The VPLS service is affected by two MTU values: port MTUs and the VPLS service MTU. The MTU on each physical port defines the largest Layer 2 packet (including all DLC headers) that may be transmitted out a port. The VPLS has a service level MTU that defines the largest packet supported by the service. This MTU does not include the local encapsulation overhead for each port (QinQ, Dot1Q, TopQ, or SDP service delineation fields and headers) but does include the remainder of the packet.
As virtual ports are created in the system, the virtual port cannot become operational unless the configured port MTU minus the virtual port service delineation overhead is greater than or equal to the configured VPLS service MTU. Therefore, an operational virtual port is ensured to support the largest packet traversing the VPLS service. The service delineation overhead on each Layer 2 packet is removed before forwarding into a VPLS service. VPLS services do not support fragmentation and must discard any Layer 2 packet larger than the service MTU after the service delineation overhead is removed.
When an IP interface is associated with a VPLS service, the IP-MTU is based on either the administrative value configured for the IP interface or an operational value derived from VPLS service MTU. The operational IP-MTU cannot be greater than the VPLS service MTU minus 14 bytes.
If the configured (administrative) IP-MTU is configured for a value greater than the normalized IP-MTU, based on the VPLS service-MTU, then the operational IP-MTU is reset to equal the normalized IP-MTU value (VPLS service MTU – 14 bytes).
If the configured (administrative) IP-MTU is configured for a value less than or equal to the normalized IP-MTU, based on the VPLS service-MTU, then the operational IP-MTU is set to equal the configured (administrative) IP-MTU value.