1. The following are true about Frame Relay:
Frame Relay is purely, a Layer 2 standard.
Frame Relay DLCIs have local significance.
Cisco supports three types of LMIs (Link Management Interface): cisco, ansi, and q933a
Cisco supports two types of Frame Relay encapsulation: cisco, and ietf. When you are connecting a Cisco router with a non-Cisco router, use ietf as the encapsulation method.
2. Frame Relay supports two types of virtual circuits (VCs):
Permanent Virtual Circuits (PVCs): these are permanently established connections that are used for frequent and consistent data transfers between DTEs across a Frame Relay cloud.
Switched Virtual Circuits (SVCs): these are temporary connections used in situations requiring only occasional data transfers between DTEs across Frame Relay cloud.
The terms "Call Setup", "Data Transfer", "Idle", and "Call Termination" are associated with SVCs. Frame Relay SVCs are not widely supported by manufacturers.
3. Frame-Relay support point-point and multipoint connection types. In point-to-point connection type, a single sub interface establishes a PVC connection to another physical interface or sub-interface. In multipoint connection type, a single sub-interface is used to establish multiple PVC connections to several physical interfaces or sub-interfaces. In multipoint Frame-Relay network, split horizon rule is applicable to broadcast traffic.
Another important thing to note when configuring Frame-Relay using sub-interfaces: The physical interface on which sub-interfaces are configured would not be assigned any IP address. Even if one is assigned, it should be removed prior to configuring Frame-Relay. Note that if an IP address is assigned to a physical interface, the sub-interfaces defined within the physical interface will not receive any frames.
4. When the sub-interfaces on a serial interface are to be configured for Frame Relay, each sub interface needs to be assigned individual DLCI.
The following command assigns a dlci of 100 to any sub-interface:
R(config-if)#frame-relay interface-dlci 100
Note that prior to issuing the above command; issue the following command to get into proper sub interface configuration mode:
R(config)#interface serial0.1 point-to-point
5. Given below are salient features of Frame Relay DLCIs:
DLCIs (Data Link Connection Identifier) have only local significance. It means, the end devices over FR network can have different DLCI numbers.
DLCI number is provided by the FR service provider. DLCI number is mapped to Layer 3 protocol address using 'frame-relay map' statement.
DLCI numbers must be unique on a router.
6. In Frame Relay NBMA networks, if no sub-interfaces are defined, then the routers will not be able to communicate routing information due to split horizon rule.
Split horizon is a method of preventing a routing loop in a network. The basic principle is simple: Information about the routing for a particular packet is never sent back in the direction from which it was received.
To overcome the split horizon, sub-interfaces can be configured on NBMA networks. A sub interface is a logical way of defining an interface. The same physical interface can be split into multiple logical interfaces, with each sub interface being defined as point-to-point.
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