CCNP (Cisco Certified Network Professional) Route Certification Exam Cram Notes

2. Layer2 Technologies

2.2 Explain Frame Relay

2.2.b Frame Relay connection Types

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.

The correct command syntax is:

Router1(config-if)# frame-relay map <protocol> <protocol-address> <dlci> [broadcast][ietf | cisco]

Here, the protocol-address specifies the destination network protocol address.

Example: Router1(config-if)# frame-relay map ip 192.168.36.9 200

The following are true about Multipoint, and point-to-point Frame-Relay configuration at sub-interfaces:

1. In multipoint, all interfaces use same subnet, whereas, in point-to-point each pair requires its own subnet.

2. No IP address is defined at the physical interface, if sub-interfaces are used for frame-relay configuration.

3. Point-to-point sub-interfaces act as point-to-point leased lines.

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1. You must specify either multipoint or point-to-point when configuring the frame-relay interface. There is no default configuration assumed.

2. Point-to-point configuration is good for star and partial mesh topologies, where as multipoint is suitable for full mesh topologies.

Characteristics of Frame-Relay point-to-point subinterface:

Only one DLCI can be configured per point-to-point subinterface.

The command "frame-relay interface-dlci" associates the selected point-to point subinterface with only one DLCI.

Sub-interfaces were originally created to take care of split-horizon issues from distance vector routing protocols over non-broadcast multiple access networks, because split horizon prevents routing updates received on one interface from retransmitting out onto the same interface. This is true even if the routing update is received on one frame relay PVC destined out to another frame relay PVC. By partitioning the frame relay network into numerous point-to point networks using subinterfaces; each new point-to-point subnetwork gets their own network number assigned. Therefore, the routed protocol views each subnetwork as if it was located on a separate interface.

  • Since only one DLCI is assigned per subnet in a logical point to point interface, there is no need for Inverse ARP, since both the DLCI and IP addresses are already known.
  • The IP subnet is mapped across a single virtual circuit, so only one DLCI is mapped per subinterface.
  • Frame-relay map command is needed when multiple virtual circuits are being configured on one physical interface. When logical subinterfaces are used, the "frame-relay interface-dlci" command is used, not this command. In short, "Frame-relay interface-dlci" is used for point-to-point subinterfaces and "frame-relay map" is used for multipoint subinterfaces.

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