ICND2 – Frame Relay
If you are not sure about Frame Relay, please read my Frame Relay tutorial.
SPLIT HORIZON: A router never sends information about a route back in same direction which is original information came, routers keep track of where the information about a route came from. Means when router A sends update to router B about any failure network, router B does not send any update for same network to router A in same direction.
Therefore in order to resolve split-horizon issue, we can create a full-mesh topology (a network topology in which there is a direct link between all pairs of nodes) so that all the routers can learn all the routes advertised by the neighbors -> B is correct.
Configuring Point-to-point subinterfaces is a good way to resolve the split-horizon issue because each subinterface is treated as a separate interface so an interface can send and receive information about a route -> D is correct.
Cisco supports two Frame Relay encapsulation types: the Cisco encapsulation and the IETF Frame Relay encapsulation, which is in conformance with RFC 1490 and RFC 2427. The former is often used to connect two Cisco routers while the latter is used to connect a Cisco router to a non-Cisco router. You can test with your Cisco router when typing the command Router(config-if)#encapsulation frame-relay ? on a WAN link. Below is the output of this command (notice Cisco is the default encapsulation so it is not listed here, just press Enter to use it).
Note: Three LMI options are supported by Cisco routers are ansi, Cisco, and Q933a. They represent the ANSI Annex D, Cisco, and ITU Q933-A (Annex A) LMI types, respectively.
HDLC is a WAN protocol same as Frame-Relay and PPP so it is not a Frame Relay encapsulation type.
This command is described in detail at http://www.9tut.com/frame-relay-tutorial/2. If you don’t get this question please read it.
When configuring on a point-to-point subinterface, the command frame-relay interface-dlci associates the selected point-to-point subinterface with a DLCI. But remember that the DLCI number in this command is the local DLCI. An example of using this command is shown below:
|R1(config)#interface Serial0/0.1 point-to-point
R1(config-subif)#ip address 192.168.1.1 255.255.255.252
R1(config-subif)#frame-relay interface-dlci 1
An example of the output of “show frame-relay map” command is shown below:
We can see the IP address 172.16.3.1 is associated with the DLCI 100.
Committed information rate (CIR): The minimum guaranteed data transfer rate agreed to by the Frame Relay switch. Frames that are sent in excess of the CIR are marked as discard eligible (DE) which means they can be dropped if the congestion occurs within the Frame Relay network.
Note: In the Frame Relay frame format, there is a bit called Discard eligible (DE) bit that is used to identify frames that are first to be dropped when the CIR is exceeded.
A main advantage of configuring Frame Relay multipoint compared to point-to-point subinterfaces is we can assign IP addresses on the same subnets/networks to the interfaces of Frame Relay switch, thus saving the subnets/networks you have.
The “show frame-relay map” command displays the current map entries and information about the connections, including encapsulation type.
You can check Table 33 in the following link: http://www.cisco.com/en/US/docs/ios/12_2/wan/command/reference/wrffr4.html#wp1029343
It clearly states there is a Field which can be Cisco or IETF, which “indicates the encapsulation type for this map”. We quote that Table 33 here for your quick reference (you will see what we want to imply in bold):
|Serial 1 (administratively down)||Identifies a Frame Relay interface and its status (up or down).|
|ip 188.8.131.52||Destination IP address.|
|dlci 177 (0xB1,0x2C10)||
DLCI that identifies the logical connection being used to reach this interface. This value is displayed in three ways: its decimal value (177), its hexadecimal value (0xB1), and its value as it would appear on the wire (0x2C10).
|static||Indicates whether this is a static or dynamic entry.|
|CISCO||Indicates the encapsulation type for this map; either CISCO or IETF.|
|TCP/IP Header Compression (inherited), passive (inherited)||Indicates whether the TCP/IP header compression characteristics were inherited from the interface or were explicitly configured for the IP map.|
The “show frame-relay lmi” gives us information about the LMI encapsulation type used by the Frame Relay interface, which can be ANSI, CISCO or Q933a. Therefore it is not what the question requires (CISCO or IETF).