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connected to the QMAC ASIC.
QMAC
The QMAC uses four protocol handlers to support four Token Ring physical interfaces directly connected to
the QTP ASIC. Together, these two ASICs provide support for early token release (ETR) and Token Ring
Full Duplex (FDX) concentrator and adapter modes for dedicated Token Ring.
Bridging Types
In the early 1980s, IBM developed a non−routable protocol called NetBIOS as part of its implementation
strategy. NetBIOS joined other non−routable protocols that came into wide use, such as System Network
Architecture (SNA) and Local Area Transport (LAT). IBM also developed a physical network topology called
Token Ring. With Token Ring came a bridging technology called Source Route Bridging (SRB).
The SRB algorithm for Token−Ring LAN bridging became the IEEE 802.5 Token Ring LAN specification.
SRB has various combinations, which will be discussed in more detail in the next chapter:
Source Route Bridging (SRB)•
Source Route Transparent Bridging (SRT)•
Source Route Translational Bridging (SR/TLB)•
Transparent Bridging (TB) is another bridging technology that was developed later by DEC, and which is now
used in Ethernet networks. Although it was developed for DEC, it is the primary bridging algorithm for
today’s switches and routers. It maintains a routing table, building tables composed of destination addresses.
It has the ability to switch network packets based upon a match of the destination address, such as those used
with IP, IPX, and AppleTalk.
TB tables are built differently than routing tables. Whereas routing tables rely heavily on routing protocols to
learn about foreign networks, TB tables learn the location of each MAC address by logging the port from
which the frame arrived. Thus, assuming that the network the frame arrived from is attached to the port of
entry, TB logs the information along with a maximum age or TTL. When this maximum is reached, TB
removes the entry from the table.
Let’s take a look at each bridging type.
Source Route Bridging
SRB is a method of bridging used to connect Token Ring segments. It makes all forwarding decisions based
upon data in the RIF. It cannot acquire or look up MAC addresses. Therefore, SRB frames without a
corresponding RIF are not forwarded.
With SRB, every port on the switch is assigned a ring number. The switch itself is assigned one or more
bridge numbers. SRB uses this information to build the RIF; it then searches the RIF to determine where to
forward incoming frames.
SRB frames are modified when they arrive using explorer frames. Explorer frames are typically one of two
types: All Routes Explorer (ARE) or Spanning−Tree Explorer (STE). SRB bridges copy ARE and STE
frames and modify the RIF with their own routing information.
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