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packet fragmentation over Frame Relay


I was wondering if anyone might have any advice.

I have a remote site that i've configured for Frame Relay. (it is a standard DS1 over a channalized DS3 back to a cisco 8510 with a frame relay DS3 adapter.)I noticed that if I send standard ICMP packet accross the serial interface I get "normal" response times. But once I start sending larger packets accross the line, my ping times increase exponentially. Just so happens that if I send packets 128 bytes or less, ping times are fine, 129bit packets and my ping times double, 257 bytes or higher and my ping times double again. 1500 bytes and my ping times are really bad. My MTU is set for 1500 byte packets.

My thought was packets are possibly fragmenting.

I don't know if this is enough information but any thoughts?

Any advise would be greatly appreciated.

thank you.

2 Replies 2


You could try to send some ICMP with DF bit set to 1. If you dont get a response at all , then you have a fragmentation issue.

You extended ping command to accomplish this task.

check the link:

Set DF bit in IP header? [no]:

Specifies whether or not the Don't Fragment (DF) bit is to be set on the ping packet. If yes is specified, the Don't Fragment option does not allow this packet to be fragmented when it has to go through a segment with a smaller maximum transmission unit (MTU), and you will receive an error message from the device that wanted to fragment the packet. This is useful for determining the smallest MTU in the path to a destination. Default : no.

change this to yes

Hope this help a bit,

if it does please rate this post.


Thank you for your reply. I was able to resolve my issue. Thanks for you input as I learned something new.

My solution was this:

There was a misconfiguration on the Cisco 8510. When the Frame Relay to pvc mapping was done, a connection table was not selected. This caused all the connections to have a PCR (Peak Cell Rate) of 34000bps.

Thanks again!

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