BGP Weight

Answered Question
Oct 20th, 2009
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hello friends,


i have some little querry regarding weight


R1 R2 and R3 are IBGP nei with each other,,R2 and R4 are Ebgp nei,,R5 and R3 are Ebgp nei, when i configured wieght on R1 which is connected to R2 and R3 with ibgp nei, R1 is configured higher weight attribute with R2, which R2 should be preffered for outgoing traffic flow.


issue->


when i am trying to see output in sh ip bgp command, some routes are tracing from R2 and some are from R3, why is so that???? because if i am configuring higher weight towards R2 then all routes should go via R2 only.. isn't it??


on the next side, same R1, if i am configuring EBGP neigh with R2 and R3, its working fine..why is so that??


pls exlain..




Correct Answer by Giuseppe Larosa about 7 years 9 months ago

Hello Sourabh,

most details are missing.

not totally clear, if I understand correctly in last case R2 and R3 are eBGP peers and not iBGP peers as in initial scenario.


let's see first scenario:

R1 prefers routes received by R2 for its increased weight.

However, R2 and R3 are iBGP peers and they exchange routes learned respectively by R4 (for R2) and by R5 (for R3).


let's suppose there is a prefix A advertised only by R5 to R3.

R2 will learn the prefix A from iBGP session with R3.

R2 is not allowed by iBGP split horizon rule to re-advertise the prefix A to R1.


in this case only R3 propagates prefix A to R1.

not receiving a copy of the advertisement from R2 it cannot increase the weight.


when all sessions are eBGP R2 is allowed to propagate to eBGP peer R1 what has learned by eBGP peer R3.

R1 receives two copies of every advertisements and prefers the advertisement coming from R2 regardless of other attributes.


Hope to help

Giuseppe


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Correct Answer
Giuseppe Larosa Tue, 10/20/2009 - 10:19
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Hello Sourabh,

most details are missing.

not totally clear, if I understand correctly in last case R2 and R3 are eBGP peers and not iBGP peers as in initial scenario.


let's see first scenario:

R1 prefers routes received by R2 for its increased weight.

However, R2 and R3 are iBGP peers and they exchange routes learned respectively by R4 (for R2) and by R5 (for R3).


let's suppose there is a prefix A advertised only by R5 to R3.

R2 will learn the prefix A from iBGP session with R3.

R2 is not allowed by iBGP split horizon rule to re-advertise the prefix A to R1.


in this case only R3 propagates prefix A to R1.

not receiving a copy of the advertisement from R2 it cannot increase the weight.


when all sessions are eBGP R2 is allowed to propagate to eBGP peer R1 what has learned by eBGP peer R3.

R1 receives two copies of every advertisements and prefers the advertisement coming from R2 regardless of other attributes.


Hope to help

Giuseppe


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