BGP confederation lab question

Answered Question
Jun 9th, 2007
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Hi, forum


This senario comes form Routing TCP/IP volume 2, page 298.

When I finished configuring all 8 routers, I have the following output form the router named Whitetooth:


r6#sh ip bgp 10.35.5.0

BGP routing table entry for 10.35.5.0/24, version 14

Paths: (2 available, best #2, table Default-IP-Routing-Table)

Advertised to non peer-group peers:

10.35.255.1

(65000 65534)

10.35.255.1 (metric 782) from 10.35.255.1 (10.35.255.1)

Origin IGP, metric 0, localpref 100, valid, confed-external

(65000 65534)

10.34.255.2 (metric 782) from 10.34.255.2 (10.34.255.2)

Origin IGP, metric 0, localpref 100, valid, confed-internal, best

r6#


My question is: why confed-internal route is selected as the best rather than the confed-external one? On page 115 of the same book, the No. 7 of path selection criterion states that:


...,prefer EBGP routes over confederation EBGP routes, and prefer confederation EBGP routes over IBGP routes.


which is conflicting with my lab result.


Any idea? Thanks

SSLIN

Correct Answer by Harold Ritter about 9 years 10 months ago

SSLIN,


there is no distinction between a confederation internal and external path (step 7 in the included URL). The second path is selected based on the fact that it has the lowest RID of both path.


Please refer to the following URL for more details on the BGP Best Path Selction process.


http://www.cisco.com/warp/customer/459/25.shtml


Hope this helps,

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Correct Answer
Harold Ritter Sat, 06/09/2007 - 16:44
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SSLIN,


there is no distinction between a confederation internal and external path (step 7 in the included URL). The second path is selected based on the fact that it has the lowest RID of both path.


Please refer to the following URL for more details on the BGP Best Path Selction process.


http://www.cisco.com/warp/customer/459/25.shtml


Hope this helps,

CSCO10892433 Sun, 06/10/2007 - 19:53
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hi, harold


Thank you for helping me clarify this . I redo the lab ,change the router-id and the result is changed accordingly.


Now I'm clear at step 7 of path selection criteria and it is simply EBGP over IBGP. But how about Jeff Doyle's advocate in his famous book(EBGP over confed EBGP over IBGP)? Was it an old version of PSC but obslete now? Or it was simply wrong from the beginning? I also find that Sam Halabi has the same description in his another famous book.


Best Regards

SSLIN

Harold Ritter Mon, 06/11/2007 - 07:24
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"EBGP over confed EBGP over IBGP"


I think it is a mistake as it wouldn't be possible to have a confederation configured on a router and to have an iBGP path (not confederation internal or external) on that same router.


Hope this helps,

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