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BGP Question

Mavrick25
Level 1
Level 1

‎03-10-2008 06:19 AM - edited ‎03-03-2019 09:03 PM

As a general rule...

BGP selects the shorts prefix match or the longest prefix match..

And could you provide me with some documentation to support your answer..

Thanks

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7 Replies 7

royalblues
Level 10
Level 10

‎03-10-2008 06:37 AM

Any routing protocol would always prefer the longest mask or the more specific route

This is mentioned in the RFC

Have a look at section 9.1.2.1. Route Resolvability Condition (RFC 4721)

http://www.ietf.org/rfc/rfc4271.txt

The route resolvability condition is defined as follows:

1) A route Rte1, referencing only the intermediate network address, is considered resolvable if the Routing Table contains

at least one resolvable route Rte2 that matches Rte1's ntermediate network address and is not recursively resolved directly or indirectly) through Rte1. If multiple matching routes are available, only the longest matching route SHOULD be considered.

HTH

Narayan

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Mavrick25
Level 1
Level 1
In response to royalblues

‎03-10-2008 06:49 AM

Narayan,

Thanks for the information..

I could have sworn that it was the shortest prefix length..

I made mistake..

Thanks

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Harold Ritter
Cisco Employee
Cisco Employee
In response to royalblues

‎03-10-2008 08:41 AM

Narayan,

Actually the routing protocols as such don't really care about shorter or longer prefixes. If a shorter and longer prefix are learnt by a routing protocol, they are considered as two different entities and are both installed in the RIB.

It is at the data plane level that the distinction is made and that the longest prefix match will apply.

Cheers,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Mavrick25
Level 1
Level 1
In response to Harold Ritter

‎03-10-2008 09:12 AM

Thanks again..

I just want to get this straight..

If R1 is populating a route through aggregation...

And

R2 is populating a BGP without aggregation..

The non-aggregated route will be perfered down the chain..

correct..

0 Helpful

vaisharm
Cisco Employee
Cisco Employee
In response to Mavrick25

‎03-10-2008 08:37 PM

You are right.

~Vaibhav

0 Helpful

Mavrick25
Level 1
Level 1
In response to vaisharm

‎03-11-2008 01:15 AM

Thanks again for all your help

0 Helpful

Harold Ritter
Cisco Employee
Cisco Employee
In response to Mavrick25

‎03-11-2008 01:36 PM

That is correct. If both routes are installed in the RIB, the non-aggregated route will be used for forwarding packets, assuming the destination address matches this non-aggregated routes obviously.

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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