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

[Bill Blackford]

If I understand your question correctly,  the /30's would be the infrastructure links? If this is the case, then they would be connected routes. If they are not then a static route between you and your client would suffice as no one is adding a /30 to the global announcements.

-b

From: cc loo [mailto:s00664233 at gmail.com]
Sent: Thursday, July 02, 2009 7:56 PM
To: Bill Blackford
Cc: cisco-nsp mailing list
Subject: Re: [c-nsp] [BGP] Multiple peering sessions with same ASN/prefixes possible ?

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<mailto: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> [mailto: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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