From: Alexei Monastyrnyi (alexeim@orcsoftware.com)
Date: Tue Apr 11 2006 - 05:02:17 GMT-3
since all other criteria are the same, your RR will prefer route with
the lowest IGP metric, as per #10 of BGP path selection from here.
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfbgp.htm#wp1000898
Rack1R5#sh ip route 204.12.1.254
Routing entry for 204.12.1.0/24
Known via "eigrp 100", distance 170, _*metric 2692096*_, type external
...
Rack1R5#
Rack1R5#sh ip route 54.1.1.254
Routing entry for 54.1.1.0/24
Known via "ospf 1", distance 110, _*metric 20*_, type extern 2, forward metric
65
....
A.
on 11/04/2006 06:57 san wrote:
> Hi,
>
> Question:
> -------------
> Trying to find on what basis did R5 select the best path for network
> 112.0.0.0 ?
>
> Topology:
> -------------
> BB3----ebgp---R3 (RRC)--------(RR)R5---------(RRC)R6-----ebgp----BB1
>
> network 112.0.0.0 is a loopback network of BB1. & comes into BGP process
> via "network" command in BB1.
>
>
> R5 is selecting path via 54.1.1.254 via R6. Would like to know why ? Is it
> because of Metric to reach advertised next-hop. 54.1.1.254 is lower ? than
> 204.12.1.254.
>
> Is my guess correct ? I am guessing this because MED, Local Pref, as-path
> list etc are same.
>
>
> Output:
> ---------
>
>
>
> Rack1R5#sh ip bgp
> BGP table version is 14, local router ID is 150.1.5.5
> 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
> *>i28.119.16.0/24 54.1.1.254 0 200 0 54 i
> *>i28.119.17.0/24 54.1.1.254 0 200 0 54 i
> * i112.0.0.0 204.12.1.254 0 100 0 54 50 60 i
> *>i 54.1.1.254 0 100 0 54 50 60 i
> * i113.0.0.0 204.12.1.254 0 100 0 54 50 60 i
> *>i 54.1.1.254 0 100 0 54 50 60 i
> *>i114.0.0.0 54.1.1.254 0 200 0 54 i
> *>i115.0.0.0 54.1.1.254 0 200 0 54 i
>
> Rack1R5#
> Rack1R5#sh ip bgp 112.0.0.0
> BGP routing table entry for 112.0.0.0/8, version 8
> Paths: (2 available, best #2, table Default-IP-Routing-Table)
> Advertised to update-groups:
> 1 2
> 54 50 60, (Received from a RR-client)
> 204.12.1.254 (metric 2692096) from 183.1.0.3 (150.1.3.3)
> Origin IGP, metric 0, localpref 100, valid, internal
> 54 50 60, (Received from a RR-client)
> 54.1.1.254 (metric 20) from 150.1.6.6 (150.1.6.6)
> Origin IGP, metric 0, localpref 100, valid, internal, best
> Rack1R5#
> Rack1R5#
> Rack1R5#sh ip route 112.0.0.0
> Routing entry for 112.0.0.0/8
> Known via "bgp 100", distance 200, metric 0
> Tag 54, type internal
> Last update from 54.1.1.254 00:26:06 ago
> Routing Descriptor Blocks:
> * 54.1.1.254, from 150.1.6.6, 00:26:06 ago
> Route metric is 0, traffic share count is 1
> AS Hops 3
> Route tag 54
>
> Rack1R5#
> Rack1R5#sh ip route 183.1.0.3
> Routing entry for 183.1.0.0/24
> Known via "connected", distance 0, metric 0 (connected, via interface)
> Redistributing via eigrp 100
> Advertised by eigrp 100
> Routing Descriptor Blocks:
> * directly connected, via Serial0/0
> Route metric is 0, traffic share count is 1
>
> Rack1R5#sh ip route 150.1.6.6
> Routing entry for 150.1.6.6/32
> Known via "ospf 1", distance 110, metric 66, type inter area
> Redistributing via eigrp 100
> Advertised by eigrp 100 metric 10000 100 255 1 1500
> Last update from 183.1.0.4 on Serial0/0, 23:55:35 ago
> Routing Descriptor Blocks:
> * 183.1.0.4, from 150.1.4.4, 23:55:35 ago, via Serial0/0
> Route metric is 66, traffic share count is 1
>
> Rack1R5#
> Rack1R5#sh ip route 204.12.1.254
> Routing entry for 204.12.1.0/24
> Known via "eigrp 100", distance 170, metric 2692096, type external
> Redistributing via ospf 1, eigrp 100
> Advertised by ospf 1 subnets
> Last update from 183.1.58.8 on FastEthernet0/0, 23:33:48 ago
> Routing Descriptor Blocks:
> * 183.1.58.8, from 183.1.58.8, 23:33:48 ago, via FastEthernet0/0
> Route metric is 2692096, traffic share count is 1
> Total delay is 40400 microseconds, minimum bandwidth is 1544 Kbit
> Reliability 255/255, minimum MTU 1500 bytes
> Loading 1/255, Hops 5
>
> Rack1R5#
> Rack1R5#sh ip route 54.1.1.254
> Routing entry for 54.1.1.0/24
> Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric
> 65
> Redistributing via eigrp 100
> Advertised by eigrp 100 metric 10000 100 255 1 1500
> Last update from 183.1.0.4 on Serial0/0, 1d00h ago
> Routing Descriptor Blocks:
> * 183.1.0.4, from 150.1.6.6, 1d00h ago, via Serial0/0
> Route metric is 20, traffic share count is 1
>
> Rack1R5#
>
> --
> Thanks & Rgds
> SAN
>
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