I am having an issue with the following setup in a lab (soon to be deployed).
Site A and Site B are running in different AS's peering with the same provider. Site B is a backup for site A prefixes by means of AS-path-prepending route-maps.
When A goes down, site B should become become the preferred route to site A prefixes (site A and B are connected internally via a seperate link running OSPF).
I originally had this working and the routers I am using to simulate the provider would show routes to the site A prefix, both through site A and site B.
Since making a number of changes (over a period of months), I am receiving the following in a "debug ip bgp updates" on the carrier router:
"DENIED due to: AS-PATH contains our own AS"
The AS path indicates this is because:
* Site A advertises 10.1.0.0/24 to the provider
* The provider advertises 10.1.0.0/24 to site B
* Site B then advertises 10.1.0.0/24 to the provider (as it is the backup for Site A)
* The provider rejects the route as it seems the AS-path as "SiteA --> Provider --> Site B --> Provider
Site B is advertising the 10.1.0.0/24 prefix it is learning via BGP instead of the prefix it is learning via OSPF and hence has the original AS-path in it, resulting in the provider rejecting it.
I could request the provider to enable "allowas-in" but feel there must be a better way. All of the other options I have looked at involve provider-side changes when using MPLS-VPNs etc. I would like to have Site B advertise the routes from OSPF and not BGP or otherwise have Site B strip the ASs when it advertises the prefix.