> On 03/06/11 11:30, Sam Wilson wrote:
> > We've tripped over an oddity in our routing set up and I'm baffled.  I
> > hope this is a suitable list to ask for assistance on.
> >
> > We use BGP on our metropolitan network with private ASs.  At the core
> > are two Cat 6500s running 12.2(33)SXI1.  They run IBGP between them with
> > no sync and, obviously, no IGP on a dedicated point-to-point 2x10GE
> 
> You say "obviously no IGP" but in my experience iBGP is almost always 
> accompanied by an IGP. Can you expand on this?

Yes.  You can run iBGP with an IGP, in which case BGP will only use
routes which have already been installed in the routing table by the
IGP, or you can run "no synchronization" where it doesn't have that
requirement.  For ASs where the border routers are separated by more
than a single hop the IGP makes sense; where they are adjacent not so
much. 

> > What seems to happen is that each router advertises itself as the next
> > hop for prefixes in the IBGP.  Each router has a connected route for the
> 
> Again, not a common config in my experience; iBGP routers do not 
> normally alter the next-hop, and usually let the IGP resolve it.

As I said, no IGP and it works fine for IPv4.

> > /126 between them and a local /128 for its own interface address.  Each
> > router then installs an IBGP route for the /128 of its neighbour routed
> > via that same address.  That presumably overrides the /126 of the
> 
> Where is this /128 IPv6 route coming from? IOS boxes will list Connected 
> and Local routes for a /126:
> 
> C   2001:630:0:9001::198/126 [0/0]
>       via Vlan3709, directly connected
> L   2001:630:0:9001::19A/128 [0/0]
>       via Vlan3709, receive

s-pop2#sh ipv6 route 2001:630:3c0::3840/126 longer-prefixes 
IPv6 Routing Table - Default - 60 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
       B - BGP, R - RIP, I1 - ISIS L1, I2 - ISIS L2
       IA - ISIS interarea, IS - ISIS summary, D - EIGRP, EX - EIGRP external
       O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
       ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
C   2001:630:3C0::3840/126 [0/0]
     via Vlan16, directly connected
B   2001:630:3C0::3841/128 [200/0]
     via 2001:630:3C0::3841%Default
L   2001:630:3C0::3842/128 [0/0]
     via Vlan16, receive

> ...but I would not normally expect them to advertise the Local route. 
> Certainly our 6500s (12.2(33)SXI5) are not.

Certainly, and it doesn't happen in IPv4.  We could filter out the local
/128 prefixes on that peering, but it just seems wrong.

> Can you send:
> 
> sh run part router bgp
> sh run int <the p2p>

I'll do that under separate cover.

Thanks,


Sam

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