BGP route selection

Unanswered Question
Jul 22nd, 2007
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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

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cjnwodo Mon, 07/23/2007 - 00:05
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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 Mon, 07/23/2007 - 01:47
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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 Mon, 07/23/2007 - 14:58
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Dear Cjnwodo


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



Kelvin Cheung

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