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iBGP and eBGP question

trinawong
Level 1
Level 1

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,

20 Replies 20

n.nandrekar
Level 4
Level 4

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)

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

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

Niranjan:

Just my 2 cents...

The AD will be the deciding BGP attribute for route selection only when all previous attributes are equal.

Victor

Victor,

AD will never be used between two BGP paths. AD is only used to prefer one protocol over the other.

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

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

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Harold:

Thank you for that great explanation.

Much appreciated...

Victor

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

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.

Marwan ALshawi
VIP Alumni
VIP Alumni

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

Marwan, you are wrong.

Read Ritters post -- and mine.

Thanks

VL

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

It is BGP feature behaviour( BGP route selecetion criteria) that BGP path would be prefer over IBGP

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