BGP REGEX + IBGP
Posted on 2011-03-22
I am just looking at a config of bgp on my branch router. There is something i do not understand. The setup is basically a leased line to a main office and then a isdn backup to the main office. The main office propgates a default route. At the branch office Hrsp is running on the two routers lan facing the prod router has higher priority. So if this prod router goes down traffic goes to the isdn router. Then on the isdn router there is a static route to the dialer to iniate isdn in the case of failure. Under bgp on the on both routers There is a network statement advertising the address block used on site + the address used on the wan connection side. So that is all fine.
There is an ibgp session between the production router and the backup router. On the ibgp neighbor statement there is a filter list. Which does
neighbor x.x.x.x- ibgp neighbor filter-list 1 out
ip as-path access-list 1 deny ^$
ip as-path access-list 1 permit .*
This is what i do not understand why do you not want in IBGP to advertise ^$ which as i understand it is saying to not advertise anything originated by the local as but advertise everything else.