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New Member

How do I disable fragmentation on a 2901 router

How do I disable fragmentation on a 2901 router?  I want it to simply drop packets oversized packets.

In my lab, I am trying to test various MTU issues.  I'm trying to use a 2901 router to simulate the WAN equipment that my WAN provider would deploy in production.  In production i'm expecting the WAN to only support an MTU of 1320 with no fragmentation at all.  I need to configure my test router to also NOT fragment in order the simulation the real life situation.

2 ACCEPTED SOLUTIONS

Accepted Solutions
New Member

Re: How do I disable fragmentation on a 2901 router

In production, if the ISP is fragmenting with a lower MTU.. Why would you not do the same?

If it's causing inherant issues, then that's an issue. If no then it's not a problem, so some ISP's will enforce a maximum packet length unit. It will be re-assmbled back to the correct IP Packet data size.

The issues you have is slight memory / cpu overhead from the amount of PPS and restructuring. With e.g. GRE you can have PMTUD, MTU and MSS issues if not set up correctly..

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml

Like shown above, but unless it is causing an issue. I wouldn't worry about it.

How do I disable fragmentation on a 2901 router

I think you need to apply ACL that will filter out ICMP "destination unreachable - fragmentation needed " messages so PMTUD mechanism will not work.

Please rate if it was helpful. "Correct answer" will be also pleasant. Thank you.

2 REPLIES
New Member

Re: How do I disable fragmentation on a 2901 router

In production, if the ISP is fragmenting with a lower MTU.. Why would you not do the same?

If it's causing inherant issues, then that's an issue. If no then it's not a problem, so some ISP's will enforce a maximum packet length unit. It will be re-assmbled back to the correct IP Packet data size.

The issues you have is slight memory / cpu overhead from the amount of PPS and restructuring. With e.g. GRE you can have PMTUD, MTU and MSS issues if not set up correctly..

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml

Like shown above, but unless it is causing an issue. I wouldn't worry about it.

How do I disable fragmentation on a 2901 router

I think you need to apply ACL that will filter out ICMP "destination unreachable - fragmentation needed " messages so PMTUD mechanism will not work.

Please rate if it was helpful. "Correct answer" will be also pleasant. Thank you.

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