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Ospf integration with cisco

Roberto Cremasco
Occasional Advisor

Ospf integration with cisco

We are integrating routing via ospf between a HP 5300 and a cisco 3550.

The problem is than from the cisco side the mtu is 1504 and the HP reject the ospf session by bad mtu size. On cisco devices is available a command "ip ospf mtu-ignore", there is any chance to set it also un HP?

An other question is, how to set the ospf neighbor as point to point ?

thanks,
RoC
6 REPLIES
Mohieddin Kharnoub
Honored Contributor

Re: Ospf integration with cisco

Hi

Thats an interesting issue, because what we know is that the 5300 accepts MTU = 1522 bytes IP packets, so i have a suspect that its not an OSPF related issue unless ...

To clear this out, can you turn on debugging on the 5300 for OSPF and forward this to a Syslog server, capture the OSPF calculation and attach it here with a little explanation about your scenario (connectivity, OSPF areas, Vlans between the cisco and 5300 ... ).

Good Luck !!!
Science for Everyone
OLARU Dan
Trusted Contributor

Re: Ospf integration with cisco

Maybe you can change the MTU setting on the cisco 3550 to match the HP value? Be careful, though, since "system mtu" is a global config command that applies to all interfaces of the device, so you might lose connectivity with other upstream devices that c3550 connects to. Maybe you can change only the MTU setting for the interface connecting to the HP in question (this feature pretty much depends on the IOS version, I would suppose).

See this:

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/1219ea1/ol208902.htm#xtocid14
Roberto Cremasco
Occasional Advisor

Re: Ospf integration with cisco

attached the syslog output for the hp device.

some more info: the ospf area on the hp site is configured as area backbone. On this area tha configured also 2 routed for wan connection with other sites. Now we are connecting a new data center via a mpls network, and the cpe of new connection is a cisco 3550. The telecom provider says me that the mtu size of 1504 is due to mlps network that needs 4 more bytes for tagging, so is not possible to lower the mtu on the cisco side.

The other question is that the provider wonts to configure the neighbor as point to point.

Thanks again,
RoC
Mohieddin Kharnoub
Honored Contributor

Re: Ospf integration with cisco

Hi

I hoped you enable the Syslog then you re-run the OSPF process on both switches from the beginning so we you can capture more information about adjacency and OSPF topology building in its initial process.

Anyway

Use the following link to troubleshoot your situation, and i hope that will help you to define it and correct it.

http://www.cisco.com/warp/public/104/trouble_main.html#trouble_mtu_check

Come back to us with the result and share your information.

Good Luck !!!
Science for Everyone
Olaf Borowski
Respected Contributor

Re: Ospf integration with cisco

Hi Roberto,

The service provider should not be sending MPLS tags to you. The MPLS network is really on their side and you should be unaware of this. The connection to the 3550 should be a "normal" ethernet connection, no MPLS tags. Again, they should be stripping off the tag which they send the packet to you.
You only have to deal with the MPLS tag if you are part of the providers MPLS backbone and I don't believe that is the case.

Regarding OSPF point-to-point: Not supported on the 5300.

Hope this helps,
Olaf
Roberto Cremasco
Occasional Advisor

Re: Ospf integration with cisco


more info: the service provider is not sending me the mpls tags, but the cisco switch has the MTU size (which is a global paramter) set to 1504 because it handle also the mpls side of the network.

I don't have the syslog info from the provider side but only my side.

There is any command that can change/ignore the MTU size on the ospf session ?

Thanks to all,
RoC