Frame Relay (FR) is a high-performance WAN protocol. It is a packet switching protocol that operates at the physical and data link layers of the OSI reference model. It is cost effective, involves less investment, is reliable and provides guaranteed bandwidth at any given time (CIR - Committed information rate).
Frame Relay is much cheaper than point-to-point leased line. In case of point-to-point leased line we need different physical interface on the router to connect to different branch offices. With frame relay a single interface is sufficient to connect multiple branch offices.
Frame relay is also called a NBMA (non broadcast multi access) protocol which means broadcasts are not forwarded on frame relay network by default. The multi access means the frame relay network will be shared by multiple devices.
Frame relay cloud will contain many or several hundreds of frame relay switches which provide reliable multi paths between two destinations at any given time. The FR cloud is managed by service provider.
FR switch is always configured as DCE which provides clocking to all the routers connected to it. The customer routers are configured as DTE. By default Cisco routers are set to DTE.
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 VC's.
There are two ways to establish frame Relay VC's.
- Permanant virtual circuit (PVC) : is a connection that is available at all times. Preconfigured by the service provider that operates only on data, transfers and idle modes.
- Switched virtual circuits (SVCs) are generally set up on a per-call basis and are disconnected when the call is terminated. Eg: ISDN. It operates in call set up, data transfer, idle and call termination.
These virtual circuits are identified by DLCI numbers (Data link connection identifiers) which are normally assigned by the Frame Relay service provider (ISP).
DLCI is a frame relay layer 2 address which is locally significat to each router only. DLCI is not advertised to each router.
DLCI is a frame relay layer 2 address which is locally significat to each router only. DLCI is not advertised to each router.
LMI ( local management interface ) : It is the heart beat of the frame relay network. LMI messages are sent between the DTE and the DCE which are the keep alives for the frame relay connection every 10 seconds. The frame relay connection drops if keep alives are not received. LMI also indicates the PVC status (ACTIVE/INNACTIVE)
There are three types of LMI. We should always verify the LMI type should match between DTE and DCE for the PVC to be established.
There are three types of LMI. We should always verify the LMI type should match between DTE and DCE for the PVC to be established.
- Cisco, ANSI and Q933
Frame Relay Topologies
- Full Mesh : where every router will have a logical connection to every other router in the frame relay network.
- Partial Mesh : where a single router has logical connection to every other router. This is also called as hub and spoke network. The hub router will have logical connection to all other spokes and spokes need to reach the hub to communication with any other spokes.
There are two types of frame mappings.
- Dynamic : Dynamic address mapping relies on the Frame Relay Inverse Address Resolution Protocol (Inverse ARP). The Frame Relay router sends out Inverse ARP requests on its Frame Relay PVC to discover the protocol address of the remote device connected to the Frame Relay network. The responses to the Inverse ARP requests are used to populate an address-to-DLCI mapping table on the Frame Relay router. On Cisco routers, dynamic Inverse ARP is enabled by default for all network layer protocols enabled on the physical interface.
- Static mapping : With static mapping, we choose to override dynamic Inverse ARP mapping by supplying a manual static mapping for the next hop protocol address to a local DLCI. When static address mapping is used, dynamic Inverse ARP is automatically disabled. Static mapping should be used when the router at the other side of the Frame Relay network does not support dynamic Inverse ARP. On a hub-and-spoke Frame Relay network, static address mapping should be used on the spoke routers to provide spoke-to-spoke reachability. Because the spoke routers do not have direct connectivity with each other and dynamic Inverse ARP would not work between them. Dynamic Inverse ARP only works between hub and spoke.
The optional broadcast keyword at the end of the frame map statement instructs the router to send a copy of any broadcast or multicast packet through the DLCI specified in the map statement. This can be called "pseudo-broadcast", because it isn't really broadcast - just replicated unicast.
Related Topics : Frame Relay Interface Types Frame Relay Switch Conguration
Related Topics : Frame Relay Interface Types Frame Relay Switch Conguration