An Ethernet Pseudowire allows Ethernet packets to transport over MPLS cloud. By using this service customers simply extend their local area networks without loosing the information. The spanning tree will work and end to end connectivity will be on same subnet. Otherwords, we can say it a virtual leased circuit. Customer end to end connectivity is aka as Emulated Service which operates over Pseudowire which further operates over Packet Switched Network (MPLS Network). Entire end to end communication reference model is depicted below.
Modes Of Ethernet Pseudowire
An Ethernet Pseudowire operates in two modes: raw mode and tagged mode. In tagged mode, each frame must have 802.1q tag and that tag is meaningful to the local and end point router. It should be noted that if the VLAN identifier is modified by the egress PE, the Ethernet spanning tree protocol might fail to work properly. If this issue is of significance, the VLAN identifier MUST be selected in such a way that it matches on the attachment circuits at both ends of the PW. It means the identifier or vlan tag should be used same not different. This mode use Pseudowire type 0x0004. Every frame sent on the PW must have a service-delimiting VLAN tag (Different vlans for different customers). If the frame as received by the PE from the attachment circuit does not have a service-delimiting VLAN tag, the PE must prepend the frame with a dummy VLAN tag before sending the frame on the PW.
But in case of raw mode, tag may or may not be added in the frame and is not meaning to the end points. Though the frame is forwarded transparently. This service corresponds to PW type 0x0005. If an Ethernet PW is operating in raw mode, service-delimiting tags are NEVER sent over the PW. If a service-delimiting tag is present when the frame is received from the attachment circuit by the PE, it mUST be stripped from the frame before the frame is sent to the PW.
Why Customer Needs Extended Lan Services
1. In case of Data Center, customers having geographically seperated DC and want to replicate the storage over Fiber Channel IP aka FCIP. FCIP works only on same broadcast domain.
2. Need to run dynamic routing protocol between the sites.