If 2 routers are eBGP neighbours, and:
- Router A advertises a class B subnet: 10.26.0.0/16
- Router B has an aggregate statement for this subnet: aggregate-address 10.26.0.0 255.255.0.0 summary-only
> Is router B injecting a NULL0 route (pref. 200) due to this 10.26.0.0/16 being received from router A ?
> Or will router B only inject the NULL0 route just in case a more specific route is learned from router A ?
This quesiton to known better the working of BGP in this configuration. Because last time we connected a new router C to router B,
which was advertising more specific (router C) routes within subnet 10.26.0.0/16.
And router B's routing table included this 10.26.0.0/16 to NULL0 interface.
As a concequense the more specific networks in range 10.26.0.0/16 to which router A is connected, where NOT KNOWN anymore (or reachable) from router B point of view.
if a BGP router receives only the same class B for which it has an aggregate/summary defined, it will not create NULL0 route for this aggregate/summary.
An interesting case is when it receives both that prefix plus some more specific.
In that case it creates the NULL0 route which beats the same aggregate prefix recieved!