Correct Answer: D
Explanation/Reference:
Explanation:
Frame-Relay (a Layer 2 protocol) uses Inverse-Arp to map know Layer 2 Address (DLCI) to unknown
Layer 3 Address.
Dynamic Mapping
Dynamic address mapping relies on the Frame Relay Inverse Address Resolution Protocol (Inverse ARP),
defined by RFC 1293, to resolve a next hop network protocol address to a local DLCI value. The Frame
Relay router sends out Inverse ARP requests on its Frame Relay PVC to discover the protocol address of
the remote device connected to the Frame Relay network. The responses to the Inverse ARP requests are
used to populate an address-to-DLCI mapping table on the Frame Relay router or access server. The
router builds and maintains this address-to-DLCImapping table, which contains all resolved Inverse ARP
requests, including both dynamic and static mapping entries.
When data needs to be transmitted to a remote destination address, the router performs a lookup on its
routing table to determine whether a route to that destination address exists and the next hop address or
directly connected interface to use in order to reach that destination. Subsequently, the router consults its
address-to-DLCI mapping table for the local DLCI that corresponds to the next hop address. Finally, the
router places the frames targeted to the remote destination on its identified outgoing local DLCI. On Cisco
routers, dynamic Inverse ARP is enabled by default for all network layer protocols enabled on the physical
interface. Packets are not sent out for network layer protocols that are not enabled on the physical
interface. For example, no dynamic Inverse ARP resolution is performed for IPX if ipx routing is not
enabled globally and there is no active IPX address assigned to the interface. Because dynamic Inverse
ARP is enabled by default, no additional Cisco IOS command is required to enable it on an interface.
Example 4-16 shows the output of the show frame-relay map privileged EXEC mode command.
The address to-DLCI mapping table displays useful information. The output of the command shows that
the next hop address 172.16.1.2 is dynamically mapped to the local DLCI 102, broadcast is enabled on the
interface, and the interface's status is currently active.
NOTE
After enabling Frame Relay on the interface, the Cisco router does not perform Inverse ARP until IP
routing is enabled on the router. By default, IP routing is enabled on a Cisco router. If IP routing has been
turned off, enable IP routing with the ip routing command in the global configuration mode. After IP routing
is enabled, the router performs Inverse ARP and begins populating the address-to-DLCI mapping table
with resolved entries.
Reference:
http://www.ciscopress.com/articles/article.asp?p=170741&seqNum=4