Frame Relay Address Mapping, How it works
DLCI ( Data Link Connection Identifier )
In Frame Relay, the end of each connection has a number to be identified with; this number is called Data Link Connection Identifier (DLCI).
For a Cisco router to transmit data over Frame Relay, it needs to know which local DLCI maps to the Layer 3 address of the remote destination. Cisco routers support all network layer protocols over Frame Relay, such as IP, IPX, and AppleTalk. This address-to-DLCI mapping can be accomplished either by static or dynamic mapping.
Any network location can connect with any other simply by stating the address of that location and DLCI number of the line it needs to use. All the data from all the configured DLCIs flows through the same port of the router.
Frame Relay Dynamic Address Mapping.
Frame Relay dynamic address mapping relies on Inverse ARP to resolve a next hop network protocol address to a local DLCI value. The Frame Relay router sends out Inverse ARP requests on its PVC to discover the protocol address of the remote 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 or access server. The router builds and maintains this mapping table, which contains all resolved Inverse ARP requests, including both dynamic and static mapping entries.
Static Address Mapping.
A network administrator can compute a manual static mapping for the next hop protocol address to a local DLCI. A static map works similarly to dynamic Inverse ARP by associating a specified next hop protocol address to a local Frame Relay DLCI. You cannot use Inverse ARP and a map statement for the same DLCI and protocol. A static frame Relay map can also be used in a situation in which the router at the other side of the Frame Relay network does not support dynamic Inverse ARP for a specific network protocol.
The Inverse Address Resolution Protocol (ARP) obtains Layer 3 addresses of other stations from Layer 2 addresses, such as the DLCI in Frame Relay networks. It is primarily used in Frame Relay and ATM networks, where Layer 2 addresses of VCs are sometimes obtained from Layer 2 signalling, and the corresponding Layer 3 addresses must be available before these VCs can be used. In essence, ARP translates Layer 3 addresses to Layer 2 addresses, Inverse ARP does the opposite.
Local Management Interface (LMI).
LMI is a keepalive mechanism that provides status information about Frame Relay connections between the router (DTE) and the Frame Relay switch (DCE). Cisco routers support three LMI types: Cisco, ANSI, and Q933-A.By default LMI type for Cisco routers is Cisco.
Every 10 seconds or more, the end device requests a dumb sequenced response or link status information. If the network does not respond with the requested information, the user device may consider the connection to be down. When the network responds with a FULL STATUS response, it includes status information about DLCIs that are allocated to that line. The end device can use this information to determine whether the logical connections are able to traffic data.
The connection between DTEs is called a Virtual Circuit-VC. It’s called virtual because there is no direct electrical connection from end to end but logical.
There are two ways to establish frame Relay VCs:
i. Switched Virtual Circuits (SVC): dynamic connection by sending signalling messages to the network, it operates in call set up, data transfer, idle and call termination.
ii. Permanent Virtual Circuits (PVC): connection preconfigured by the carrier that operates only on data, transfers and idle modes
VCs provide a two-way communication path from one device to another. VCs are identified by DLCI values, which are normally assigned by the Frame Relay service provider (ISP). A DLCI identifies a VC to the equipment at remote end.
A DLCI value has no significance beyond the single link, this means DLCIs is locally significant to the physical link or channel which they reside. Two devices connected by a VC may use a different DLCI value to refer to the same connection.
As frames move from one network to another, Frame Relay labels each VC with a DLCI. The DLCI is stored in the address field of every frame transmitted to inform the network how the frame should be routed.
The Frame Relay service provider or ISP assigns DLCI numbers or values. Usually, DLCIs 0 to 15 and 1008 to 1023 are reserved for special purposes. However, service providers or ISPs basically assign DLCIs in the range of 16 to 1007.
In the figure above, there is a VC between the sending and receiving networks. The VC follows the path A, B, C, and D. Frame Relay creates a VC by storing input-port to output-port mapping in the memory of each Frame Relay switch and thus links one switch to another until a continuous path from one end of the circuit to the other is identified. A VC can pass through any number of intermediate devices (switches) located within the Frame Relay network.
Frame Relay Encapsulation Options
The default encapsulation type on a serial interface on a Cisco router is the Cisco proprietary version of HDLC. This can be changed from encapsulation HDLC to Frame Relay, use the encapsulation frame-relay cisco or ietf command. Cisco routers use either Cisco or ietf as default and if you are connecting non-Cisco devices use the ietf. The IETF encapsulation type complies with RFC 1490 and RFC 2427.
Frame Relay Broadcast Keyword
Frame Relay, ATM, and X.25 are non-broadcast multiple access (NBMA) networks. NBMA networks allow only data transfer from one computer to another over a VC or across a switching device. NBMA networks do not support multicast or broadcast traffic, so a single packet cannot reach all destinations. This requires you to broadcast to replicate the packets manually to all destinations especially when running routing protocols like RIP, EIGRP andOSPF.