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Chapter 10

Wide Area Networking Protocols

new LAN frame to be delivered to the destination host. The frame on
the LAN will have the final destination hardware address in the
header. This was found in the router's ARP cache, or an ARP broad-
cast was performed. Whew!

The user and server do not need to know, nor should they know, everything

that happens as the frame makes its way across the Frame Relay network. The
remote server should be as easy to use as a locally connected resource.

Frame Relay Encapsulation

When configuring Frame Relay on Cisco routers, you need to specify it as an
encapsulation on serial interfaces. There are only two encapsulation types:
Cisco and IETF (Internet Engineering Task Force). The following router out-
put shows the two different encapsulation methods when choosing Frame
Relay on your Cisco router:

RouterA(config)#int s0

RouterA(config-if)#encapsulation frame-relay ?

ietf Use RFC1490 encapsulation

<cr>

The default encapsulation is Cisco unless you manually type in IETF, and

Cisco is the type used when connecting two Cisco devices. You'd opt for the
IETF-type encapsulation if you needed to connect a Cisco device to a non-
Cisco device with Frame Relay. So before choosing an encapsulation type,
check with your ISP and find out which one they use. (If they don't know,
hook up with a different ISP!)

Data Link Connection Identifiers (DLCIs)

Frame Relay virtual circuits (PVCs) are identified by DLCIs. A Frame Relay
service provider, such as the telephone company, typically assigns DLCI val-
ues, which are used by Frame Relay to distinguish between different virtual
circuits on the network. Because many virtual circuits can be terminated on
one multipoint Frame Relay interface, many DLCIs are often affiliated with it.

For the IP devices at each end of a virtual circuit to communicate, their

IP addresses need to be mapped to DLCIs. This mapping can function as a
multipoint device--one that can identify to the Frame Relay network the
appropriate destination virtual circuit for each packet that is sent over the
single physical interface. The mappings can be done dynamically through
IARP or manually through the Frame Relay map command.

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