From: Mohamed M Moustafa (mmma@gawab.com)
Date: Mon Oct 22 2007 - 20:47:05 ART
Hi All,
You are all very welcomed :), i sent the reply for all of you but i think
there is something wrong with the system, or with my email.
Gary, "bgp bestpath as-path ignore" skips the AS-Path length as a BGP route
selection step, as the BGP decision process implemented in the Cisco IOS
takes into consideration the length of the AS_PATH attribute when choosing
between multiple routes to the same destination. However, RFC 1771 does not
include this step. As a result, a Cisco BGP speaker occasionally is peered
with another vendor's router that does not consider the AS_PATH length. If
this situation creates the potential for inconsistent routing decisions,
you can instruct the Cisco router to ignore the AS_PATH length in its
decision process with the command bgp bestpath as-path ignore.
BR,
Mohammed Mahmoud.
Gary Duncanson <gary.duncanson@googlemail.com> wrote on 23 Oct 2007, 01:48
AM:
Subject: Re: maximum-paths in bgp
>Spasiba Andrew :-)
>
>Yes, looks like a workaround with some mileage. I recall bgp bestpath
>as-path ignore but I need to look at this one.
>
>Thanks Mohamed.
>----- Original Message -----
>From: "Usankin, Andrew" <Andrew.Usankin@twtelecom.com>
>To: "Mohamed M Moustafa" <mmma@gawab.com>
>Cc: "Gary Duncanson" <gary.duncanson@googlemail.com>; "Roman Rodichev"
><roman@iementor.com>
>Sent: Tuesday, October 23, 2007 12:32 AM
>Subject: RE: maximum-paths in bgp
>
>
>
>WOW, that's a finding I call. Thanks a lot Mohamed, I'll try to lab it
>ASAP.
>I guess this info is good for Gary and Roman too, so I copy it to them
>also.
>
>Andrew
>
>-----Original Message-----
>From: Mohamed M Moustafa [mailto:mmma@gawab.com]
>Sent: Monday, October 22, 2007 5:15 PM
>To: Gary Duncanson
>Cc: Usankin, Andrew; ccielab@groupstudy.com
>Subject: Re: maximum-paths in bgp
>
>Hi,
>
>For the maximum-paths to operate, all the routes attributes must be
>identical and the next-hop must be different (will only load share if
>routes are received from the same upstream to have identical AS-PATH).
>However you can use the hidden command "bgp bestpath as-path
>multipath-relax" to allow the router to load-share across multiple BGP
>paths even if the as-path is different and can be helpful when dual
>homed from multiple providers.
>
>BR,
>Mohammed Mahmoud.
>
>
>Gary Duncanson <gary.duncanson@googlemail.com> wrote on 23 Oct 2007,
>12:36
>AM:
>Subject: Re: maximum-paths in bgp
>>Hi Andrew,
>>
>>Interesting finding there. Doyle vol II version 2 pp 305 has
>something of
>>
>>an example of behavior *without* maximum-paths i.e router forced to
>>choose one of two equal paths available thanks to iBGP from a route
>reflector.
>>
>>Looks like BGP multipath can be used to install multiple paths in the
>>routing table providing they are learned via the same neighboring AS so
>
>>presumably a problem if you are multihomed. Check out example in
>>Halibi pp380 if you haven't already.
>>
>>Regards
>>Gary
>>----- Original Message -----
>>From: "Usankin, Andrew" <Andrew.Usankin@twtelecom.com>
>>To: <ccielab@groupstudy.com>
>>Sent: Monday, October 22, 2007 5:36 PM
>>Subject: maximum-paths in bgp
>>
>>
>>> Hi all!
>>>
>>> I stumbled yesterday across what appears to be a simple thing - BGP
>>> command "maximum-paths"
>>>
>>> There is almost nothing in DocCD on this command and in the book
>>> "Cisco
>>> BGP-4 command and configuration handbook" all I found useful is this
>>> phrase:
>>> "...If all the attributes of the paths are equal, such as MED, Local
>>> Preference, and Weight, the route that will be installed is the one
>>> learned from the router with the lowest router ID... The
>>> maximum-paths command allows up to six paths to the same destination
>>> to be installed in the IP routing table."
>>>
>>> Well I had to lab it up just to find out that BGP won't install route
>
>>> in the routing table if AS path is different! Yes MED is the same,
>>> Weight and Local preference is the same, but route is not in the
>routing table.
>>> Because my as-path is different :( Stupid me, I should have guessed
>>> it up!
>>>
>>> Can anybody recommend "yet another" great book about BGP? Because I'm
>
>>> not sure that maximum paths is part of RFCs, so I'm out of references
>
>>> on this one.
>>>
>>> ------------------------
>>> A#sh ip bgp
>>> BGP table version is 6, local router ID is 192.168.13.1 Status codes:
>
>>> s suppressed, d damped, h history, * valid, > best, i - internal,
>>> r RIB-failure, S Stale
>>> Origin codes: i - IGP, e - EGP, ? - incomplete
>>>
>>> Network Next Hop Metric LocPrf Weight Path
>>> * 0.0.0.0 192.168.12.2 0 0 200 ?
>>> *> 192.168.13.3 0 200 ?
>>> * 172.16.1.0/24 192.168.12.2 0 0 200
>12000
>>> 300 i
>>> *> 192.168.13.3 0 0 200
>44000
>>> 300 i
>>> *> 192.168.1.0 0.0.0.0 0 32768 i
>>> * 192.168.2.0 192.168.12.2 0 0 200 i
>>> *> 192.168.13.3 0 200 i
>>> *> 192.168.3.0 192.168.13.3 0 0 200 i
>>> A#
>>> A#sh run | s bgp
>>> router bgp 100
>>> no synchronization
>>> bgp log-neighbor-changes
>>> network 192.168.1.0
>>> neighbor 192.168.12.2 remote-as 200
>>> neighbor 192.168.12.2 prefix-list A-2 in neighbor 192.168.13.3
>>> remote-as 200 maximum-paths 2 no auto-summary !
>>> address-family nsap
>>> maximum-paths 2
>>> no synchronization
>>> exit-address-family
>>> A#sh ip ro
>>> Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
>>> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
>>> N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
>>> E1 - OSPF external type 1, E2 - OSPF external type 2
>>> i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS
>>> level-2
>>> ia - IS-IS inter area, * - candidate default, U - per-user
>>> static route
>>> o - ODR, P - periodic downloaded static route
>>>
>>> Gateway of last resort is 192.168.13.3 to network 0.0.0.0
>>>
>>> C 192.168.12.0/24 is directly connected, Serial1/0
>>> C 192.168.13.0/24 is directly connected, Serial1/1
>>> 172.16.0.0/24 is subnetted, 1 subnets
>>> B 172.16.1.0 [20/0] via 192.168.13.3, 00:02:44
>>> C 192.168.1.0/24 is directly connected, FastEthernet0/0
>>> B 192.168.2.0/24 [20/0] via 192.168.13.3, 00:02:44
>>> [20/0] via 192.168.12.2, 00:02:44
>>> B 192.168.3.0/24 [20/0] via 192.168.13.3, 00:02:44
>>> B* 0.0.0.0/0 [20/0] via 192.168.13.3, 00:02:44
>>> [20/0] via 192.168.12.2, 00:02:44 A#
>>>
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