Hello,
I have 3 routers:-
150.150.150.0/24
-------------------------------------------------------------------
|
|
|
|
R1 ---------------------------- R2----------------------------R3
(AS1) (AS2)
(AS3)
R1 - R2, R2 - R3 are using EBGP
I injected the network 150.150.150.0/24 into BGP from both R1 & R3. I have
enabled bgp always-compare-med to allow R2 to compare the MED from AS 1 & AS
3. I have also set a route-map on R3 so that when it advertise
150.150.150.0/24 to R2, it would have a metric of 20, as compared to 0 from
R1.
The thing I'm confused is that after I have enabled the command "bgp
bestpath med missing-as-worst", according to CCO, the MED of 0 from R1 would
now be considered as infinity (as confirmed by the debug messages), but why
is R2 still prefers the route from R1 instead of from R3?
I realized that Weight, Local Pref, AS-Path, Origin are used before MED for
best path selection by BGP. However, they are all left at default and are
the same on both R1 & R3/
Best Regards,
Hunt
RouterB#sh ip bgp 150.150.150.0/24
BGP routing table entry for 150.150.150.0/24, version 3
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
10.1.1.2
1
172.17.1.2 from 172.17.1.2 (20.20.20.1)
Origin IGP, metric 0, localpref 100, valid, external, best
3
10.1.1.2 from 10.1.1.2 (15.15.15.1) 10.1.1.2 to
main IP table
02:58:54: BGP: 172.17.1.2 rcv message type 4, length (excl. header) 0
02:58:54: BGP(0): 172.17.1.2 computing updates, afi 0, neighbor version 0,
table version 2, starting at 0.0.0.0
02:58:54: BGP(0): 172.17.1.2 send UPDATE (format) 150.150.150.0/24, next
172.17.1.1, metric 0, path 3
02:58:54: BGP(0): 172.17.1.2 1 updates enqueued (average=47, maximum=47)
02:58:54: BGP(0): 172.17.1.2 update run completed, afi 0, ran for 12ms,
neighbor version 0, start version 2, throttled to 2
02:58:54: BGP: 10.1.1.2 rcv message type 4, length (excl. header) 0
02:58:54: BGP(0): 172.17.1.2 rcvd UPDATE w/ attr: nexthop 172.17.1.2, origin
i, metric 0, path 1
02:58:54: BGP(0): 172.17.1.2 rcvd 150.150.150.0/24
02:58:54: BGP(0): Revise route installing 150.150.150.0/24 -> 172.17.1.2 to
main IP table
02:59:17: BGP: Performing BGP general scanning
02:59:17: BGP(0): scanning IPv4 Unicast routing tables
02:59:17: BGP(IPv4 Unicast): Performing BGP Nexthop scanning for general
scan
02:59:17: BGP(1): scanning VPNv4 Unicast routing tables
02:59:17: BGP(VPNv4 Unicast): Performing BGP Nexthop scanning for general
scan
02:59:17: BGP(2): scanning IPv4 Multicast routing tables
02:59:18: BGP(IPv4 Multicast): Performing BGP Nexthop scanning for general
scan
02:59:22: BGP(0): 10.1.1.2 computing updates, afi 0, neighbor version 1,
table version 3, starting at 0.0.0.0
02:59:22: BGP(0): 10.1.1.2 send UPDATE (format) 150.150.150.0/24, next
10.1.1.1, metric 0, path 1
02:59:22: BGP(0): 10.1.1.2 1 updates enqueued (average=47, maximum=47)
02:59:22: BGP(0): 10.1.1.2 update run completed, afi 0, ran for 12ms,
neighbor version 1, start version 3, throttled to 3
02:59:23: BGP(0): 172.17.1.2 computing updates, afi 0, neighbor version 2,
table version 3, starting at 0.0.0.0
02:59:23: BGP(0): 172.17.1.2 send unreachable 150.150.150.0/24
02:59:23: BGP(0): 172.17.1.2 send UPDATE 150.150.150.0/24 -- unreachable
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=46321&t=46321
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