Multiple BGP sessions between the same router pairs at IXPs

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1

Anyone feel like sanity checking this for me:

As far as I know, if you have this config

             RTRA BGP ID 4.4.4.4 ASN 4
    2.2.2.4 / \ 3.3.3.4
2.2.2.0/24 | | 3.3.3.0/24
    2.2.2.5 \ / 3.3.3.5
             RTRB BGP ID 5.5.5.5 ASN 5

and you configure on router A

router bgp 4
neighbor 2.2.2.5 remote-as 5
nieghbor 3.3.3.5 remote-as 5

and on router B

router bgp 5
neighbor 2.2.2.4 remote-as 4
nieghbor 3.3.3.4 remote-as 4

then only one peering session will come up, as routers only
support one BGP session with any given router ID.

We hit this problem when renumbering the LINX, I think, in that
you have to make sure you take down old sessions to get new
sessions to work.

This would also affect people who set up new peerings over
the ATM MAE's, who also have existing peerings on the FDDI
fabric between the same routers, right?

2

Alex Bligh wrote:

then only one peering session will come up, as routers only
support one BGP session with any given router ID.

You can get both sessions up, but you'll only use one set of
routes, which if you don't toy with any knobs, will be the
one with the lower router ID. However, you can use BGP max
paths and set it to 2, and both routes will be sent to
the route table. Or you could use eBGP multihop.

/Sean Butler

3

I had the same situation and ended up dropping a static to the loopback of
the remote router at both interfaces and then peering loop to loop.. (the
remote end would have to static your loopback out their's too, of course) Added
benefit of very good load balancing on the links.., I'm using CEF so CPU
load is nill even on per-packet load sharing..

Tim