BGP question

john55657 · ‎12-30-2007

I am currently studying BSCI and cannot find an answer to this one: I know the routes prefered for BGP are the following:

1 - Weight

2 - Local pref

3 - Orig route

4 - AS Path

5 - MED

But in many questions from Testking, I get an output, and the preferred routes are always the ones with the highest Local Pref... I am confused here. Is it because there are more than 1 route to the destination that it will select the highest local pref, even if the weight is lower on it?

Thanks for any help!

marikakis · ‎12-30-2007

Hello,

Only if the weight attribute values of 2 routes that cover the same prefix (pay attention to the network as well as the mask) are equal, will the BGP best path selection algorithm proceed to check the local preference values. If local preference values are also equal, the origin will be checked and so on.

Just in case, have a look at the following documentation on the weight attribute:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#weight

"The weight attribute is a Cisco-defined attribute. This attribute uses weight to select a best path. The weight is assigned locally to the router. The value only makes sense to the specific router. The value is not propagated or carried through any of the route updates. A weight can be a number from 0 to 65,535. Paths that the router originates have a weight of 32,768 by default, and other paths have a weight of 0.

Routes with a higher weight value have preference when multiple routes to the same destination exist."

Some tests do have mistakes, but let's first pay attention to all the details before concluding on this.

Kind Regards,

M.

john55657 · ‎01-01-2008

Thanks very much! I also realised that the ">" sign is also there to identify the best path in the summary output.