From: Joe Chang (changjoe@earthlink.net)
Date: Tue Nov 23 2004 - 13:23:11 GMT-3
> The fact that you are a transit doesn't really have any
> bearing on the route redistribution.
The scenario where a transit AS might require restrictions on redistribution is this:
Say all three are BGP routers in the same AS. All three routers also have EBGP peers with different provider systems:
A-----B
\ /
--C--
The IBGP peers are not directly physically connected. Synchronization is not to be disabled. EBGP routes must be redistributed into the IGP.
Say redistribution is configured on each peer with no restrictions. e.g.:
router ospf 1
redistribute bgp 1 subnets
Say there is a route 10.0.0.0 that C receives from its EBGP peer. This route would be redistributed into the IGP at peers A, B as well as C.
There is an intermediate, non-BGP router D that needs to find a route to 10.0.0.0:
A B
\ /
D
|
C
D must use the IGP to determine in which direction 10.0.0.0 is. Instead of determining C being the closest route, the IGP might determine route A to be closer. Router D then sends packets destined for 10.0.0.0 to A. Router A receives these packets, and finds in its table the closest route is in the direction of C. Router A sends these packets to router D. The packets are caught in a routing loop.
That's why I'm thinking redistribution must be restricted in this scenario. EBGP routes must be redistributed into the IGP at the same point where they are received from the EGP peer. But then my undestanding of BGP isn't that solid, and I haven't found anything written that concerns this scenario.
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