Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 320

Any Transport over MPLS Chapter 21 Configuring Multiprotocol Label Switching These sections describe AToM: • AToM Load Balancing, page 21-14 • Understanding EoMPLS, page 21-14 • EoMPLS Guidelines and Restrictions, page 21-14 • Configuring EoMPLS, page 21-16 AToM Load Balancing EoMPLS does not support load balancing at the tunnel ingress; only one Interior Gateway Protocol (IGP) path is selected even if multiple IGP paths are available, but load balancing is available at the MPLS core. Understanding EoMPLS EoMPLS is one of the AToM transport types. AToM transports Layer 2 packets over a MPLS backbone using a directed LDP session between edge routers for setting up and maintaining connections. Forwarding occurs through the use of two level labels that provide switching between the edge routers. The external label (tunnel label) routes the packet over the MPLS backbone to the egress PE at the ingress PE. The VC label is a demuxing label that determines the connection at the tunnel endpoint (the particular egress interface on the egress PE as well as the VLAN identifier for an Ethernet frame). EoMPLS works by encapsulating Ethernet PDUs in MPLS packets and forwarding them across the MPLS network. Each PDU is transported as a single packet. Note Use OSM-based EoMPLS when you want local Layer 2 switching and EoMPLS on the same VLAN. You need to configure EoMPLS on the SVI; the core-facing card must be an OSM. When local Layer 2 switching is not required, use PFC3B-based EoMPLS configured on the subinterface or physical interface. EoMPLS Guidelines and Restrictions When configuring EoMPLS, follow these guidelines and restrictions: • Ensure that the maximum transmission unit (MTU) of all intermediate links between endpoints is sufficient to carry the largest Layer 2 packet received. • EoMPLS supports VLAN packets that conform to the IEEE 802.1Q standard. The 802.1Q specification establishes a standard method for inserting VLAN membership information into Ethernet frames. • If QoS is disabled globally, both the 802.1p and IP precedence bits are preserved. When the QoS is enabled on a Layer 2 port, either 802.1q P bits or IP precedence bits can be preserved with the trusted configuration. However, by default the unpreserved bits are overwritten by the value of preserved bits. For instance, if you preserve the P bits, the IP precedence bits are overwritten with the value of the P bits. The PFC3B provides a new command that allows you to trust the P bits while preserving the IP precedence bits. To preserve the IP precedence bits, use the no mls qos rewrite ip dscp command. 21-14 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY OL-11439-03