External OSPF route selection

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Jun 12th, 2009
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Dear all,


This is a generic question , is there any reason that router selects an ospf route with E1 type metric 40000 over another ospf route type E1 with metric 101


the first route is coming from MPBGP redistribution to OSPF ( PE >>> CE) , while the second route is coming from redistribute static to ospf.


Thanks & BR

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Harold Ritter Fri, 06/12/2009 - 09:22
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Moamen,


I suppose you mean E1 with a metric 40000 being preferred over E2 with a metric of 101.


An ospf E1 is always preferred over an E2 as it's metric comprises both the external value (as the route is redistributed in the AS) and the internal cost (cost to the ASBR), whereas the E2 only comprises the external cost.


The OSPF order of preference is as follow:


Intra-area

Inter-area

External type 1

External type 2


Regards

moamen.elhefnawy Fri, 06/12/2009 - 09:44
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Hello ,


Thanks for your reply, but both of them are E1 , that why it seems strange to me.


I will post the config once I have access.


Thanks & BR

Moamen

Harold Ritter Fri, 06/12/2009 - 10:08
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Hi Moamen,


Thanks for the feedback. It is strange indeed, hence me thinking that it might have been a typo.


Are these two routes received from two different ospf processes. If so, the route type (intra-area, inter-area, e1, e2) will not be compared nor will the metrics. Only the administrative distance would come into play in this context and since it is equal by default , the first one received would be installed in the RIB.


Regards

moamen.elhefnawy Fri, 06/12/2009 - 11:07
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Hello,


Thanks for your support. I have checked and they are one ospf process, is anything realted to MPBGP can lead to that ?


find the show ip route in 2 cases


1- the path to the higher metric route is shut , so we can see the one with better metric coming from the PE.


"instead of the " XXXX" there is the As number "


Rouetr #sh ip route 172.20.216.0

Routing entry for 172.20.216.0/22

Known via "ospf 1", distance 110, metric 120

Tag Complete, Path Length == 1, AS "XXXX", , type extern 1

Last update from 172.20.3.2 on Vlan3, 00:00:05 ago

Routing Descriptor Blocks:

* 172.20.3.2, from 172.20.3.17, 00:00:05 ago, via Vlan3

Route metric is 120, traffic share count is 1

Route tag 3489669380


After We Open the Link to the other path which has higher metric value, the router put it in the routing table and remove the better one.


Router#sh ip route 172.20.216.0

Routing entry for 172.20.216.0/22

Known via "ospf 1", distance 110, metric 40421, type extern 1

Last update from 172.20.2.226 on Vlan2, 00:00:00 ago

Routing Descriptor Blocks:

* 172.20.2.226, from 20.20.20.20, 00:00:00 ago, via Vlan2

Route metric is 40421, traffic share count is 1


May I missed something here , I don't know.


Thanks & BR


Harold Ritter Fri, 06/12/2009 - 13:54
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Hi Moamen,


Could you please post the output from the "show ip ospf da ext 172.20.216.0".


Regards

moamen.elhefnawy Fri, 06/12/2009 - 15:05
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Hello ,


That what I have for now .


Link State ID: 172.20.216.0 (External Network Number )

Advertising Router: 20.20.20.20

LS Seq Number: 80000004

Checksum: 0xED49

Length: 36

Network Mask: /22

Metric Type: 1 (Comparable directly to link state metric)

TOS: 0

Metric: 20

Forward Address: 0.0.0.0

External Route Tag: 0



Routing Bit Set on this LSA

LS age: 162

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 172.20.216.0 (External Network Number )

Advertising Router: 172.20.3.17

LS Seq Number: 80000001

Checksum: 0xAFAD

Length: 36

Network Mask: /22

Metric Type: 1 (Comparable directly to link state metric)

TOS: 0

Metric: 118

Forward Address: 0.0.0.0

External Route Tag: 3489669380


sorry but I don;t have direct access to the routers, and I'm troubleshooting exactly like you , please advise if this is enough for you.


Thanks & BR

Moamen

Harold Ritter Sun, 06/14/2009 - 17:09
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Moamen.


Could you check that the E1 route received from BGP was not originally redistributed from ospf into BGP and then back into ospf.


Regards

moamen.elhefnawy Mon, 06/15/2009 - 23:44
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Hello,


Thanks a lot , yes it is originally distributed from OSPF to BGP then to OSPF, this becuse the routeis coming from remote site to backbone then to the local site


ospf---- BGP--- OSPF


anyway we discovered that when we see the route with high metric preferred , in that time the router does not receive the route from BGP at all , as the Backbone router receive the ospf route first and now he has 2 routes one from BGP and one from OSPF ( lower admin distance ) and he preferrs the ospf .


That's why he removes the BGP route from the routing table and will not redistribute it to OSPF.


Thanks a lot guys for your help and valuable support.


Thanks & BR


Giuseppe Larosa Fri, 06/12/2009 - 09:22
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Hello Moamen,

it shouldn't happen unless you are using multiple OSPF processes: in that case the first to propose the route to the ip routing table domain wins.


You can attach the configuration parts of interest to get better help.


Hope to help

Giuseppe



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