Content 5.1 Frame Relay Concepts 5.1.7 Stages of Inverse ARP and LMI operation LMI status messages combined with Inverse ARP messages allow a router to associate network layer and data link layer addresses. When a router that is connected to a Frame Relay network is started, it sends an LMI status inquiry message to the network. The network replies with an LMI status message containing details of every VC configured on the access link. Periodically the router repeats the status inquiry, but subsequent responses include only status changes. After a set number of these abbreviated responses, the network will send a full status message. If the router needs to map the VCs to network layer addresses, it will send an Inverse ARP message on each VC. The Inverse ARP message includes the network layer address of the router, so the remote DTE, or router, can also perform the mapping. The Inverse ARP reply allows the router to make the necessary mapping entries in its address to DLCI map table. If several network layer protocols are supported on the link, Inverse ARP messages will be sent for each.
Content 5.2 Configuring Frame Relay 5.2.1 Configuring basic Frame Relay This section explains how to configure a basic Frame Relay PVC. Frame Relay is configured on a serial interface and the default encapsulation type is the Cisco proprietary version of HDLC. To change the encapsulation to Frame Relay use the encapsulation frame-relay [cisco | ietf] command. cisco Uses the Cisco proprietary Frame Relay encapsulation. Use this option if connecting to another Cisco router. Many non-Cisco devices also support this encapsulation type. This is the default. ietf Sets the encapsulation method to comply with the Internet Engineering Task Force (IETF) standard RFC 1490. Select this if connecting to a non-Cisco router. Cisco’s proprietary Frame Relay encapsulation uses a 4-byte header, with 2 bytes to identify the data-link connection identifier (DLCI) and 2 bytes to identify the packet type.Set an IP address on the interface using the ip address command. Set the bandwidth of the serial interface using the bandwidth command. Bandwidth is specified in kilobits per second (kbps). This command is used to notify the routing protocol that bandwidth is statically configured on the link. The bandwidth value is used by Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF) to determine the metric of the link. This command also affects bandwidth utilization statistics that can be found by using the show interfaces command. The LMI connection is established and configured by the frame-relay lmi-type [ansi | cisco | q933a] command. This command is only needed if using Cisco IOS Release 11.1 or earlier. With IOS Release 11.2 or later, the LMI-type is autosensed and no configuration is needed. The default LMI type is cisco. The LMI type is set on a per-interface basis and is shown in the output of the show interfaces command.These configuration steps are the same, regardless of the network layer protocols operating across the network. Lab Activity Lab Exercise: Configuring Frame Relay In this lab, the student will configure a router to make a successful connection to a local Frame Relay switch. Lab Activity e-Lab Activity: Configuring Frame Relay In this activity, a router will be configured to make a successful connection to a local Frame Relay switch. Web Links Configuring Basic Frame Relay http://www.cisco.com/en/US/tech/ tk713/tk237/technologies_configuration_ example09186a008009421c.shtml
Content 5.2 Configuring Frame Relay 5.2.2 Configuring a static Frame Relay map The local DLCI must be statically mapped to the network layer address of the remote router when the remote router does not support Inverse ARP. This is also true when broadcast traffic and multicast traffic over the PVC must be controlled. These static Frame Relay map entries are referred to as static maps. Use the frame-relay map protocol protocol-address dlci [broadcast] command to statically map the remote network layer address to the local DLCI. Lab Activity Lab Exercise: Configuring Frame Relay PVC In this lab, the student will configure two routers back-to-back as a Frame Relay permanent virtual circuit (PVC). Lab Activity e-Lab Activity: show frame-relay map In this activity, the student will demonstrate how to use the show frame-relay map command to display the current map entries and information about the connections. Web Links Configuring Dynamic and Static Mapping for Multipoint Subinterfaces http://www.cisco.com/en/US/tech/ tk713/tk237/technologies_configuration_ example09186a008009421f.shtml
Content 5.2 Configuring Frame Relay 5.2.3 Reachability issues with routing updates in NBMA By default, a Frame Relay network provides non-broadcast multi-access (NBMA) connectivity between remote sites. An NBMA environment is viewed like other multiaccess media environments, such as Ethernet, where all the routers are on the same subnet. However, to reduce cost, NBMA clouds are usually built in a hub-and-spoke topology. With a hub-and-spoke topology, the physical topology does not provide the multi-access capabilities that Ethernet does. The physical topology consists of multiple PVCs. A Frame Relay NBMA topology may cause two problems: Split-horizon updates reduce routing loops by not allowing a routing update received on one interface to be forwarded out the same interface. If Router B, a spoke router, sends a broadcast routing update to Router A, the hub router, and Router A has multiple PVCs over a single physical interface, then Router A cannot forward that routing update through the same physical interface to other remote spoke routers. If split-horizon is disabled, then the routing update can be forwarded out the same physical interface from which it came. Split-horizon is not a problem when there is a single PVC on a physical interface. This would be a point-to-point Frame Relay connection. Routers that support multiple connections over a single physical interface have many PVCs that terminate in a single router. This router must replicate broadcast packets such as routing update broadcasts, on each PVC, to the remote routers. The replicated broadcast packets can consume bandwidth and cause significant latency to user traffic. It might seem logical to turn off split-horizon to resolve the reachability issues caused by split-horizon. However, not all network layer protocols allow split-horizon to be disabled and disabling split-horizon increases the chances of routing loops in any network. One way to solve the split-horizon problem is to use a fully meshed topology. However, this will increase the cost because more PVCs are required. The preferred solution is to use subinterfaces. Lab Activity e-Lab Activity: Configuring Frame Relay Subinterfaces In this activity, the student will configure three routers in a full-mesh Frame Relay network. Web Links Problems with Running OSPF in NBMA Mode over Frame Relay http://www.cisco.com/en/US/tech/ tk826/tk365/technologies_tech_ note09186a0080094051.shtml
Content 5.2 Configuring Frame Relay 5.2.4 Frame Relay subinterfaces To enable the forwarding of broadcast routing updates in a hub-and-spoke Frame Relay topology, configure the hub router with logically assigned interfaces. These interfaces are called subinterfaces. Subinterfaces are logical subdivisions of a physical interface. In split-horizon routing environments, routing updates received on one subinterface can be sent out another subinterface. In a subinterface configuration, each virtual circuit can be