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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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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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