Application Overview; Bridging - 3Com 3C63100-AC-C - PathBuilder S600 Bridge/router Reference Manual

Application Overview

Bridging

This section describes the following applications that are supported by the
PathBuilder S600:
Bridging
n
Spanning Tree
n
CBR application
n
Ethernet and voice application
n
Frame application
n
See the following subsections for details.
The bridge function is an integral part of the Ethernet module. The PathBuilder
S600 supports ATM permanent virtual circuits (PVCs) for access to the ATM
network. These PVCs connect an PathBuilder S600 Ethernet port through the ATM
network to another PathBuilder S600 Ethernet port. All the PVCs configured for a
port will form a virtual Bridged Ethernet network to all other ports at the other
end of the PVC.
The bridge operation is equivalent to a multiport bridge. The two Ethernet ports
on the module are treated separately unless they are connected through the ATM
network via PVCs. The ATM Layer operates transparently under the datalink layer
to allow higher level protocols to run without changes.
The CPU examines each packet's headers while they are in memory and performs
the bridging operations. A bridge is configured and maintained for each slot and
port in the PathBuilder S600. The bridge maintains the following:
A forwarding table in its memory that lists the source and destination addresses
n
contained in each packet that passes through the PathBuilder S600
An aging timer that removes addresses from the forwarding table if an address
n
is not used for a configured time
A configured list of protocols, source addresses, and destination addresses that
n
will be rejected by the bridge. For information on configuring bridging filters,
see "Configuring the Bridge"" in Chapter 5.
The bridge learns and builds forwarding tables for every PVC that is tied to that
port. When a packet is received on the port or any PVC tied to that port, the
Source MAC address is learned and kept in the forwarding table until the aging
timer expires. You can also use the user interface to add static forwarding
addresses that the bridge will not delete after the aging timer expires.
When the packet arrives, the bridge looks up the destination MAC address to
determine the destination of the packet from the forwarding tables. If the
destination is found, the bridge forwards the packet to the correct destination. If
the destination is not found, the bridge broadcasts or floods the packet on all
PVCs that are tied to the particular port. If you have assigned a multicast PVC for
that particular port, the bridge will forward the packet to the port multicast VC
instead of broadcasting it.
The PathBuilder S600 performs LLC-based multiplexing per RFC 1483 and
Ethernet frames are bridged. In the first phase, PathBuilder S600 does not support
frames bigger than Ethernet maximum frame size (1518) or IEEE 802.3 frame size
Application Overview 83