Change MTU size in a campus network

[pcolot]

Hi,

I'd like to implement 802.1q tunneling in my network for a subsidiary which needs to connect two sites trough our network. In order to achieve it, I have to modify the mtu size to 1504 on all switches in my network. Has anybody some experience on it ? Is there well-known problems associated with this change ?

For me, only the traffic initiated to or by the switch could have some trouble but not with servers...

Am i wrong ?

Patrick

[lgijssel]

Dear Patrick,

You are wrong. You do not need to adjust anything, it will work with an MTU of 1500 on the LANs.

The only place where you might observe larger frames is on the trunks, and they adjust automatically.

Regards,

Leo

[colotpa]

Hi,

Tanks for your info but in the following document "http://www.cisco.com/en/US/customer/products/hw/switches/ps646/products_configuration_guide_chapter09186a0080115937.html" they mention "The default system MTU for traffic on the Catalyst 3550 switch is 1500 bytes. You can configure the switch to support frames larger than 1500 bytes by using the system mtu global configuration command. 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" Could you explain their statement ?

[lgijssel]

Glad too,

This text is about service providers. For a service provider it might be nescessary to forward tagged frames over his infrastructure.

This enables customers to use vlans spanning multiple locations. This does not apply to you if you are not a service provider.

Leo

[weaver.lance]

Patrick,

It sounds like you want to carry your subsidiary's vlan information across your network. In this instance you are acting like a service provider and the document you are referencing does apply.

I have changed MTU size many times before without issue although not to the extent that you will need to.

Lance

[Darren Ramsey]

We have a 10 MB L2 Metro pipe and send multiple tags across it. The provider uses Cisco gear and I had them to bump up the MTU in order to support this. They are running CatOS, but I believe this is possible in IOS natively with QinQ support or Baby Giants.