Operation
3-2
Transparent Bridging is accomplished by building a Source Address Table (SAT)
from source MAC/physical addresses and using the SAT to make forwarding
decisions.
The 9T122-24 prevents unnecessary network traffic from passing through the
module by implementing two separate filtering processes — IEEE 802.1d or
Cabletron’s Special Filtering Database. These processes may be used 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 9T122-24 first goes on-
line, it initially forwards all packets across the bridge. After receiving a packet on
the bridge port, the 9T122-24 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 9T122-24 does not forward across the Token
Ring link.
The second filtering process, the Cabletron Systems Special Filtering Database,
provides an additional step in the filter/forward decision. Through Remote
Management, you can define up to 10 additional filtering parameters for
incoming network traffic. These parameters include (but are not limited to) the:
• destination address
• source address
• type field (protocol)
• 64 bytes of the data field (using a data offset)
For example, using this process, you can ensure that the 9T122-24 always filters or
forwards packets with a specific protocol or address.
Source Route Bridging operates by transmitting frames over a designated route.
Unlike Transparent Bridging, all devices in a Source Route Bridged (SRB) network
“know” the locations of other stations within the network. Using a portion of the
MAC frame header known as the RIF (Routing Information Field), the source
device determines the route for the frames it sends.
In order for source routing to work, the source station must determine the proper
route to reach the desired destination. To accomplish this:
1. The source station sends out frames called All Route Explorer (ARE) or Single
Route Explorer (SRE) frames onto the network. All Source Route Bridges
recognize these frames and forwards them to their outbound ports.