The following table describes the default MTU values that are dependent upon the (sub-) port type, mode, and encapsulation.
Port type |
Mode |
Encap type |
Default (bytes) |
|---|---|---|---|
Ethernet |
access |
null |
1514 |
Ethernet |
access |
dot1q |
1518 |
Port mode |
access |
qinq |
1522 |
Fast Ethernet |
network |
— |
1514 |
Other Ethernet |
network |
— |
9212 |
Ethernet |
hybrid |
— |
9212 |
Notes:
The no service-mtu-check command disables service MTU check. Disabling the service MTU check allows packets to pass to the egress if the packet length is less than or equal to the MTU configured on the port. The length of the packet sent from a SAP is limited only by the access port MTU. In case of a pseudowire, the length of the packet is limited by the network port MTU (including the MPLS encapsulation).
In 7210 SAS, length of the SAP tag (or service-delimiting tag, for a packet received over a pseudowire) is included in the computation of the packet length before comparing it with the service-MTU configured for the service. Packet length= Length of IP packet + L2 header + length of SAP tag
For example, if the IP packet received over a dot1q SAP is 1500 and the service-MTU configured is 1514, the service MTU validation check fails as:
Packet length=1500 (Length of IP packet) +14 (L2 header) +4 (length of SAP tag) =1518. The packet is dropped as packet length is greater than the service MTU configured.
See the 7210 SAS release notes for other restrictions with regards to MTU checking and processing on each of the platforms.