SoO and eBGP

Mukarram Jah Raheel · ‎06-01-2010

SoO is used to prevent a route from being readvertised back to the site from which it originated...

but in following case when eBGP is the PE/CE routing protocol, PE3 will prefer the eBGP route from CE-2 instead of iBGP route from PE1 or PE2 unless routing loop is intentionally created by preferring the iBGP route through weight or other attributes....

so when does the SoO comes into play with eBGP....

Laurent Aubert · ‎06-04-2010

Hi,

SoO is required with BGP when you also configured as-override. In this case the AS PATH can't be used anymore to detect loop so you need something else to identify from where this path is coming from.

In usual case, PE3 will send the update to CE2 which will drop it as it sees its own AS in the AS-PATH. With as-override, CE2 is not seeing its own AS anymore.

PE3 will still prefer its eBGP route from CE2 regardless if you configured SoO or not.

HTH

Laurent.