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New Member

Cisco 3640 MTU Locked?

I am unable to change the MTU on a Cisco 3640. It says the: "Interface Ethernet0/0 does not support user settable mtu."

Does anyone know why this isn't allowed, and why the router only appears to accept 1272byte packets and fragments the rest?

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Bronze

Re: Cisco 3640 MTU Locked?

What does the router say the mtu is. Do sho int.

Its probably the mtu in your pc thats the problem. Looks like it is set at 1300. Ip hdr 20 + icmp hdr 8 + 1272 = max 1300.

3 REPLIES
New Member

Re: Cisco 3640 MTU Locked?

Are you trying to change the MTU size of IP packets on the interface or the maximum packet size on the interface?

While you're at it, what IOS version are you running on your 3640?

New Member

Re: Cisco 3640 MTU Locked?

I am trying to change the MTU size for packets. From what I understand ethernet generally has a default MTU of 1500bytes. If I ping the router with the don't fragment flag set and the buffer size is above 1272 it will drop the packet.

I'm running IOS 12.2(17a)

Please let me know if you need further information.

Bronze

Re: Cisco 3640 MTU Locked?

What does the router say the mtu is. Do sho int.

Its probably the mtu in your pc thats the problem. Looks like it is set at 1300. Ip hdr 20 + icmp hdr 8 + 1272 = max 1300.

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