Cisco routers support all network layer protocols such as IPv4, IPv6, IPX, and AppleTalk over Frame Relay. For transmitting data over Frame Relay, it needs to know which local DLCI maps to the Layer 3 address of the remote destination. In simple words Frame Relay address mapping matches a next-hop IP address that sits on the Frame Relay network with the local DLCI used to send frames to that next-hop device. It is working just like ARP on a LAN. This address-to-DLCI mapping can be accomplished both by static or dynamic mapping. Before discussing the Frame Relay mapping, we will discuss the Inverse ARP.
The Inverse Address Resolution Protocol (ARP) is the primary tool for Frame Relay. Inverse ARP is the opposite of the ARP. ARP translates Layer 3 addresses to Layer 2 addresses. But Inverse ARP get Layer 3 addresses of other stations from Layer 2 addresses, such as the DLCI in Frame Relay networks. Inverse ARP is usually used in Frame Relay and ATM networks.
Frame Relay for IPv6 uses Inverse Neighbor Discovery to get a Layer 3 IPv6 address from a Layer 2 DLCI. A Frame Relay router sends an Inverse Neighbor Discovery Solicitation message to request a Layer 3 IPv6 address corresponding to a Layer 2 DLCI address of the remote Frame Relay router. At the similar time the Inverse Neighbor Discovery Solicitation message provides the sender’s Layer 2 DLCI address to the remote Frame Relay router.
Dynamic Frame Relay Address Mapping
Frame Relay Dynamic address mapping depends on Inverse ARP to determine a next-hop IPv4 address to a local DLCI value. The Frame Relay router sends out Inverse ARP requests on its PVC to find out the address of the far-end device connected to the Frame Relay network. The router uses the responses to populate an address-to-DLCI mapping table on the Frame Relay router. The Cisco Routers enabled Inverse ARP by default, no additional command is required to configure dynamic Frame Relay mapping on an interface.
The router make and maintains this mapping table, which included all determined Inverse ARP requests with both dynamic and static mapping entries. Cisco routers have enabled Inverse ARP, by default, for all protocols on the physical interface. Inverse ARP packets are not sent out for protocols that are not enabled on the interface.
Static Frame Relay Address Mapping
A network administrator can compute a manual static mapping for the next hop address and can override dynamic Inverse ARP mapping by supplying a manual static map address to a local DLCI. A static Frame Relay mapping works similar to dynamic Inverse ARP by associating a particular next-hop protocol address to a local Frame Relay DLCI. We cannot use Inverse ARP and a map statement for the same DLCI and protocol.
If the router of the remote side does not support dynamic Inverse ARP for a specific network, then the network administrator can use static Frame Relay mapping. Static Frame Relay mapping will complete the remote network layer address to local DLCI resolution in the above example.
Another use of static Frame Relay mapping is a hub-and-spoke Frame Relay network. Network administrator uses static Frame Relay address mapping on the spoke routers to provide spoke-to-spoke reach ability.
We earlier discussed the Hub and Spoke, and we know that the spoke routers do not have direct connectivity with each other so, dynamic Inverse ARP would not work between them. Dynamic Inverse ARP work if there are direct point-to-point connections available between two ends. In the Hub and Spok network, the dynamic Frame Relay mapping will only work between the Hub and Spoke, and the spokes require static mapping to provide connectivity to each other. In the next article we will discuss the configuration of static Frame Relay mapping.