GroupStudy.com GroupStudy.com - A virtual community of network engineers
 Home  BookStore  StudyNotes  Links  Archives  StudyRooms  HelpWanted  Discounts  Login
can iBGP advertise second best route? posted 11/14/2006
[Chronological Index] [Thread Index] [Top] [Date Prev][Date Next] [Thread Prev][Thread Next]


Hi Group!

I remember reading that BGP advertises only the best route out.

I came across a scenario where one RR talks to two clients peering the same remote AS and getting the same route sets from outside. Simple enough - like a multi-homing with one ISP.

RR (R5) learns both sets of routes from the clients and advertises one best set, so that one RR client R6 has a redundant path in its BGP table but another one (R1) doesn't. I understand that as soon as that R1 looses its EBGP connectivity, it will get routes from RR. But at that stable point BGP routing tables no R1 and R6 are different.

RR has multipath enabled.

R5#sh ip bgp 199.172.4.0
BGP routing table entry for 199.172.4.0/24, version 50
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Multipath: eBGP iBGP
 Advertised to peer-groups:
    RRC
 254, (Received from a RR-client)
   150.100.5.254 (metric 100) from 5.5.1.1 (5.5.1.1)
     Origin IGP, metric 0, localpref 100, valid, internal, multipath, best
 254, (Received from a RR-client)
   150.102.5.254 (metric 100) from 5.5.6.6 (5.5.6.6)
     Origin IGP, metric 0, localpref 100, valid, internal, multipath

Is is possible to advertise a valid non-best route? I came to conclusion that it is not, but I might miss some fine point.

TIA,
A.