BGP decision question

Unanswered Question
Nov 9th, 2007

Hi, I'm looking at some BGP topo entries and don't see why this particular path is being as "best."


Here's our setup:


We have 2 core routers in our network, one in ATL and one in PHIL.


Each router, ATL & PHIL, has IBGP connections to 2 outlying routers which are both have EBGPconnections to the same AS and are both receiving EBGP advertisements for the same prefixes.


These routers at the AS edge that are doing EBGP are coded as RRclients.


This following was taken from ATL:

32471

65.168.87.33 (metric 297246976) from 172.31.192.2 (172.31.255.2)

Origin IGP, metric 1, localpref 100, valid, internal

Community: 32471:160

Originator: 172.31.255.12, Cluster list: 172.31.255.2


32471, (Received from a RR-client)

65.168.87.37 (metric 297246976) from 172.31.192.90 (172.31.255.13)

Origin IGP, metric 1, localpref 100, valid, internal

Community: 32471:160


32471, (Received from a RR-client)

65.168.87.33 (metric 297246976) from 172.31.192.82 (172.31.255.12)

Origin IGP, metric 1, localpref 100, valid, internal, best

Community: 32471:160


It shows the bgp advertisement that it received from PHIL from 172.31.192.2(loopback 172.31.255.2)


And it shows the advertisements it received from the 2 route-reflectors, 172.31.192.90 and 172.31.192.82.


I don't see why it picked the route that it did; i thought it should have picked the rouute from PHIL, 172.31.192.2


Reasoning:

Next hop and Sync - OK

Weight - 0 for all 3

Local pref - 100 for all 3

#4 AS path - same for all 3


#5 Origin - IGP for all 3


#6 MED - either 297246976 or 1, not sure how to read it, but equal for all 3


#7Prefer EBGP over IBGP - all are IBGP


#8 Prefer the path with the lowest IGP

metric to the BGP next hop - is this shown in the output? This may be the decision point -- not sure how this is measured, as going thorugh PHIL the cost to the endpoint is the highest, but since this is EIGRP internally, does this use the eigrp topo table to check and see if it is using that particular neighbor to get to the BGP next hop?


#9 - multipath


#10 When both paths are external, prefer the path that was received first (the oldest one). not applicable


#11 Prefer the route that comes from the BGP router with the lowest router ID. picks 172.31.255.12 instead of .12 (but I'm still not sure why it didn't pick the advertisement originally from .12 and out of our PHIl core router).










Thanks!!!

Lisa G


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royalblues Fri, 11/09/2007 - 12:02

yes for point number 8 it uses the IGP metric which will be EIGRP in your case. Can you check what is the metric calculated by EIGRP to the next hop


BTW can you try to post a network topology indicating the IP address?


Narayan

lgontarsk Fri, 11/09/2007 - 12:44

Thanks!!


Show ip eigrp topo for the next hops:


r001-atl-crx>show ip eigrp topo 65.168.87.32 255.255.255.252

IP-EIGRP (AS 1): Topology entry for 65.168.87.32/30

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 297246976

Routing Descriptor Blocks:

172.31.192.82 (Tunnel10), from 172.31.192.82, Send flag is 0x0

Composite metric is (297246976/28160), Route is Internal

Vector metric:

Minimum bandwidth is 9 Kbit

Total delay is 500100 microseconds

Reliability is 255/255

Load is 48/255

Minimum MTU is 1440

Hop count is 1

172.31.192.2 (Tunnel0), from 172.31.192.2, Send flag is 0x0

Composite metric is (310046976/297246976), Route is Internal

Vector metric:

Minimum bandwidth is 9 Kbit

Total delay is 1000100 microseconds

Reliability is 255/255

Load is 253/255

Minimum MTU is 1440

Hop count is 2

r001-atl-crx>show ip eigrp topo 65.168.87.36 255.255.255.252

IP-EIGRP (AS 1): Topology entry for 65.168.87.36/30

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 297246976

Routing Descriptor Blocks:

172.31.192.90 (Tunnel11), from 172.31.192.90, Send flag is 0x0

Composite metric is (297246976/28160), Route is Internal

Vector metric:

Minimum bandwidth is 9 Kbit

Total delay is 500100 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1440

Hop count is 1

172.31.192.2 (Tunnel0), from 172.31.192.2, Send flag is 0x0

Composite metric is (310046976/297246976), Route is Internal

Vector metric:

Minimum bandwidth is 9 Kbit

Total delay is 1000100 microseconds

Reliability is 255/255

Load is 190/255

Minimum MTU is 1440

Hop count is 2



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