Giants in a MPLS vlan

[bjornarsb]

Hi,

We are running MPLS between cisco 7206 routers connected via two 6509 core switches. The interface on the 6509 report giants . We have enabeled jumbo frames support and we have configured :

tag-switching mtu 1516 on the 7206 routers. We have nothing else in this vlan except from the 7206 routers running MPLS and we cannot see why we get giants. We have checked that the module in the 6509 (WS-X6348-RJ-45) supports jumbo frames. up to 8092

Any ideas?

Best regards

Bjornarsb

[mounir.mohamed]

Dear,

Baby giants: The baby giants feature allows a switch to pass or forward packets that are slightly larger than the IEEE Ethernet MTU. Otherwise, the switch declares big frames as oversize and discards them

When you enable MPLS on an interface, MPLS can also augment the frame size of a packet, depending on the number of labels in the Label stack for an MPLS-tagged packet. The total size of a label is four bytes. The total size of a label stack is n x 4 bytes. If a label stack is formed, the frames can exceed the MTU.

The default MTU size is 9216 bytes after you enable jumbo frame support on the individual port. However, an Application Specific Integrated Circuit (ASIC) limitation requires that you limit the MTU size to 8092 bytes, you module is one of the supported modules WS-X6348-RJ-45.

As a workaround, you can enable a port for trunking in order to support baby giants. When you enable a port for 802.1q trunking, the switch automatically assumes that an extra four bytes of data are appended on, and increment the frame size of the L2 packet. Note that ISL encapsulation is not supported on these platforms

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Best Regards,

Mounir Mohamed

[bjornarsb]

Hi,

Thank you for your answer but we are still experiencing giants.

Regards

Bjornarsb

[mounir.mohamed]

Dear,

Would you increase the Interface MTU, to minimum 1518 or higher because it's seems you using more than one lable.

Please rate helpful posts.

Best Regards,

Mounir Mohamed

[Harold Ritter]

Bjornar,

This is normal behavior. The Pinnacle ASIC on the WS-X6348-RJ-45 considers anything over 1518 bytes as a giant frame. In other words, the giant counter is not necessarily an error counter.

If you receive frames larger than 1518 bytes, the giant counter will increment no matter what value you set the mtu on the interface. As long as you have enabled jumbo frame support and set mpls mtu to the proper value, everything should work just fine and the giant counter should keep incrementing.

There is a refernce to this behavior in the following document:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml

Hope this helps,