External typer-2 route preference

Unanswered Question

Hi All

I have network 148.172.0.0/16 in Singapore. I have two links India to Singapore and India-Sydney. We are receiving 172.16.0.0/16 in India as external type-2 routes fron Sydney and Singapore since it is getting reditributed in Singapore and Sydney both. So to reach Singapore traffic is going through Sydney link also.


Now i want to select Sigapore route as prefered route for 148.172.0.0/16 traffic.


gur_voipcorert3a>sh ip route 148.172.145.25

Routing entry for 148.172.0.0/16

Known via "ospf 4000", distance 110, metric 20, type extern 2, forward metric 50

Last update from 10.139.129.83 on Vlan10, 01:21:42 ago

Routing Descriptor Blocks:

10.139.129.83, from 148.172.218.134, 01:21:42 ago, via Vlan10

Route metric is 20, traffic share count is 1

* 10.139.129.82, from 148.172.217.1, 01:21:42 ago, via Vlan10

Route metric is 20, traffic share count is 1


Regds

Om

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Giuseppe Larosa Thu, 04/09/2009 - 02:11
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Hello Om,

you can have singapore to have the route redistributed as OSPF E 1 that is preferred.


Hope to help

Giuseppe


lamav Thu, 04/09/2009 - 03:39
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Om:


Maybe its just me, but I am a bit confused.


"I have network 148.172.0.0/16 in Singapore."


OK.



"I have two links India to Singapore and India-Sydney. We are receiving 172.16.0.0/16 in India as external type-2 routes fron Sydney and Singapore since it is getting reditributed in Singapore and Sydney both."


Why would your ASBRs in 2 different cities be advertising the same /16 prefix?


And what does that OE2 route for 172.16.0.0 in India have to do with reaching 148.172.0.0 in Singapore?



"So to reach Singapore traffic is going through Sydney link also."


Can you clarify this?


Thanks






Giuseppe Larosa Thu, 04/09/2009 - 04:24
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Hello Om,

good OSPF design rules suggests to use OSPF type 1 when where are multiple ASBRs advertising the same routes.


this is because the OSPF type 1 sums the seed metric to the metric to reach the ASBR.


So moving to OSPF O E1 is recommended in a scenario like yours.


Then if you want to prefer one ASBR in a selective way you need to use some distance command.


see


http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_pi1.html#wp1011754


Hope to help

Giuseppe


lamav Thu, 04/09/2009 - 04:30
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Om:


Giuseppe is right.


If you cant make changes to the Singapore or Sydney routers, you're out of luck because you cannot influence how India will see the 172.16.0.0/16 route coming to it by making configuration changes in India. This is not BGP, where you can manipulate attributes for any inbound/outbound prefix you want.


HTH


Victor

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