eBGP vs iBGP (Path selection vs Admin Distance)

[awoog]

Take a router with both eBGP and iBGP: According to the best path selection algorithm, if we set local pref higher on the neighboring iBGP router, we can make some of the iBGP routes preferred. However doesnt it conflict with the rule of the admin distance (admin distance always favourise eBGP)?

[stephtchoko]

The administrative is used to choose the best routing among many routing, if there 2 path (ebgp path and ibgp path); the ebgp path will be choose, because it distance administrative is 20 against 200 for ibgp.

The local preference is a kind of metric use by on BGP call attribute.

Don't miss metric and distance administrative.

You can look at this :http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml

Best regards

[awoog]

Tks for your answer. But how do you interpret that in: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml point 7 (eBGP vs iBGP) comes after point 1/2 (weight/local pref) ?

[gaurav_saxena]

As per the BGP path selection algorithm, higher local preference IBGP route will be preferred over lower local preference EBGP route. Please refer "BGP Path Selection" topic in following url for details. Metric comparision comes on 8th number during deciding of best route in bgp.

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/bgp.htm

[awoog]

Hasn't metric in "best path selection" criteria more to do with MED attribute than with administrative distance ?

[Harold Ritter · Accepted solution]

You are correct. The admin distance never comes into play when two BGP paths are compared. The BGP best path selection process will determine the best path and then the admin distance (20 or 200) will be used to select the route to install in the RIB if the same prefix has been learnt from another routing protocol (or static).

Hope this helps,

[awoog]

All clear, tks a lot

[rsijoria]

This is exactly the behavior I noticed on my GNS3 setup, I have 2 iBGP neighbors R1 and R2, eBGP peered with VRRP SDWAN(for sake R3), R1 is prepending  AS+AS+AS towards R3 and R3 is prepending AS+AS+AS towards R1 and normal towards R2, R1 learns about same prefix (X/24)from eBGP R3 and iBGP R2, I was worried R1 would prefereBGP over iBGP since it has better AD, but it is preferring R2 as it has better AS_Path..deploying tomorrow night!

[crazy.xploring]

Though its old thread, thought of leaving a note.

I did stumbled and was wondering the same for a moment and realized, administrative distance is IP routing table attribute to install best route. It never could be an attribute of BGP to install a prefix in the BGP table. It just follows N WLLA OMNI to decide what to be installed in the BGP table.