From: Usankin, Andrew (Andrew.Usankin@twtelecom.com)
Date: Mon Oct 22 2007 - 13:36:02 ART
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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