what is difference between BGP commands "set as-path <as path.>" and as-path prepend last as <no.>? when should one use?

[MukeshGwari]

What is difference between BGP commands "set as-path <as path.>" and as-path prepend last as <no.>? when should one use? 

and how outbound prepending work with outbound filter list ?

[milan.kulik]

Hi,

IMHO, the first syntax is for use when redistributing some prefixes to BGP only.

So you are creating a new AS PATH.

The correct syntax is "set as-path tag" though, you can't put multiple AS numbers into the AS PATH this way.

See http://www.cisco.com/c/en/us/td/docs/ios/iproute_bgp/command/reference/irg_book/irg_bgp4.html#wp1147170

for details.

Regarding prepending, here is a nice explanation:

http://wiki.nil.com/AS-path_prepending_%28technical_details%29

To answer your question shortly:

Use "set as-path..." when redistributing prefixes into BGP and "as-path prepend..." when modifying AS PATH  of prefixes existing in the BGP table already.

Best regards,

Milan