Here's how AS-override works:
- this is used when two or more CEs for a customer use the same BGP AS# (quite common)
- the PE looks at the first AS# in the AS-PATH of the route being advertised to a CE. If this is equal to the AS# of the CE it is advertising the route to, it is replaced by the provider's own AS#. This works even if there are multiple occurences of the As# (due to ASPATH prepending).
- the impact of the above will be that the ASPATH of the route received by the CE will have at least two occurences of the provider's own AS#.
Here's how allowas-in works:
- this is used in situations where a customer site links 2 VPNS e.g. a site has 2 links,where each terminates on a different VRF on the PE
- this is also applicable in cases where a CE is multi-homed to 2 PE routers (same VRF)
- now, when the CE advertises a route learned from one of the PEs to another, the PE will drop the route because it contains it's own AS# (since the route was learned from another PE in the first place).
- the use of allowas-in disables this check on the PE
- you can specify the maximum amount of occurences of the PE router's AS# in the ASPATH to prevent loops
You can also use the Site-of-Origin attribute to prevent advertising routes out to a site from which they were originally learned.
Hope that helps - pls rate the post if it does.
Paresh
