From: Bob Sinclair (bob@bobsinclair.net)
Date: Fri Nov 04 2005 - 12:45:27 GMT-3
LiLi,
The definitive answer is always going to be the result of trying it in the
lab. What is true is what works!
Barring that, I would go with the more recent documentation. You will note
that the admin distance of confederation external routes is 200, the same as
IBGP routes. My bet would be that R2 will have two paths that appear to be
internal: the one from R5 and the one from R6. As you point out, lowest IGP
metric would point to R5 as the nearest exit.
HTH,
Bob Sinclair
CCIE #10427, CCSI 30427
www.netmasterclass.net
----- Original Message -----
From: Li Li
To: Cisco certification
Sent: Friday, November 04, 2005 9:35 AM
Subject: bgp best path selection within confederation
Routing TCP/IP volumn2 and Internet Routing Architectures both said
that EBGP routes is preferred over confederation external routes,which
is better than IBGP routes.
But in Cisco latest topic of BGP Best Path Selection
Algorithm(http://www.cisco.com/warp/public/459/25.shtml), No.7 is:
Prefer eBGP over iBGP paths. If bestpath is selected, go to Step 9
(multipath). Note: Paths that contain AS_CONFED_SEQUENCE and
AS_CONFED_SET are local to the confederation. Therefore, these paths
are treated as internal paths. There is no distinction between
Confederation External and Confederation Internal.
Consider following scenario:
R2 (confed 65002) --- R5 (confed 65002) ---networkA(as100)
| |
|-------------------------- R6 (confed 65001) -----|
R2,R5,R6 are all in AS200. R5 and R6 are both EBGP to AS100. If R2 to
R5 cost is 10, R2 to R6 cost is 20, R2 will pick which path to
networkA?
According to the books, R2 should choose path via R6, which is the
confederation external route. But if follow that Cisco document, path
via R5 is better, becasue it has the lower IGP cost.
So which one is true?
Thanks,
Li
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