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MPLS MTU problem of OSR's dot1q trunk link?

Unanswered Question
Jan 4th, 2004
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ge-wan ge-wan ge-wan


|FE |FE | FE | FE


As the above figure,I have four OSR connected using

OSM-4GE-WAN's ge-wan interface,at each OSR I use one

L2 GE port connected two one of the 4 GE-WAN interface,

the L2 GE port use 802.1q trunk.

Each OSR use a FE link to connect PE router, the FE links

are access link of a vlan which was trunked by the L2 GE port

to the LE GE-WAN interface.

The GE-WAN subinterface and the L2 GE/FE interface are

all configured to pass jumbo frames.

Everything seems ok and after the VPN and all network run

normally for almost a month,packets bigger than 1500 bytes

can't pass through P1 or P2, and no events logged .

The case never happened to P3 and P4 .

The hardware and IOS version and mpls vpn configuration

are the same at the four OSR.

Who can give me some help or advice about that?


mail:[email protected]

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ipotts Tue, 01/06/2004 - 06:07
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Would you please clarify what the MTU problem is that you have? It it IP MTU? Are there any firewalls that could be blocking dynamic MTU discovery?

td.wu Thu, 01/08/2004 - 20:31
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There is no firewall.The IP mtu is 1526 and the layer2 fastethernet mtu is 9216.Everything is ok for the mpls ,but after running for almost a month,there is a OSR can't allow the big packet pass through.

romccallum Thu, 01/08/2004 - 04:58
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  • Silver, 250 points or more

Away from your issue here you have one which is even worse. P switching and MPLS on the OSM-GIG-WAN modules is not supported. You require the osm+ cards along with the sup720+ module. If you dont have this then you will run into endless problems with blackholing etc etc. I found this out the hard way also so you are not alone.

td.wu Thu, 01/08/2004 - 20:26
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But I do use the OSM-GIG-WAN as the P router,the cisco document hasn't said that OSM can't running as P in mpls vpn.If it's true as you said,I'll come into a nightmare.


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