DLCIto Address Mappings

To pass data over the Frame Relay circuit, you must associate each local DLCI with a destination address. This association, or mapping, tells the router which DLCI to use when packets are destined for the remote address. For example, referring to the figure, an administrator would map the IP address of the destination Frame Relay interface (10.1.1.1) to DLCI 500, which is the PVC to that remote router. Any routes that point to 10.1.1.1 as the next-hop IP address will use this mapping that the PVC identified as DLCI 500, and forward packets to the remote site.

On Cisco routers, the address mapping can be either configured manually or dynamically assigned. With dynamic address mapping, Frame Relay Inverse Address Resolution Protocol (Inverse ARP) is used to dynamically discover the protocol address of the remote device associated with a given PVC. During initial link establishment, the router sends an Inverse ARP packet out each active DLCI and requests the next-hop protocol addresses from the device at the other end of the connection. The remote device responds with the protocol addresses associated with that PVC. The router then updates its mapping table and uses the information to forward packets on the correct route.

When packets are sent across the network, the intermediate switches look up the DLCI in the map table and perform the following

■ If the DLCI is defined on the link, the switch forwards packets toward their destination.

■ If the DLCI is not defined on the link, the switch discards the frame.

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