Fragmented Packets And Mtu

[Parrotgeek7]

What can cause packets to bcome fragmented. We had a 24 port 3-Com switch that, when monitored showed more then half of the packets were fragmented ( even if smaller then the MTU)

The ISP admin swears its the router, but as soon as we swapped out the switch for a newer one, the fragmentation stopped. (this is one of their switches, not ours)

the network goes like this

Outside>>>Cisco 1700 router>>>24 port switch>>>port 7,8 or 14>>>Linksys router>>>linksys 8 port switch>>office pcs

>>>ports other then 7,8, and 14 all go to a patch panal and out into the community.

There ere no problems with the communty switchs (forced 10 full duplex) but problems on the 3 autosync ports (7,8 and 14)

Its running like a champ now...but was it the switch or could it be something else?

Any ideas would be welcome.

[12lostcause12]

According to my networking text-book, from last fall, talking about services provided as a part IP as a transport protocal:

Fragmentation: Large IP packets may be split into smaller packets if an intermediary network is able to process only smaller packets.  IP will transparently reassemble the smaller packets into the original large one.

This would lead me to assume that the first switch thought that it was sending the packets on to a smaller network and breaking them up, or that the switches have a diffrent through-put speed and the fragments are needed for that.

[Parrotgeek7]

According to my networking text-book, from last fall, talking about services provided as a part IP as a transport protocal:

Fragmentation: Large IP packets may be split into smaller packets if an intermediary network is able to process only smaller packets.  IP will transparently reassemble the smaller packets into the original large one.

This would lead me to assume that the first switch thought that it was sending the packets on to a smaller network and breaking them up, or that the switches have a diffrent through-put speed and the fragments are needed for that.

<{POST_SNAPBACK}>

I'm thinking that the two had different MTU's and that may have been the cause, since I have only access to one side of the equation I have to assume that theirs was not set to 1500 (which is default for Linksys) as our was, and when the new switch went in, they didn't change it from the factory default so maybe they both shipped with 1500 as the magic number.

Could be.....

[Parrotgeek7]

CC PM me and said it could be the routers not having the same size MTU or possible a switch that doesnt handle large frames (have large frame support enabled)

What think?