Origin attribute in BGP decision process?

[hanyawad]

Hi,

in the BGP decision process the fifth tiebreaker is the Origin PA, which is prefere IGP routes over EGP routes which are in turn prefered over incomplete routes. my question is how the BGP preferes the IGP like ( OSPF, EIGRP, RIP) over EGP like (EBGP) although EBGP is AD of 20 and the other IGP routing protocols have a higher AD numbers.

or i misunderstood the explanation of that attribute.

thanks for your response.

Makar

[cadet alain · Accepted solution]

Hi,

Yes you are misunderstanding the atribute.

origin is igp if it was injected in the BGP RIB with a network command or an aggregate command(in some cases)

origin of  egp means originated byEGP which is the predecessor of BGP so you can only find this if the origin has been tampered with a route-map

origin of incomplete means injected by redistribute or aggregate( in some cases) or default-information originate.

Again all these can be set within a route-map.

The origin is for electing a best route in the BGP table so it has nothing to do with distance which is only used to install a route learned by multiple sources in the RIB.

Regards.

Alain

[hanyawad]

Hi Alain,

in the tiebreaker number 3 which preferes locally injected routes ( offcourse through network or aggregate commands) over iBGP/eBGP learned.

so if this step is a tie so why it is used again in the origin step.

i think i'm a bit confused!!!

Labib

[cadet alain]

Hi,

tiebreaker 3 should never be used except in some special case because locally generated routes have a weight of 32768 and others have a weight of zero by default.

furthermore a locally generated route can have an origin of igp or incomplete depending how it was injected into the BGP table.

Regards.

Alain

[hanyawad]

Hi Alain,

thank you very much for your clarification, i've read some documents for that topic and i found that EGP shouldn't occer today and is not even supported in current IOS versions. so the locally route injected that has ORIGIN code of i better than ORIGIN code of ?.

and also The origin is for electing a best route in the BGP table which has no thing to do with distance as you said.

thanks!

Regards,

Makar