Frame Relay IP Routing
Page 29-15

Frame Relay IP Routing

Frame Relay routing is different than standard LAN IP Routing. In normal LAN IP Routing MAC
addresses are used as source and destination addresses. In Frame Relay IP Routing, no MAC
addresses are included in a routed frame. In fact, the only address in a routed Frame Relay
frame is the DLCI, or virtual circuit identifier. The DLCI is the main indentifier for source and
destination addresses.
Because Frame Relay uses 10-bit DLCIs as the main addressing units, routed Frame Relay
frames require less overhead than LAN IP frames, which use LAN standard 48-bit addresses.
However, due to the nature of DLCIs on a WAN, Frame Relay routing requires a special
version of the IP protocol. The DLCI for a single VC may or may not be different on both
sides of a Frame Relay connection. That’s why Frame Relay uses the Inverse Address Resolu-
tion Protocol (InARP) to resolve DLCI issues and to automatically learn the IP addresses of
remote routers.
The InARP protocol ensures that before any data passes between two Frame Relay routers,
those routers notify each other of their IP addresses and associated DLCIs. So, the first
communication over a routed Frame Relay network is normally initiated by InARP.
Frame Relay InARP Protocol
R
WSX WSX sends Router a message inform-
ing of its IP address and DLCI (“IP
111.22.33.44 on DLCI 24”).
Router returns a message to WSX
with its IP address and DLCI (“IP
222.33.44.55 on DLCI 32”).
➊
➋
DLCI 24
R
WSX
DLCI 32
Frame Relay
Network