Map between WAN Interface IP Address and Link Layer Protocol Address 325
cannot receive the arp message, then possibly the error is on the Ethernet physical
layer.
Fault 2: When the interface is encapsulated with PPP or Frame Relay, the
link layer protocol status does not change to UP.
Troubleshooting: check whether the IP address of this interface is in the same
network segment as the opposite side.
Fault 3: After the interface borrows an IP address, the link layer protocol
status will turn to UP, but it can not ping through itself, and other ports
can not ping through this borrowed IP address either.
Troubleshooting: Check whether the lender port is UP. Only when the port
protocol of the lender is UP, will the address be added to the route table and
pinged through by other ports.
Map between WAN Interface IP Address and Link Layer Protocol Address In a router, you shall maintain both the mapping from an Ethernet interface IP
address to an MAC address, and that from a WAN interface IP address to a link
layer protocol address. Namely there are the following types:
■On a dialup interface (such as an asynchronous serial port or ISDN interface),
mapping between IP address and dialing serial port is maintained by the
command dialer route ip.
■On an interface encapsulated with X.25, the mapping between an IP address
and X.121 address is maintained by the command x25 map ip.
■On an interface encapsulated with Frame Relay, mapping between an IP
address and a virtual circuit number (DLCI) is maintained by the command fr
map ip.
The above mapping tables are also called second routing tables, which are
essential for the normal working of the router. For details, refer to related chapters
in Link Layer Protocol.