next-hop-check bgp

Hi all

I've worked out the problem in this particular scenario but need help
understanding it..

Topology

R1 ---- R2 ----- R4 ---- R5
---------iBGP------eBGP----

R1 also connects to another iBGP peer = R3 via another interface which
injects a default-route via redistribute-static into BGP

Ok so R1-R2 are RR clients of each other, for which R5 originates an
eBGP route to R4, now R4 to R2 iBGP there is no next-hop-self so the
updates that R2 receives from R4 is the /30 of R5 i.e next hop
unchanged (to fix it simply I have to add next-hop-self on R4's
peering to R2)

But what I don't understand without this command is that R1 receives
the update from R2 with the original next-hop in-tact i.e R5's
interface BUT R1 installs the route into the routing table!!?

On the premise that it has a default route 0.0.0.0 via R1's interface to R3

The original prefix received from the RR R2 & the next-hop /30 of R5
is not reachable via R1's interface to R3 nor via it's interface to R2
but R1 none-the-less creates a CEF entry in it's routing table of the
prefixes (/30 of R5 & original prefix) to point to R3's interface who
originated a default route....

My question is therefore the BGP next hop reachable check, what does
it do behind the scenes at this has got me thinking and confused...

Sorry for the long mail

BR

Tony

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Received on Sun Mar 31 2013 - 00:30:43 ART