A Query about PBR and MTU size.

Hi Folks,

Can PBR be used to modify the outgoing MTU size?

I have a scenario with a Cisco 827 using PPPoA and an MTU of 1500, where one particular destination site uses PPPoE (and an MTU of 1492) and their ISP seems to be blocking Path MTU Discovery from working. If I manualy adjust my MTU down I can get this working, however this has to be doen for ALL sites, and I just want it to be done for the one site. Can I use PBR to do this for just the ONE target Network? If so, would someone care to post an example?

Is there another option for resolvnig this problem?


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I am pretty sure that you are a regular here and will know already that the increase in overhead by moving from 1500 to 1492 will be so small that the effect will be hard to measure. Still I have an idea.

This may or may not work.

Set up a new loopback for this with an mtu of 1492. Make up an IP address for it. Doesn't matter at all what it is as long as there is no conflict. Make it a NAT inside if necessary.

Use PBR to send all inbound traffic from your LAN via this loopback /if/ it is heading for the PPPPoE site.

I have seen this type of thing used and work but not necesarily in this context.

I very much like tcp mss-adjust, you may be able to use that too.

Please feed back the outcome.

Good luck, I wouldn't bother tho':-)

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Yep, been following this group for many years now.........;-)

I agree, performance wise the difference in MTU size is trivial, however when some brain dead ISP decides to turn off PMTUD the result can be (ultimately) fatal for such things as FTP transfers for files over around 80KB. On the odd times I needed it, I set all traffic to an MTU of 1492 and the transfers completed fine.

Thanks, I will see if I can get it to work.



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