[c-nsp] [BGP] Multiple peering sessions with same ASN/prefixes possible ?

[cc loo]

Hi Bill,

Thanks for your kind explanation.

So far we discussed about having 2 peering links advertising the same
prefix, however the routing table would only choose _1_ out of 2 links to
send packets.

We have a requirement that a client must advertise prefix > /24 only.

Does this impose a limitation that if
- a /30 is only reachable via the first link only
- another /30 is only reachable via second link only
(Both /30 are subnets of the advertised /24)

Both /30 wouldnt be reachable at the same time, is that right ? (assuming
that the 2 routers for both links are not inter-connected, as the latter is
used for testing purposes only.)

On Fri, Jul 3, 2009 at 10:29 AM, Bill Blackford <BBlackford at nwresd.k12.or.us
> wrote:

> <snip>
> Wouldnt my router be confused with 2 peers advertising the same prefixes ?
> How would it decide which peer to send to ?
> </snip>
>
> No. BGP will see both routes in the RIB and select the best path and insert
> into the forwarding table.
> If all else is equal, chances are the forwarding decision will be based on
> the lower IP address, or which peer has the longest uptime.
>
> You should be fine
>
> If you issue a 'sh ip bgp' you will see two routes to your clients
> destination. One with a ">" indicating the best path which is the one
> selected for the forwarding table. My explanation may not be as eloquent as
> it should be, but I hope the content of the information is helpful.
>
> -b
>
>
> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net [mailto:
> cisco-nsp-bounces at puck.nether.net] On Behalf Of cc loo
> Sent: Thursday, July 02, 2009 6:49 PM
> To: cisco-nsp mailing list
> Subject: [c-nsp] [BGP] Multiple peering sessions with same ASN/prefixes
> possible ?
>
> Hi all,
>
> my company's network has a peering connection with a client.
> Recently, they requested us to set up another concurrent peering link (for
> testing purposes).
>
> The 2 BGP routers will be advertising the same ASN and prefixes.
> As i have limited knowledge in BGP, i wondered if such a set up would screw
> up the entire routing for this client ?
>
>
> Wouldnt my router be confused with 2 peers advertising the same prefixes ?
> How would it decide which peer to send to ?
>
> Appreciate your kind advise. Thanks.
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