BGP and multiple AS and ISPs

[paul.pink]

My company has multiple IT groups, AS # and ISP's. Yes, the age of mergers and acquisitions. For simplicity, Company A advertises it's publicy routed network through AS# 1 and ISP X. Company B advertises it's publicly routed networks through AS# 2 and ISP y. Company A now wants to share company B's internet pipe while still retaining it's own AS# and publicly routed server, in essence, using company B as a transit. The question is, will this be possible? Many ISP's only allow only contractual AS# to traverse it's network. Will company B's ISP allow company A's AS# to pass through its network? Would it be easier to eliminate Company A's AS# and migrate any DMZ server to Company B's network?

[pkhatri]

Hi,

There's a nifty little feature that is designed exactly for the circumstances you described here. It allows the use of a local AS# that is different to that expected by the ISP. Here's a document with more details:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122sb/newft/122sbc27/fsbgpdas.pdf

Paresh

PS> Pls do remember to rate posts

[jackyoung]

What I think if co. A want to share co. B Internet link, the routes from A will be learned by B then advertise to the ISP Y. You have to configure the B to advertise A's routes but not A's AS#.

You can check the AS-Override to override the AS# of A when advertise from B to ISP Y.

For the ISP side, you have to check w/ the ISP will they allow to advertise transit AS#.

Hope this helps.