Cisco ASR1002 Configuration Guide - Page 379

Configuring MPLS Layer 2 VPNs Overview of L2VPN Interworking • Connectivity services-An example of this is an enterprise that has different sites running an Internal Gateway Protocol (IGP) that has incompatible procedures on broadcast and non broadcast links. This enterprise has several sites that run an IGP, such as Open Shortest Path First (OSPF) or Intermediate System-to-Intermediate System (IS-IS), between the sites. In this scenario, some of the procedures (such as route advertisement or designated router election) depend on the underlying L2 protocol and are different for a point-to-point ATM connection versus a broadcast Ethernet connection. Therefore, the bridged encapsulation over ATM can be used to achieve homogenous Ethernet connectivity between the CE routers running an IGP. IP or Routed Interworking IP interworking is also called routed interworking. The CE routers encapsulate the IP on the link between the CE router and the PE router. A new VC type is used to signal the IP pseudowire in MPLS. Translation between the L2 and IP encapsulations across the pseudowire is required. Special consideration needs to be given to the address resolution protocol operation and routing protocol operation, because these are handled differently on different L2 encapsulations. The IP interworking mode is used to provide IP connectivity between sites, regardless of the L2 connectivity to these sites. It is different from a Layer 3 VPN because it is point-to-point in nature and the service provider does not maintain any routing information pertaining to customers. Address resolution is encapsulation dependent as specified here: • Ethernet uses Address Resolution Protocol (ARP) • ATM uses inverse ARP • PPP uses IP Control Protocol (IPCP) Therefore, address resolution must be terminated on the PE router. Also, the end-to-end address resolution is not supported. Routing protocols operate differently over broadcast and point-to-point media. For Ethernet, the CE routers must either use static routing or configure the routing protocols to treat the Ethernet side as a point-to-point network. In routed interworking, the IP packets that are extracted from the ACs are sent over the pseudowire. The pseudowire works in the IP Layer 2 transport (VC type 0x000B) like-to-like mode. The interworking function at the network service provider's (NSP) end completes the required adaptation based on the AC technology. The non-IPv4 packets are dropped. In routed interworking, the following considerations must be kept in mind: • ARP, inverse ARP, and IPCP are punted to the routing protocol. Therefore, the PE router at the NSP end must provide the following address-resolution functionalities for the Ethernet and ATM and Frame Relay point-to-point subinterface attachment circuits: - Ethernet-The PE device acts as a Proxy ARP server to all the ARP requests from the CE router. The PE router responds with the MAC address of its local interface. - ATM and Frame Relay point-to-point subinterface-By default, inverse ARP does not run in the point-to-point Frame Relay or ATM subinterfaces. The IP address and subnet mask define the connected prefix; therefore, configuration is not required in the CE devices. • Interworking requires that the MTUs in both the ACs must match for the pseudowire that is to come up. The default MTU in one AC must match the MTU of other AC. Table 1 lists the range of MTUs that can be configured for different ACs. Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide 3