[c-nsp] Peering + transit backup between 2 ISPs

[Justin M. Streiner]

On Thu, 24 Nov 2005, Vincent De Keyzer wrote:

> let ISP A and ISP B, which both have routers in a given room. They are good
> friends and they would like to achieve the following:
>
> 1.	set-up a peering to exchange own AS routes
> 2.	serve as backup transit one for the other: each ISP would have one
> upstream only, and when the unique upstream goes down, it would then (and
> only then) use the other ISP for transit

> Is this feasible? I can't really figure out how this could be achieved
> without having both ISPs offering transit to each other all of the time.

It could be done, and it would probably look something like this:

ISP A and ISP B send each other customer routes, routes they originate, 
and a default route.  Each side has an inbound route-map or 
policy-statement (if either is using vendor J) to decrease the 
local-preference on just the default route.  ISP A will also need to 
announce ISP B's routes to their transit provider and vice versa. 
Either the AS path on those routes will need to be artifically lengthened 
with AS prepends, or the transit providers will need to provide 
communities that can be used to selectively alter the local-preference of 
routes announced to them.

If ISP A's primary transit goes down, their lower local-preference default 
route will start to push traffic across the peering connection to ISP B.
The only path that ISP B's transit provider will see to reach ISP A 
will be to go through ISP B since ISP A will no longer be announcing 
routes to their primary transit provider and hence, to the 'rest of the 
Internet'.

jms