There needs to be a way to link the layer 2 identifiers (DLCI) to layer 3 (Network layer) addresses.
Mapping
provides a mechanism to link one or more network addresses to a DLCI. Remember that
Frame Relay works only at the Data Link layer (layer 2 of the OSI model) and does not understand
IP addressing. In fact, to communicate via IP (because it could just as easily be IPX and AppleTalk
instead of IP), you need to convert the destination IP address to a destination DLCI (PVC) number.
The frame switch uses only DLCI numbers to communicate, not IP addresses.
Mappings can be done either statically by an administrator or dynamically via the router. If
you are mapping a static Network layer address to a DLCI number, you use the
frame-relay
map
command. It is necessary to create static mappings when the remote router does not support
dynamic addressing or when you’re using OSPF in some network configurations. It’s also necessary
even if you want to control broadcasts over your Frame Relay network.
To understand how to use static mappings, look at Figure 29.3. Figure 29.3 shows a corporate
office in Chicago connected to two other sites—one in Miami and one in New York. The
IP address of the serial interface in New York is 172.16.1.2/24, and the IP address of the Miami
serial interface is 172.16.1.3/24. It’s important to note that the Miami location is a Cisco router,
and the New York location is a non-Cisco router. A static mapping would have to be used for
different Frame Relay encapsulation methods to run under the same physical serial interface,
unless all routers used an open encapsulation type for interoperability, resulting in the ability to
use dynamic mapping.
FIGURE 2 9 . 3
Configuring Frame Relay static mappings
The following router output shows an example of how you would create static Frame Relay
mappings on the Chicago router:
Router(config)#
interface serial 0
Router(config-if)#
ip address 172.16.1.1 255.255.255.248
Router(config-if)#
frame-relay map ip 172.16.1.2 20 broadcast ietf
Router(config-if)#
frame-relay map ip 172.16.1.3 16 broadcast
Router(config-if)#
exit
The
frame-relay map
command maps the IP address of the remote location to a specific
PVC or DLCI. The first map statement tells the Chicago router that if it has an IP packet with
Chicago
Miami
Cisco router
New York
Non-Cisco router
PVC
PVC
DLCI to NY = 20
DLCI to Miami = 16
Corporate Office
172.16.1.2/24
a destination IP address of 172.16.1.2, it should use PVC 20 to get there. Also, because the New
York office is not a Cisco router (can you imagine that?), it should use the standard Internet
Engineering Task Force (IETF) encapsulation method. We’ll talk about encapsulation methods
used with Frame Relay in a minute.
Because Miami is a Cisco router, no specification of encapsulation is necessary because Cisco is
the default encapsulation method. The broadcast parameter at the end of each line specifies that
broadcasts should be forwarded over the PVC because they are not forwarded by default. The
frame-relay map
command supports many Network layer protocols, including IP, Connectionless
Network Services (CLNS), Digital Equipment Corporation’s Networking architecture (DECnet),
Xerox Network Services (XNS), and Virtual Integrated Network Service (VINES).
Dynamic addressing is turned on by default. It automatically maps Network layer addresses
to DLCI addresses rather well
.
Inverse ARP (IARP)
is used to automatically map a DLCI to a
network address (IP, IPX, and so on) without any user configuration. It provides Network
layer-to-DCLI-number translation and creates an entry in the DLCI mapping table. This table
is used by the router to correctly route outgoing traffic. No map configuration is necessary for
IARP to work.