CCNA study notes on Frame-Relay

Routed protocol - Frame-Relay

1. The following are true about Frame Relay:

2. Frame Relay supports two types of virtual circuits (VCs):

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:

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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