EoMPLS- Ethernet PW Modes

EoMPLS- Ethernet PW Modes

Definition: An Ethernet PseudoWire (PW) emulates a single Ethernet link between two endpoints (PE devices). An Ethernet PW allows Ethernet PDUs to be carried across MPLS network.

An Ethernet PW operates in one of two modes- Tagged mode or Untagged (or raw) mode

In Tagged mode, the frame must have atleast one 802.1Q/VLAN tag, and that VLAN tag has some meaning for PW edge devices (PE devices) i.e. these PE devices will have some agreement on how to process this tag.

In Untagged mode, the frame may have an 802.1Q tag, but it can pass transparently through the PE devices.

When the PE router receives an Ethernet frame with a VLAN tag, it can be one of two cases-

1) The tag is service-delimiting: This means that the tag was inserted by the SP device. For example- LANs from different customers might be connected to the same SP switch, and that switch seperates customer traffic by inserting VLAN tags and then forwards the frames to the PE device.

2) The tag is not service-delimiting: This means that the tag was inserted in the frame by a CE device and is not meaningful to the PE device.

If the Ethernet PW is running in Untagged mode at the PE device, and the PE device receives a frame with service-delimiting tag on its attachment circuit, the tag must be stripped before forwarding. The tag is never sent over the Ethernet PW.

If the Ethernet PW is running in Tagged mode at the PE device, the PE device requires to receive the frame with a service-delimiting tag. If no tag is present, the PE device prepends a dummy tag in the frame before forwarding the frame over to Ethernet PW.

In both modes, the non service-delimiting tags are sent transparently over Ethernet PW.

When the PE device receives a frame on Ethernet PW with a service-delimiting tag, it may or may not rewrite the tag in Tagged mode before forwarding to the attachment circuit. In raw mode, the tag must not be rewritten or removed before sending the frame over the attachment circuit.