corresponds to the MAC address component of the forwarding-table entry for each path. The first ESP path is computed, the B-VLAN identifier (B-VID) 89 is assigned, and the forwarding tables in the intervening Backbone Core Bridges (BCBs) (802.1 bridges) are configured with the mapping {B-VID = 89/MAC = BEB2}. The same process can be employed to create a corouted return path from BEB2 to BEB1. Note that there is no fundamental restriction from the standpoint of configuration that forces the return path to be corouted. However, asymmetric failures in Ethernet are catastrophic for spanning tree algorithms to the point where significant effort has been expended by standards delegates to ensure that failures are bidirectional, and although a configured ESP does not use a spanning tree, its 802.1ah client frequently does (recall that it is MAC-in-MAC). However, the allocation of bandwidth for each direction need not be the same. For the example illustrated, the paths can be seen to cross at a BCB, indicating that it the combination of both the B-VID and MAC that determines a unique forwarding entry. Collisions in either space, such as B-VID 89 or 90 used in combination with another MAC address or as in the example where paths 89/BEB2 and 90/BEB2 cross, are still uniquely resolved to a single egress port. This technique combines both destination-based forwarding and multiplexing, which is sometimes referred to as shared forwarding. When multiple sources are configured to send traffic to the same VLAN/destination MAC-tuple, knowledge of the source is pre- served in the form of the source MAC address, and this is present for both bearer and data Chapter 14 Next Generation Ethernet plane OAM frames. This differs from other "lossy" multipoint-to-point approaches such as merging, where forwarding is based on a fixed length locally significant label and, on merging, visibility of the source is lost. This can be retrieved only by going to a higher layer in the protocol stack. Preserving information regarding the source of a fr