Team,
I found an example of BGP "next-hop" Attribute from Internet Routing Arch
(by Halabi) - Any explanation will be appreciate.
Router A, B & C are all in the same AS, while Router D is in a separate AS.
Not the best diagram :)
Basically:- Router A & Router B are running IBGP
Router B & Router D are running EBGP
Router B is attached to a stub network (192.212.1.0 /24)
Router D is attached to a stub network (128.213.1.0 /24)
Router B is learning route 128.213.1.0 /24 from Router D
Router B is also injecting route 192.212.1.0 /24 into
BGP
IBGP
EBGP
2.2.2.2
1.1.1.1
A ----------------------------
B ----------------------D -----------------| 128.213.1.0 /24
Serial 0 \ / |
\ / |
\ / |
\ / |
\ / ------------
3.3.3.3. \ / 192.212.1.0 /24
C
I understand that in BGP table of Router A, it would have 2 entries:-
Destination 192.212.1.0 /24 Next-hop: 2.2.2.2
(Next-hop is 2.2.2.2 since for IBGP sessions, the Next-hop will be the IP of
the IBGP neighbor who advertised the route, which is Router B)
Destination 128.213.1.0 /24 Next-hop: 1.1.1.1
(Next hop is 1.1.1.1 since for routes coming into an AS via EBGP, the next
hop learned from EBGP is carried unchanged into IBGP - in this case, it's
Router D).
What I am confused it's the Routing Table of Router A,
Destination 192.212.1.0 /24 Next-hop 2.2.2.2 (Best Route inserted
from BGP table) - cool
Destination 128.213.1.0 /24 Next-hop: 1.1.1.1 (Best Route inserted
from BGP table) - cool
Destination 3.3.3.0 /24 Next-hop: Directly Connected via Serial
0 - cool
Destination 2.2.2.0 /24 Next-hop: 3.3.3.3 (why?? isn't this
directly connected via IBGP, shouldn't it be directly connected?)
Destination 1.1.1.0 /24 Next-hop: 3.3.3.3 (why?? shouldn't
this have 2 routes, one via 3.3.3.3, and the other via 2.2.2.2?)
Also, I thought that in such a scenario, one would want to make sure that
every BGP Next-Hop can be reached via either IGP or static routes - However,
Halabi says it's only needed for IBGP Next-hop, is it correct?
Thanks,
Hunt
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