MPLS MTU problem

jeffrey.zhou · ‎08-09-2003

If a Cisco IOS router will adjust its interface MTU when the tag swtiching is enabled on it? if it do, what is the Maximum MTU value can the interface pass?

mazhar71 · ‎08-10-2003

Hi,

No , IOS router doesn't adjust MTU automatically when tag switching is enabled. But you can simply set it using "tag-switching mtu" command. MTU values change according to the type of the interface. According to the todays implementation , maximum label stack is 5 so 5*20bytes=100 bytes. It depends on the MPLS implemetation of course.

Best Regards

jeffrey.zhou · ‎08-11-2003

Maximun label stack is 5 so 5*4=20byes? (i think eack label is 4-byte length)

Through my lab, I found something. First, the IOS may change the actual tag mtu value according to your "tag-switching mtu , in my lab, when I configured "tag-switching mtu 1511", the router may actually use 1504 to fragment the packets, and when I configured "tag-switching mtu 1512 or 1519", the router use 1512 to fragment the packets, when I configured "tag-switching mtu 1523"the router use 1520 to fragment the packets, and when I configured "tag-switching mtu 1529", the router use 1528 to fragment the packets. The large packets (above 1600 bytes) ping successfull when the tag mtu value was below or equal 1512, and failed if the tag mtu value was larger than 1512. through the lab, I found the IOS router increased 12 bytes (3 label statcks) for its interface MTU size, and it can transmit 1508-byte (plus 4-byte label) icmp packets without fragmentation. All above was verified by debug ip packet on the router. But unfortunately, I cannot found any doucments on CCO which had accurate information regarding this.

Regards