BGP as-path question

[jean tang]

Hi,guys

here's a BGP as-path configuration questions

BB1(254)-------(R1-------R2---------R3)----------BB2(254)

R1/R2/R3 belong to AS 64512

1.1.1.1----R1's loopback

150.1.1.254------BB2's add

R3: show ip bgp

Network                      next-hop             metric      local    weight                path
*>197.68.0.0/19            0.0.0.0               32768                                              i
s  197.68.1.0           150.1.1.254                                                           254  253  i
s>                              1.1.1.1                                                         (64512)   254 i

I understand the path received from R1 with AS-path (64512) 254 i

But in what condition, R3 receive 197.68.1.0 will insert a AS-number (253) from BB2,  I remeber local-as can insert AS-number on the left....

the output should be like above, how can i configure to do it ?

thanks in advance

[Mohamed Sobair]

Hi,

Your understanding is correct! The (Local-as) insert the as on the left, However, the behaviour you are examining is due to as-path prepending.

BB2 is prepending the Network 197.68.1.0 with (AS 253) when its advertised twords R3, thats why you see 254 253 in the AS path.

HTH

Mohamed

[jean tang]

Hi,Mohamed,thanks

so, I can't configure this on R3, only on BB2?

[Mahesh Gohil]

Hello Jean,

Yes you can configure local-as for peer only. It means for R3 local-as configured on BB2 and vice versa.

but similar output is also possible if there is AS253 exists behind AS254

Regards

Mahesh

[Mohamed Sobair · Accepted solution]

Hi Jean,

Correct, According to your output from (sh ip bgp), this is only configured on BB2.

HTH

Mohamed

[cadet alain]

Hi mohamed,

I thought it was a bad idea prepending another AS and that when you are doing AS_PATH prepending you should prepend your own AS and more than 1 time.

Can you explain to me please ?

Regards.

[Mohamed Sobair]

Its indeed a bad idea to prpend AS other than your own AS, the reason is because the traffic could be dropped if AS 253 exist in the path due to BGP split horizon rule.

Traffic will never reach AS 253 if its exist in the path. So its better to prepend your own AS Always. However, prepending could be done more than one time , it depends on the configuration and the requirement.

Prepending is usually done whenever you want to influence the return path. Shortes AS-path is prefered from the upstream and internet IF (Local preference and Weight) are not configured to influence outgoing path selection.

HTH

Mohamed

[milan.kulik]

Hi,

prepending AS number different from the router's own one might be sometimes useful for some special prefix tagging within complicated enterprise networks, .e.g.

You can define some private AS numbers used for this purpose only.

You might say communities should be used for this purpose.

But they might not be exported somewhere, while AS-PATH is rarely manipulated.

BR,

Milan