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802.1q and MTU

Hi

I have been asked to configure a VLAN trunk accross a LAN extension circuit but have no specification for the LES circuit.

I notice the ports on a 2950 and also a 3500XL all have an MTU of 1500. When you configure a port as a trunk port does it automatically increase the MTU to say 1504 for 802.1q?

Thanks

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VIP Purple

Re: 802.1q and MTU

Hello,

you have to use the global command ´system mtu´ to increase the size to 1504 or higher for frames larger than 1500 bytes.

Here is the official Cisco explanantion:

The default system MTU for traffic on the Catalyst 3550 switch is 1500 bytes.Because the 802.1Q tunneling feature increases the frame size by 4 bytes when the metro tag is added, you must configure all switches in the service-provider network to be able to process maximum frames by increasing the switch system MTU size to at least 1504 bytes. The maximum allowable system MTU for Catalyst 3550 Gigabit Ethernet switches is 2000 bytes; the maximum system MTU for Fast Ethernet switches is 1546 bytes.

Regards,

Georg

2 REPLIES
VIP Purple

Re: 802.1q and MTU

Hello,

you have to use the global command ´system mtu´ to increase the size to 1504 or higher for frames larger than 1500 bytes.

Here is the official Cisco explanantion:

The default system MTU for traffic on the Catalyst 3550 switch is 1500 bytes.Because the 802.1Q tunneling feature increases the frame size by 4 bytes when the metro tag is added, you must configure all switches in the service-provider network to be able to process maximum frames by increasing the switch system MTU size to at least 1504 bytes. The maximum allowable system MTU for Catalyst 3550 Gigabit Ethernet switches is 2000 bytes; the maximum system MTU for Fast Ethernet switches is 1546 bytes.

Regards,

Georg

New Member

Re: 802.1q and MTU

Thanks :)

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