Jumbo frames are supported on all Ethernet ports.
The maximum MTU size for a jumbo frame on the 7705 SAR is 9732 bytes. The maximum MTU for a jumbo frame may vary depending on the Ethernet encapsulation type, as shown in Table: Maximum MTU (or MRU) per Ethernet Encapsulation Type . The calculations of the other MTU values (service MTU, path MTU, and so on) are based on the port MTU. The values in Table: Maximum MTU (or MRU) per Ethernet Encapsulation Type are also maximum receive unit (MRU) values. MTU values are user-configured values. MRU values are the maximum MTU value that a user can configure on an adapter card that supports jumbo frames.
Encapsulation |
Maximum MTU (bytes) |
|---|---|
Null |
9724 |
Dot1q |
9728 |
QinQ |
9732 |
For an Ethernet adapter card, all frames received at an ingress network or access port are policed against the MRU for the ingress adapter card, regardless of the configured MTU. Any frames larger than the MRU are discarded and the ‟Too Long Frame” and ‟Error Stats” counters in the port statistics display are incremented.
At network egress, frames are checked against the configured port MTU. If the frame exceeds the configured port MTU and the DF bit is set, then the ‟MTU Exceeded” discard counter will be incremented on the ingress IP interface statistics display, or on the MPLS interface statistics display if the packet is an MPLS packet.
For example, on adapter cards that do not support an MTU greater than 2106 bytes, fragmentation is not supported for frames greater than the maximum supported MTU for that card (that is, 2106 bytes). If the maximum supported MTU is exceeded, the following occurs.
An appropriate ICMP reply message (Destination Unreachable) is generated by the 7705 SAR. The router ensures that the ICMP generated message cannot be used as a DOS attack (that is, the router paces the ICMP message).
The appropriate statistics are incremented.
Jumbo frames offer better utilization of an Ethernet link because as more payload is packed into an Ethernet frame of constant size, the ratio of overhead to payload is minimized.
From the traffic management perspective, large payloads may cause long delays, so a balance between link utilization and delay must be found. For example, for ATM VLLs, concatenating a large number of ATM cells when the MTU is set to a very high value could generate a 9-kbyte ATM VLL frame. Transmitting a frame that large would take more than 23 ms on a 3-Mb/s policed Ethernet uplink.