How I can change size of Ethernet frame ???

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Apr 17th, 2007
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How I can change size of Ethernet frame at cisco 871 ?

I have channel (point to point) from my ISP.

Topology of this channel is "Bridge".

This schema doesn't work :

cisco871 <- ADSL modem -> ISP <-- ADSL modem --> cisco 871

But this schema work propertly:

cisco2611xm <- ADSL modem -> ISP <-- ADSL modem --> winodws2003(routing on)<--> cisco 871

My ISP said me: "Your cisco871 have outgoing Ethernet-frame more 1508 and ISP reject its. "

I don't understand why cisco2611xm work propertly so cisco871 doesn't when i use ISP's "Bridge".

I'm can't change MTU at FastEthernet inetface:


pervomay(config)#interface fastEthernet 4


pervomay(config-if)#mtu ?

<64-17940> MTU size in bytes

pervomay(config-if)#mtu 1450

% Interface FastEthernet4 does not support user settable mtu.


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c.lemaire Tue, 04/17/2007 - 05:02
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You should try with the 'system mtu xxx' command in global config or with the 'ip tcp adjust-mss xxx' command in interface config.

ksn77 Tue, 04/17/2007 - 05:11
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As I understand in this device where is no ability to execute command 'system mtu xxx':




I tryed ip 'tcp adjust-mss xxx' and 'ip mtu xxxx' but no result was given.

scottmac Tue, 04/17/2007 - 06:10
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Try "ip mtu xxxx" frmo the interface config prompt.

Also, relatively few frames are actually that large. For most networks, the majority of traffic is in the 128-512 byte range.

If the ISP is seeing frames larger than 1508, then you may also have some kind of tagging / VLAN active or there's a proxy tag ... some other information is being added to the ethernet frame.

Good Luck


Ahmede Tue, 04/17/2007 - 10:52
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My understanding is that on the 871 the FE ports is a switch port not a router port, so you won't be able to change the MTU size on that port. You will be able to change on the router port, i.e any port you can assign IP address to..


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