BGP route selection

[cheung.kelvin]

Dear Community

Pls see the "show ip bgp". For me the path No.2 and No.4 have the same conditions, why path No.4 is the best. What is the mechanism to choose it?

R1>sh ip bgp 10.20.0.0/19

BGP routing table entry for 10.20.0.0/19, version 18108

Paths: (4 available, best #4, table Default-IP-Routing-Table)

Not advertised to any peer

17668 64513 64513 64513 64513 64513, (aggregated by 64513 10.20.4.129)

10.255.66.9 from 10.255.66.9 (202.168.55.242)

Origin IGP, localpref 100, valid, external, atomic-aggregate

17668 64513 64513 64513, (aggregated by 64513 10.20.4.129)

10.255.66.2 from 10.255.66.2 (202.168.55.243)

Origin IGP, localpref 100, valid, external, atomic-aggregate

17668 64513 64513 64513 64513 64513, (aggregated by 64513 10.20.4.129)

10.255.66.10 from 10.255.66.10 (202.168.55.243)

Origin IGP, localpref 100, valid, external, atomic-aggregate

17668 64513 64513 64513, (aggregated by 64513 10.20.4.129)

10.255.66.1 from 10.255.66.1 (202.168.55.242)

Origin IGP, localpref 100, valid, external, atomic-aggregate, best

R1>

Thanks,

Kelvin Cheung

[cjnwodo]

Kelvin,

Looks like No. 4 was chosen because the prefix was received from a BGP neighbour with a lower router id [10.255.66.1] whereas No.2 has a router id of 10.255.66.2]. Cisco BGP best path selecyion algorithm number 11.

Here's a link to the Cisco BGP best path selection algorithm. Hopefully it will make it clearer to you.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

[royalblues]

you should check,

Which router is closer as per the IGP metric

And the router ID --(as said above lower ID wins)

HTH

Narayan

[cheung.kelvin]

Dear HTH

Thanks a lot..

You are right....

Kelvin Cheung

[cheung.kelvin]

Dear Cjnwodo

Thanks for your help. It is great....

Kelvin Cheung