08-05-2008 10:51 PM - edited 03-03-2019 11:02 PM
Hi All,
If a router running both iBGP and eBGP, iBGP local preference is 250, eBGP local preference 100, be default, what's the route it should choose? and Why?
Thank you,
08-05-2008 10:56 PM
Hi!
The e-BGP route will always be installed in the routing table as the admin distance for e-bgp is 20 while that for i-bgp is 200.
Only when selecting among routes that belong to the same admin distance (all are ibgp or all are ebgp), then the bgp route selection process is followed as given in the following doc. :
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml
Regards,
Niranjan
(pls rate helpful posts)
08-06-2008 03:42 AM
Niranjan,
The AD will not come into play in such a case. The decision will be made based on the BGP best path decision process that you are referring to.
The two paths will first be compared, the best one will be selected and then be installed in the RIB. In the particular scenario explained by the original poster, the iBGP path would be selected since its local preference is 250 vs. 100 for the ebgp learnt path.
Regards,
08-06-2008 04:44 AM
Hi Harold!
Thanks for the correction! I terribly messed it up. (Have to brush up the things again). Prefer ebgp over ibgp is itself a rule (#7) in the path selection criteria. So the local pref will play first and the ibgp path will be selected. So the ebgp and ibgp AD will come into picture only when comparing with other routing protocols. :).
Got it! Thanks.
Cheers!
Niranjan
08-06-2008 04:47 AM
Niranjan:
Just my 2 cents...
The AD will be the deciding BGP attribute for route selection only when all previous attributes are equal.
Victor
08-06-2008 04:54 AM
Victor,
AD will never be used between two BGP paths. AD is only used to prefer one protocol over the other.
Regards,
08-06-2008 01:27 PM
Harold:
The way I interpreted step 7 of the BGP route selection process, in which eBGP is selected over iBGP -- given everything else being equal, of course -- was that eBGP was being selected because it has a better (lower) AD than iBGP. Perhaps I am wrong.
If it is not the AD of iBGP and eBGP that are compared in step 7 of the BGP route selection process, why then is eBGP automatically selected over iBGP? What is being compared?
Victor
08-07-2008 06:40 AM
Victor,
Step 7 simply compares the routes based on whether they are external (eBGP) or internal (iBGP). The AD has nothing to do with it. You could for instance change the AD for eBGP to 201 (making it worst than iBGP (200)) and the BGP best path selection process would still prefer the eBGP over iBGP learnt path.
As mentioned in my previous post, AD only comes into play between protocols.
Regards,
08-07-2008 08:09 AM
Harold:
Thank you for that great explanation.
Much appreciated...
Victor
06-15-2017 06:21 AM
Hello,
I think something is wrong, given the cisco example below, the behavior is opposite of what is been described in the forum, please check this discussion
https://supportforums.cisco.com/discussion/13315956/bgp-multihoming-dual-ce-single-link-dual-pe
06-15-2017 08:30 AM
Actually nothing wrong with the example and what they described is the correct BGP route selection process. I have explained and corrected your topology in the link you provided.
I hope you find it useful.
08-06-2008 04:46 AM
simply
always if u have route reachable through IBGP and EBGP
EBGP will be selcted
and it is not recomended to change this default behivaur
rate if helpful
08-06-2008 04:48 AM
Marwan, you are wrong.
Read Ritters post -- and mine.
Thanks
VL
08-06-2008 06:57 AM
so what wrong
i am already talking about default behivour
and dont forget that bgp dose not pass route updat between IBGP peers from IBGP
unless it is from ebgp peer
so that they made it prefered by default and they dont recomend to be changed
08-06-2008 09:03 AM
It is BGP feature behaviour( BGP route selecetion criteria) that BGP path would be prefer over IBGP
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide