels, complains " not on common subnet".
>
> try using tunnel destination of "loopback0" instead of using the ip
> address of the interface.
>
> Regards,
>
> John D. Matus
> Technical Support / PAS
> Fujitsu Consulting
> 626-568-7716
> John.Matus@tokiom.com
>
>
>
> prashant shukla < shukla_cisco@yahoo.co.in> Sent by:
> nobody@groupstudy.com
> 12/29/2006 06:29 AM
> Please respond to
> prashant shukla <shukla_cisco@yahoo.co.in >
>
>
> To
> ccielab@groupstudy.com
> cc
>
> Subject
> EIGRP_over 2-Tunnels, complains " not on common subnet".
>
>
>
>
>
>
> Gurus,
>
> I have a simple 2 routers over a serial link.
>
> i create 2 Tunnels between them , if i keep the "tunnel Source" and
> "tunnel dest" to exactly same as Loopback 0 , Eigrp complains regarding
> "neigh not on common subnet" if i keep then diff , as in 2 diff
> sources/dests , its works.
>
> I know its got to do with primary IP add space, cant feel the real
> pulse
> here.
>
> config on R1:
>
> interface Loopback0
> ip address 1.1.1.1 255.255.255.255
> no ip directed-broadcast
> no clns route-cache
> !
> interface Tunnel0
> ip address 128.1.1.2 255.0.0.0
> no ip directed-broadcast
> no clns route-cache
> tunnel source Loopback0
> tunnel destination 2.2.2.2
> !
> interface Tunnel1
> ip address 129.1.1.100 255.0.0.0
> no ip directed-broadcast
> no clns route-cache
> tunnel source Loopback0
> tunnel destination 2.2.2.2
> !
> interface Serial0/0
> ip address 172.16.1.100 255.255.0.0
> no ip directed-broadcast
> no clns route-cache
>
> same config at R2 too.
>
> Shukla.
>
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