From: Gary Duncanson (gary.duncanson@googlemail.com)
Date: Mon Oct 22 2007 - 19:36:22 ART
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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