Frame Relay Network

Thre are various Frame relay network topologies exist including Hub and Spoke (star topology), partial mesh and full mesh topologies. Hub and spoke is the most common topology used due to the least number of virtual circuits being used. Router configurations may involve either the use of multipoint or point-to-point sub-interfaces to define the WAN connection topology. The following diagram shows three frame relay topologies: hub and spoke, full-mesh, and partial mesh frame relay topology.

See also Local area network topology for LAN network.

Hub and Spoke Frame Relay Topology

The figure 1 is a hub-and-spoke topology diagram. It’s a dedicated circuit point-to-point network to each branch office with the headquarter office. It is a very cost effective mechanism you can use.

Full Mesh Frame Relay Topology

The figure 2 is a full-mesh topology diagram. Every single unit has a dedicated circuit point-to-point. Every single branch office and central office has a single dedicated circuit. It’s a very expensive deployment of the frame relay. Only critical sites where no single point of failure is acceptable. If one link to the site is failure, they still can link together from other links.

Partial Mesh Frame Relay Topology

Figure 3 is a partial mesh topology diagram, it’s a less expensive than the full mesh topology where only several branch offices have a dedicated circuit to each other. Only critical sites are configured with redundant links.

Frame Relay network sub-interfaces allows multiple virtual circuits to co-exist across a single router interface. This has considerable cost savings in the provisioning of a single interface to support many WAN connections. It also circumvents many connection problems relating to the normal operation of dynamic routing protocols.

Configuration errors can cause numerous network related problems. Many of these problems can be simply overcome by following basic configuration guidelines.

If a Frame Relay network connection is experiencing a regular increase in congestion notification messages (as well as network performance problems), then the bandwidth requirements of the WAN circuit needs to be addressed.

The following is the general task to do in configuring frame relay network.

1. Enable the interface by setting the encapsulation type
2. Assign a network layer address to the interface such as an IP address
3. Configure dynamic (inverse ARP) or static (mapped) address
4. For a point-to-point sub-interface, or a multipoint sub-interface with dynamic addressing, assign a DLCI to the sub-interface
5. Configure the LMI setting (optional)

Note:

By default Cisco routers autosense the LMI type and configure themselves accordingly. You only need to set the LMI type if autosensing does not work or if you want to manually assign it.

The following is the steps in configuring the frame relay network:

1. Enable frame relay and set the encapsulation method:

Router (config-if) # encap frame

2. Assign a network layer address:

Router (config-if) # ip address 10.0.0.1 255.0.0.0

3. Enable inverse ARP (if it has been disabled)

Router (config-if) # frame inverse-arp

Or assign static addressing

Router (config-if) # frame map ip 10.0.0.3 100

4. Configure the LMI type

Router (config-if) # frame lmi-type cisco

Router (config-if) # keepalive 9

Configuring serial sub-interfaces: point-to-point

To complete a point-to-point sub-interface, complete the following steps:

1. Enable frame relay on the interface and set the encapsulation method

Router (config-if) # encap frame

2. Create the subinterface, identifying it as a point-to-point interface

Router (config-if) # int s0.1 point

3. Assign a network layer address

Router (config-subif) # ip address 11.0.0.1 255.0.0.0

4. Configure static or dynamic addressing

Router (config-subif) # frame interface-dlci 100

Note: for point-to-point subinterface, or multipoint subinterface with dynamic addressing, you must also assign a DLCI number to the subinterface.

Configuring serial sub-interface: multipoint

The following steps are configuring frame relay network serial sub-interface multipoint:

1. Enable frame relay network on the interface and set the encapsulation method

Router (config-if) # encap frame

2. Create the subinterface, identifying it as a multipoint interface

Router (config-if) # int s0.100 multipoint

3. Assign a network layer address

Router (config-subif) # ip address 13.0.0.1 255.0.0.0

4. Configure static or dynamic addressing (DLCI to the interface for inverse-arp)

Router (config-subif) # frame interface-dlci 300

Or configure static addressing

Router (config-subif) # frame map ip 13.0.0.3 300

Note: for point-to-point subinterface or multipoint subinterfaces with dynamic addressing, you must also assign a DLCI number to the sub-interface.

Configuring back-to-back frame relay 

The preceding sections described router connect to the existing frame relay network and the router was configured as DTE with the DCE being the frame relay switch at the central office. You can configure the Cisco router as DCE in a back-to-back for testing.

Complete the following steps to configure two routers back-to-back frame relay network:

1. Connect the two routers with the correct cables. Use a DTE cable for one router and a DCE cable at the other router, or use a DTE/DCE crossover cable.
2. In interface mode, set the clock rate for the interface of the router acting as the DCE
3. Complete the following configuration items for each of the connected interface:

• Enable the encapsulation method
• Assign network layer addresses
• Turn-off the keep alive interval
• Map addresses to DLCIs (use the same DLCI for both) or assign DLCI numbers to the interface (use the same DLCI for both)

The following commands configure two routers back-to-back. Router D is configured as the DCE.

RouterD (config) # int s0

RouterD (config-if) # encap frame

RouterD (config-if) # ip address 10.0.0.1 255.0.0.0

RouterD (config-if) # no keepalive

RouterD (config-if) # frame map ip 10.0.0.2 100

RouterD (config-if) # clock rate 56000

RouterE (config) # int s0

RouterE (config-if) # encap frame

RouterE (config-if) # ip address 10.0.0.2 255.0.0.0

RouterE (config-if) # no keepalive

RouterE (config-if) # frame map ip 10.0.0.1 100

You could also substitute the following command for the frame-relay map command for both routers:

Frame-relay interface-dlci 100

Frame relay monitoring

After completion of the configuration, you can use several commands to view the current configuration and operation of frame relay on the router. In addition to the show run command, use the following commands to view configuration information.

If you want to view	Use this
DLCI numbers	Show run Show frame pvc
Frame relay network encapsulation method	Show int Show run
Interface configuration (DCE or DTE)	Show frame pvc Show int
LMI information and traffic statistics	Show frame lmi Show int
Global traffic statistic	Show frame traffic
Addresses and associated DLCIs	Show frame map

See also:

• WAN Technologies
• Best wireless -n routers
• Router connections