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Old 06-14-2009, 11:38 PM
admin admin is offline
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Default Difference between BGP's 'local-as' and 'local-as no prepend'?

Can someone tell me what the difference is when using the 'local-as' command by itself, as opposed to using the 'local-as no-prepend' command? Logically I would think that when using the 'local-as' the default behavior would be to prepend the local-as before the true AS in the AS path, but when making these changes in the lab, I am not seeing this taking place. Can somone please explain the difference to me? Any assistance is greatly appreciated.

Rack1SW1(config-router)#do sh run | i 204.12.1.254
neighbor 204.12.1.254 remote-as 54
neighbor 204.12.1.254 local-as 100

BB3(config)#do sh ip bgp 100.100.100.0
BGP routing table entry for 100.100.100.0/24, version 33099
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Advertised to peer-groups:
RACKS
Advertised to non peer-group peers:
172.16.4.1
100 400 (as you can local-as and real AS is in the path)***
204.12.1.7 from 204.12.1.7 (150.1.7.7)
Origin IGP, metric 0, localpref 100, valid, external, best

after the 'no-prepend' was added I didn't see anything change?

Rack1SW1(config-router)#do sh run | i 204.12.1.254
neighbor 204.12.1.254 remote-as 54
neighbor 204.12.1.254 local-as 100 no-prepend

BB3(config)#do sh ip bgp 100.100.100.0
BGP routing table entry for 100.100.100.0/24, version 33102
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Advertised to peer-groups:
RACKS
Advertised to non peer-group peers:
172.16.4.1
100 400 (why do I still see both the local-as and the real AS still in the path?)***
204.12.1.7 from 204.12.1.7 (150.1.7.7)
Origin IGP, metric 0, localpref 100, valid, external, best

I can't see where it made any difference, unless I use the 'local-as no-prepend replace-as' command.

Rack1SW1(config-router)#do sh run | i 204.12.1.254
neighbor 204.12.1.254 remote-as 54
neighbor 204.12.1.254 local-as 100 no-prepend replace-as

BB3(config)#do sh ip bgp 100.100.100.0
BGP routing table entry for 100.100.100.0/24, version 33105
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Advertised to peer-groups:
RACKS
Advertised to non peer-group peers:
172.16.4.1
100 (now i only see the loca-as w/o the real AS)
204.12.1.7 from 204.12.1.7 (150.1.7.7)
Origin IGP, metric 0, localpref 100, valid, external, best

What am i missing?
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Old 06-17-2009, 04:54 AM
David.C David.C is offline
 
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Posts: 118
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To see these changes, look at the show ip bpg' output on SW1's neighbors (other than BB).

Local-AS lies between SW1 and BB. When advertising prefixes learned from local-AS to other peers, both real and local AS will be included. With no-prepend, SW1 will stop advertising local AS with these prefixes.

"No-prepend" does not prepend the local autonomous system number to any routes received from the eBGP neighbor.
Here's an example for you, see the following diagram:

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Old 06-17-2009, 04:56 AM
David.C David.C is offline
 
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Posts: 118
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When the " no-prepend" was removed, the as-path shows as below:
Attached Files
File Type: qmap bgplocalas.qmap (1.4 KB, 0 views)
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Old 06-17-2009, 05:57 AM
Chris Chris is offline
 
Join Date: Jun 2009
Posts: 74
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I see exactly what you were talking about. Thanks for the clarification, this really helped.
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