route metric 65536?

Unanswered Question
Nov 13th, 2008
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Hi,


I am using XR 3.6. When I sh ip route from router A to directly connected router, router B,(using loopback address), I will get route metric 65536. Thus makes this link is not prefered (as it has another path to router B).


Its connected thru FastEthernet where cost should be 1. I didnot configure any cost or metric on that paticular interface. Just wonder how I get the 65536?


Below is the sample output:


1. Normal routing condition from router A:


RA#sh route 2.2.2.2


Routing entry for 2.2.2.2/32

Known via "ospf 1", distance 110, metric 68, type inter area

Installed Nov 13 20:00:26.128 for 00:00:16

Routing Descriptor Blocks

x.x.x.x, from 2.2.2.2, via POS0/3/1/1

Route metric is 68

No advertising protos.



2. Condition when shut POS0/3/1/1

RA#sh route 2.2.2.2


Routing entry for 2.2.2.2/32

Known via "ospf 1", distance 110, metric 65536, type inter area

Installed Nov 13 20:01:05.211 for 00:00:02

Routing Descriptor Blocks

x.x.x.x, from 2.2.2.2, via FastEthernet0/3/3/5

Route metric is 65536

No advertising protos.


note: router A and router B are connected back to back thru int FastEthernet0/3/3/5. 2.2.2.2 is the loopback of router B.



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Giuseppe Larosa Thu, 11/13/2008 - 11:51
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Hello Mazlan,

there is an OSPF feature called max-route lsa

useful in some scenarios


Enabling the max-metric router-lsa command will cause a router to originate LSAs with a maximum metric (LSInfinity: 0xFFFF) through all nonstub links, which allows BGP routing tables to converge without attracting transit traffic (if there are not alternate lower cost paths around the router). The router will advertise accurate (normal) metrics after the configured or default timers expire or after BGP sends a notification that routing tables have converged.


see


http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_osp2.html#wp1012938


I think this feature is enabled on your router because it is supported on IOS XR.





Hope to help

Giuseppe


mazlan.alatif Thu, 11/13/2008 - 18:45
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Hi,


If I configure max-metric router-lsa, router A will advertise a maximum metric so that other routers do not prefer the router A as an intermediate hop for other links as well.


I tried to make Fe interface which connected directly to router B being prefered. Currently it is not due to metric 65535 on that routes. I just wonder how this can happen? its only directly connected.


I tried modified the cost, but its not helping. Still metric 65535. Hope somebody can explain


Thanks,


--mazlan


Giuseppe Larosa Thu, 11/13/2008 - 22:14
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Hello Mazlan,

what is the reference bandwidth for OSPF cost calculation ?


OSPF path metric is a 24 bit number but cost of a single link is a 16 bit number so 65535 is the maximum cost on a link for this reason I thought to max-metric lsa feature.


the total cost should be the sum of:


loopback cost

FE cost


if this doesn't happen there is something else that is changing the way the route is treated.


Hope to help

Giuseppe


mazlan.alatif Thu, 11/13/2008 - 23:14
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Hi Giuseppe,


The Default Cost of an OSPF Interface Is 100,000,000 Divided by the Interface Bandwidth.


You are right that the cost to reach the loopback interface is the cost of the FastEthernet interface plus the cost of the loopback interface, in this case should be 2.


Not sure where does 65535 comes from.

Maybe somebody has experienced this before can explain.


Thanks,


--mazlan

mazlan.alatif Fri, 11/14/2008 - 01:32
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Hi Giuseppe,


I have resolved the problem.


The problem lies with mpls ldp sync which is enabled but not synced so ospf will

advertise max metric.


After I removed mpls ldp sycn under router ospf, I now get the real cost and it is using the right path.


RA#sh route 2.2.2.2


Routing entry for 2.2.2.2/32

Known via "ospf 1", distance 110, metric 2, type intra area

Installed Nov 14 17:10:17.115 for 00:20:26

Routing Descriptor Blocks

x.x.x.x, from 2.2.2.2, via FastEthernet0/3/3/5

Route metric is 2

No advertising protos.


Thank you,


--mazlan


Giuseppe Larosa Fri, 11/14/2008 - 01:49
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Hello Mazlan,

good news to hear you have solved, the router was waiting for LDP session to advertise the same prefix and used the max-route lsa concept.



Best Regards

Giuseppe


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