INTRODUCTION
Page 1-6
1.5.1
Remote Bridging
The ETWMIM uses Point to Point Protocol (PPP) or Frame Relay
Protocol to communicate across a Wide Area link. When transmitting
with PPP or Frame Relay, the ETWMIM uses High level Data Link
Control (HDLC) as a basis for encapsulating MAC frames. When
connecting your ETWMIM to a Wide Area Network, keep the following
in mind:
•
If transmitting to a non-Cabletron device at the other side of
the WAN link, make sure that device expects to receive MAC
frames encapsulated in PPP or Frame Relay.
•
When using a router to connect networks across a WAN link,
make sure the router configuration is set to bridge all network
traffic using PPP or Frame Relay.
1.5.2
Transparent Bridging
Transparent Bridging, used in Ethernet, Token Ring, and FDDI
networks, operates by:
•
building a Source Address Table (SAT) from source MAC/
physical addresses
•
using the SAT to make forwarding decisions.
The ETWMIM prevents unnecessary network traffic from passing
through the module by implementing two separate filtering processes
— IEEE 802.1d or Cabletron’s Special Filtering Database. The
ETWMIM can use these processes individually or in tandem.
The first process, the IEEE 802.1D filtering process, begins with the
creation of a list of local node addresses in a table (the SAT). When the
ETWMIM first goes on-line, it initially forwards all packets across the
bridge. After receiving a packet on the bridge port, the ETWMIM
learns the address of the sending node from the packet and stores that
address in the SAT. In this manner, the bridge learns the address of
each node on each side of the bridge. The bridge then uses the
addresses stored in the table to compare the destination address of
each subsequent packet that travels to the bridge. If the destination
address of a packet resides on the bridge segment, the ETWMIM does
not forward across the WAN or Token Ring link.
