From: Brian Hescock (bhescock@xxxxxxxxx)
Date: Sun Mar 17 2002 - 20:18:40 GMT-3
That's the correct behavior for BGP in the newer versions of IOS, a
iBGP peer will withdraw its route if it learns it's iBGP peer has a
better path. The result is you will see iBGP peer with less routes than
you would expect (assuming that's what you're referring to below).
Brian
jonatale@earthlink.net wrote:
>Folks,
>
>I am very interested in the conclusion of this thread, but I do not have the
>time to decipher it. Could somebody please summarize.
>
>
>W. Alan Robertson wrote:
>
>>Well, that's exactly what I'm seeing, but it certainly wasn't what I
>>expected. Nor did it appear to be what our TAC engineer expected...
>>I'm going to get the case notes in the morning, and I'll share them
>>with the list.
>>
>>Thanks for mocking this up Przemek... Now if you can manage to get
>>that config time down to 20 minutes, instead of an hour, you're going
>>to kick butt in the Lab. ;)
>>
>>----- Original Message -----
>>From: "Przemyslaw Karwasiecki" <karwas@ifxcorp.com>
>>To: "W. Alan Robertson" <warobertson@earthlink.net>
>>Cc: "Peter van Oene" <pvo@usermail.com>; "Groupstudy - CCIELAB"
>><ccielab@groupstudy.com>; "Groupstudy - Cisco Certification"
>><cisco@groupstudy.com>
>>Sent: Tuesday, February 05, 2002 10:50 PM
>>Subject: Re: Undocumented iBGP Behavior (Confirmed by Cisco)
>>
>>>I have prepended it in the router in AS3.
>>>
>>>I wanted to simulate longer paths from one of the ASes,
>>>like it happens between AS1 and AS 701 in reality.
>>>
>>>Main point I wanted to prove is that initialy both routers
>>>have all routes, but after BGP converged, righ router (r6)
>>>selected routes learned via iBPG from r5, and withdrawn
>>>routes via AS3 from its advertisments to left router (r5).
>>>
>>>Przemek
>>>
>>>
>>>On Tue, 2002-02-05 at 22:40, W. Alan Robertson wrote:
>>>
>>>>How are you ending up with a greater number of AS hops for the
>>>>
>>route
>>
>>>>on R6 learned via AS3?
>>>>
>>>>>r6#sh ip bgp 10.0.0.0
>>>>>BGP routing table entry for 10.0.0.0/8, version 3
>>>>>Paths: (2 available, best #2, table Default-IP-Routing-Table)
>>>>> Advertised to non peer-group peers:
>>>>> 1.1.34.3
>>>>> 3 3 1
>>>>>
>>>> ^^^
>>>> | Why does there seem to be an as-prepend here? --Alan
>>>>
>>>>> 1.1.34.3 from 1.1.34.3 (1.1.34.3)
>>>>> Origin IGP, localpref 100, valid, external
>>>>> 2 1
>>>>> 1.1.24.2 from 2.2.2.1 (2.2.2.1)
>>>>> Origin IGP, localpref 100, valid, internal, best
>>>>>
>>>>Again, in this case, the iBGP learned route is preferred because
>>>>
>>it is
>>
>>>>only two AS hops away... The externally learned route, from peer
>>>>1.1.34.3, shows AS3 twice in the path, making this route 3 AS hops
>>>>away.
>>>>
>>>>
>>>>----- Original Message -----
>>>>From: "Przemyslaw Karwasiecki" <karwas@ifxcorp.com>
>>>>To: "W. Alan Robertson" <warobertson@earthlink.net>
>>>>Cc: "Peter van Oene" <pvo@usermail.com>; "Groupstudy - CCIELAB"
>>>><ccielab@groupstudy.com>; "Groupstudy - Cisco Certification"
>>>><cisco@groupstudy.com>
>>>>Sent: Tuesday, February 05, 2002 10:15 PM
>>>>Subject: Re: Undocumented iBGP Behavior (Confirmed by Cisco)
>>>>
>>>>
>>>>>Ok,
>>>>>
>>>>>Lab is done. I expected 20 minutes, it tooks 1 hour.
>>>>>Important lesson about time management learnt :)
>>>>>
>>>>>small legend:
>>>>>r5 and r6 are routers in AS4
>>>>>
>>>>>If anyone care I can send complete configs.
>>>>>
>>>>>Przemek
>>>>>
>>>>>r5#sh ip bgp summ
>>>>>BGP router identifier 2.2.2.1, local AS number 4
>>>>>BGP table version is 2, main routing table version 2
>>>>>1 network entries and 1 paths using 133 bytes of memory
>>>>>1 BGP path attribute entries using 60 bytes of memory
>>>>>1 BGP AS-PATH entries using 24 bytes of memory
>>>>>0 BGP route-map cache entries using 0 bytes of memory
>>>>>0 BGP filter-list cache entries using 0 bytes of memory
>>>>>BGP activity 1/9 prefixes, 2/1 paths, scan interval 60 secs
>>>>>
>>>>>Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ
>>>>>
>>Up/Down
>>
>>>>State/PfxRcd
>>>>
>>>>>1.1.24.2 4 2 23 22 2 0 0
>>>>>
>>00:18:14
>>
>>>>1
>>>>
>>>>>2.2.2.2 4 4 27 26 2 0 0
>>>>>
>>00:21:53
>>
>>>>0
>>>>
>>>>>r5#sh ip bgp 10.0.0.0
>>>>>BGP routing table entry for 10.0.0.0/8, version 2
>>>>>Paths: (1 available, best #1, table Default-IP-Routing-Table)
>>>>> Advertised to non peer-group peers:
>>>>> 2.2.2.2
>>>>> 2 1
>>>>> 1.1.24.2 from 1.1.24.2 (1.1.1.1)
>>>>> Origin IGP, localpref 100, valid, external, best
>>>>>r5#
>>>>>telnet-server#6
>>>>>[Resuming connection 6 to r6 ... ]
>>>>>
>>>>>r6#sh ip bgp summ
>>>>>BGP router identifier 2.2.2.2, local AS number 4
>>>>>BGP table version is 3, main routing table version 3
>>>>>1 network entries and 2 paths using 169 bytes of memory
>>>>>2 BGP path attribute entries using 120 bytes of memory
>>>>>2 BGP AS-PATH entries using 48 bytes of memory
>>>>>0 BGP route-map cache entries using 0 bytes of memory
>>>>>0 BGP filter-list cache entries using 0 bytes of memory
>>>>>BGP activity 1/6 prefixes, 2/0 paths, scan interval 60 secs
>>>>>
>>>>>Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ
>>>>>
>>Up/Down
>>
>>>>State/PfxRcd
>>>>
>>>>>1.1.34.3 4 3 21 20 3 0 0
>>>>>
>>00:15:20
>>
>>>>1
>>>>
>>>>>2.2.2.1 4 4 27 28 3 0 0
>>>>>
>>00:22:13
>>
>>>>1
>>>>
>>>>>r6#sh ip bgp 10.0.0.0
>>>>>BGP routing table entry for 10.0.0.0/8, version 3
>>>>>Paths: (2 available, best #2, table Default-IP-Routing-Table)
>>>>> Advertised to non peer-group peers:
>>>>> 1.1.34.3
>>>>> 3 3 1
>>>>> 1.1.34.3 from 1.1.34.3 (1.1.34.3)
>>>>> Origin IGP, localpref 100, valid, external
>>>>> 2 1
>>>>> 1.1.24.2 from 2.2.2.1 (2.2.2.1)
>>>>> Origin IGP, localpref 100, valid, internal, best
>>>>>r6#
>>>>>
>>>>>
>>>>>Once better route is selected in Local-RIB, the other,
>>>>>previously advertised is withdrawn.
>>>>>
>>>>>
>>>>>
>>>>>On Tue, 2002-02-05 at 21:15, W. Alan Robertson wrote:
>>>>>
>>>>>>If you can, build your test scenario to look like this:
>>>>>>
>>>>>>
>>>>>> [eBGP ]
>>>>>> ___[AS 1]___
>>>>>> | |
>>>>>> | |
>>>>>> [ eBGP ] [ eBGP ]
>>>>>> [ AS 2 ] [ AS 3 ]
>>>>>> | |
>>>>>> | |
>>>>>> | |
>>>>>> | |
>>>>>> [ BGP ] [ BGP ]
>>>>>> [ AS 4 ] [ AS 4 ]
>>>>>> | |
>>>>>> | |
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