ISL trunking and MTU and PMTUD

gnijs · ‎03-16-2006

All,

In the case you setup a ISL trunk from switch to a router (using logical interfaces), does anyone know if the MTU changes on the routers interface and/or the router can cope with the 30 bytes overhead of ISL. I would think that the router can cope with the extra 30 bytes, but i do never see any baby giant frames (1514 + 30 = 1544 bytes frames) in the statistics.

Geert

smahbub · ‎03-22-2006

When a port is enabled for 802.1q trunking the switch will automatically assume that there is an extra four bytes of data appended on, incrementing the frame size of the Layer 2 (L2) packet. Therefore, for implementations that require exactly only one tag to be carried (either 802.1q or Multiprotocol Label Switching (MPLS), but not both), it is possible to force the switchport to accept an extra four bytes of data by configuring it as a trunk port.