R3303-HP HSR6800 Routers Layer 2 - WAN Configuration Guide

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Virtual circuit
Virtual circuits are logical paths established between two devices. Depending on how they are set up,
virtual circuits include the following types:
• Permanent virtual circuits (PVCs)—Preconfigured by network administrators and maintain until
being manually removed.
• Switched virtual circuits (SVCs)—Analogous to dialup connections. They are dynamically set up or
cleared on an as-needed basis through protocol negotiation.
PVCs are used far more than SVCs.
Data link connection identifier
A DLCI is a unique number assigned to a virtual circuit endpoint in a frame relay network for the
addressing purpose.
A DLCI uniquely identifies a particular virtual circuit on a physical link and has local significance only to
that link. A DLCI can be used on different physical ports to address different virtual circuits and a virtual
circuit between two DTE devices can be addressed with different DLCIs at the two ends, as shown
in Figure 34.
As the v
irtual circuits in a frame relay network are connection oriented, each DLCI on a physical port is
destined for a distinct remote device. DLCIs can be regarded the frame relay addresses of remote
devices.
The maximum number of PVCs that can be created on a frame relay interface is 1024. The user
configurable DLCIs for the PVCs are in the range 16 to 1007. Other DLCIs are reserved for special
purposes. For example, DLCI 0 and DLCI 1023 are reserved for the LMI protocol to transfer control
messages.
Frame relay address mapping
Frame relay address mapping associates the protocol address of a remote device with its frame relay
address (local DLCI) so the upper layer protocol, IP for example, can locate the remote device.
Take delivering an IP packet across a frame relay network for example. After a DTE device receives an IP
packet, it looks up the IP routing table for the outgoing interface and next hop address. If the outgoing
interface is enabled with frame relay, the device must look up the frame relay address mapping table
based on the next hop IP address for the DLCI.
The address mappings can be static ones administratively created or dynamic ones created with the
InARP.
The following describes how frame relay uses InARP to create an address mapping:
Once a new virtual circuit is created, InARP sends an inverse ARP request over the circuit to request the
peer end for its protocol address. This request also conveys the local protocol address. When the peer
device receives the request, it creates an address mapping based on the protocol address in the request
and responds with its protocol address. When the local end receives the response, it creates the address
mapping for the peer.
For virtual circuits with static address mappings, InARP will not be performed regardless of whether the
mappings are correct or not. In addition, the inverse ARP request recipient does not create a mapping
based on the protocol address in the request if a static entry is already available for the address.