From: Usankin, Andrew (Andrew.Usankin@twtelecom.com)
Date: Tue Oct 23 2007 - 12:35:09 ART
Damn, everybody speaks Russian this days! :)
Yes, I checked this command in my lab yesterday and it balance even
between paths originated in different AS.
Now to the hard part, Mohamed! Where da'hell did you get this command?
It is really very hidden one, question mark doesn't show it as a list of
valid commands but it did go through and afterwards it is shown in
running config.
Andrew
-----Original Message-----
From: Gary Duncanson [mailto:gary.duncanson@googlemail.com]
Sent: Monday, October 22, 2007 5:48 PM
To: Usankin, Andrew
Cc: mmma@gawab.com; ccielab@groupstudy.com; Roman Rodichev
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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