From: V.Shekhar@GlobalAssurance.net
Date: Tue Apr 29 2008 - 12:44:15 ART
All OSPF adjacencies needs to be L3 adjacent. (Unlike BGP)
all OSPF hello packet s, be it multicast or unicast is always having a
TTL of 1, which a L3 hop will not forward.
HTH.
-V Shekhar
CCIE(sec)#17589/CISSP/RHCE.
Alexandre Ribeiro wrote:
> Hello all,
>
> I'm currently studying OSPF over FR with a bit more depth. I configured a
> simple FR scenario, with R1 as the hub and R2 and R3 as spokes. I disabled
> inarp and I statically mapped R2 and R1's ip address on R3, and R1 and R3 on
> R2.
>
> Now, the typical lab scenario would have you configure OSPF over this, with
> or without changing OSPF's network type on the interfaces, or with or
> without subinterfaces. I've done all possible lab configs and I'm over that.
> What I wanted to understand is why the hub MUST have L2 connectivity to the
> spokes, even if you're using OSPF's non-broadcast mode on the interfaces,
> with neighbor commands for the respective neighbors. In order to reach a
> conclusion, I enabled OSPF on R2 and R3 (but not on R1). I verified that
> they had connectivity (ping), and I placed a neighbor command from R2 to R3
> and vice-versa. Now, the neighbor command causes hello's, DBDs and LSA
> updates to be sent as unicast messages, so I figured that these messages
> should reach R3 from R2 and vice-versa. But no...
>
> Doing a debug ip packet showed that even though hello's were being sent
> between R2 and R3, they we're not arriving at their destination. I
> replicated this scenario in Dynamips, captured the traffic, and I saw that
> R1 (the hub) was receiving the hellos, but instead of forwarding them (it
> should, they're IP packets), it replies with an ICMP TTL exceeded. Why does
> this happen? If FR provides connectivity for IP, why doesn't it forward
> OSPF's messages when they're being unicasted over IP?
>
>
> Thanks in advance,
>
> Alex
>
>
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