Suppose I have several routers making up an iBGP mesh. Lets
suppose I have two routers (R1 and R2) which are advertising the same
set of networks: N1, N2, ... Nk.
OSPF is running underneath BGP (assume area 0). All of the N
networks are being advertised with a next-hop set to the respective
loopback's from R1 and R2.
Now consider some other BGP router in the network. It will have
received a BGP announcement for each of N1, N2, .. Nk from R1 and R2.
This third router will select one of the paths to N1, N2, etc.
and insert it into the routing table. I'd expect to see something
like:
subnet next-hop
--- ---
N1 R1-lo0
N2 R1-lo0
... ...
Nk R1-lo0
R1-lo0
R2-lo0
Now, suppose R1 goes belly up. OSPF will quickly inform all
other routers that R1 and its loopback no longer exist. I'm assuming
that this will invalidate all the routes in the routing table which
have R1-lo0 as next hop. This will therefore cause the removal of all
occurences of routes to N1, N2, ... Nk from the routing table.
The question is this: what event will trigger BGP to re-evaluate
the routes it knows about and add in routes for N1, N2, ... Nk via
R2-lo0? Will the removal of the N1 route from the routing table
inform BGP to re-evaluate? Or will the BGP timers need to timeout
and detect that R1 is dead before re-evaluating?
One other question-- does "no sync" in BGP have a role here or is that
related only to determining when to advertise a route via eBGP?
Thanks
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=57255&t=57255
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