BGP Path selection

Unanswered Question
Aug 8th, 2009

Hi experts,

I am aware of BGP Path selection. Still i am not able to understand why & how the BGP behaved in the scenario I have seen now . Please find the attached output

Why BGP has choosed 10.1.14.4 as the best, even though 10.1.12.2 is the lowest IP address (final selection when all the parameters for BEST PATH SELECTION is equal)

Hope my explanation is clear & thanks in advance

Sairam

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Jerry Ye Sat, 08/08/2009 - 17:36

Hi,

Can you post the output of show ip bgp sum?

Regards,

jerry

Edison Ortiz Sun, 08/09/2009 - 08:10

Neighbor 10.1.14.4 has the oldest route based on the 'show ip bgp sum'

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

10.1.12.2 4 200 15 22 9 0 0 00:08:34 3

10.1.14.4 4 200 16 19 9 0 0 00:07:49 3

Step # 10 on the BGP Best Path Selection.

If you clear bgp, 10.1.12.2 will have the best path.

HTH,

__

Edison.

snarayanaraju Sun, 08/09/2009 - 08:49

Hi Edison,

Thanks. perhaps I missed to remember that. It is clearly stated in BGP BEST PATH SELECTION ALGORITHM doc from Cisco

But please tell me which parameter of command "show ip bgp summ" shows that the particular route is older than all.

Thanks in advance

sairam

snarayanaraju Sun, 08/09/2009 - 08:59

Hi,

I myself took the chance and found the answer.

It is in the top to bottom order. Bottom is the oldest. Please correct me if I am wrong.

Thanks for your hints and tips

Sairam

Edison Ortiz Sun, 08/09/2009 - 11:16

Sairam,

I retract my previous statement. Based on the 'show ip bgp sum' output - R2 has been up the longest:

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

10.1.12.2 4 200 15 22 9 0 0 00:08:34 3

10.1.14.4 4 200 16 19 9 0 0 00:07:49 3

Thus based on step #10, R2 should've been chosen as the best path.

With that said, I took your lab into my GNS3 setup and found a different output from yours based on the routerID.

In your posting, the routerID on R2 reflects the physical interface and not the loopback from R2 which I find strange.

R1#sh ip bgp 3.0.0.0

BGP routing table entry for 3.0.0.0/8, version 16

Paths: (2 available, best #2, table Default-IP-Routing-Table)

Flag: 0x840

Advertised to update-groups:

1

200

10.1.12.2 from 10.1.12.2 (2.2.2.2)

Origin IGP, localpref 100, valid, external

200

10.1.14.4 from 10.1.14.4 (10.1.34.4)

Origin IGP, localpref 100, valid, external, best

and this is your output:

R1#show ip bgp 3.0.0.0/8

BGP routing table entry for 3.0.0.0/8, version 7

Paths: (2 available, best #2, table Default-IP-Routing-Table)

Advertised to update-groups:

1

200

10.1.12.2 from 10.1.12.2 (10.1.23.2)

Origin IGP, localpref 100, valid, external

200

10.1.14.4 from 10.1.14.4 (10.1.34.4)

Origin IGP, localpref 100, valid, external, best

Were you doing any changes on BGP while the session was up? Any changes on BGP can alter its behavior and to get the expected behavior on routers after a change is made, it's recommended to hard clear or soft clear BGP.

Without knowing the steps on this BGP configuration, the best path selection at that moment couldn've been influenced by anything, not just step 10.

HTH,

__

Edison.

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