Matrix N-Series DFE
General description of how jumbo ethernet frame processing works.
Here is the way that jumbo frame (1184) processing operates, on the DFE:
- Jumbo frame enablement at the port level ('set port jumbo enable <port>') changes the port's maximum receivable frame size from 1500 to 10239 bytes, and changes the Maximum Transmission Unit (MTU) setting from 1500 to 10239 bytes. As a separate matter; the router entity accepts a maximum of 1500-byte frames prior to firmware 5.31.17, and a maximum of 10239-byte frames with firmware 5.31.17 and higher.
- Note: All of these measurements represent the data portion of the packet. The additional overhead for an Ethernet_II frame is typically 18 bytes, and a VLAN Tag would add four bytes to that.
- Jumbo frames may only be received on a port which has jumbo functionality enabled. The frame is otherwise discarded, and counted as "In Discards" in the output of a 'show port counters' command.
- For devices (including the DFE) which support MTU Discovery, a Path-MTU message (IP) is sent upstream to the originator of any frame which has exceeded the local MTU (and thus is being discarded), indicating that the MTU of the packet path was exceeded and what the appropriate MTU is.
- If the DFE still has MTU Discovery globally default-enabled ('set mtu enable'), it will know of any constrained maximum frame size permitted along the path from its location to the destination. Only given this knowledge will it as necessary (and as permitted by the Do Not Fragment bit in the IP header) fragment any incoming jumbo frame to suit known downstream size requirements. When doing this, it does not consider the MTU capabilities of its local egress ports.
- Any resulting unfragmented or fragmented frame will be transmitted only if it does not violate the limits of the router entity (when routing) and the transmission port (when routing or switching). The frame is otherwise discarded, and counted as "Out Discards" in the output of a 'show port counters' command and as "Oversize Pkts" in the output of a 'show rmon stats' command.
- If MTU Discovery is enabled and the DFE must discard either on ingress or egress due to MTU considerations, it notifies upstream devices via a Path-MTU message.