From: Mohamed M Moustafa (mmma@gawab.com)
Date: Mon Oct 22 2007 - 20:14:56 ART
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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