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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Source Route Transparent Bridging

Source Route Transparent Bridging (SRT) is a combination of SRB and TB. SRT bridges make forwarding decisions based on either the Routing Information Field (RIF) for the destination or the MAC address in the frame.

Some protocols attempt to establish a connection using a frame without using a RIF. These applications send a test frame to see if the destination is on the same ring as the source. If no response is received from this test frame, then an All Routes Explorer (ARE) test frame with a RIF is sent. If the destination receives the ARE, it responds, and the spanning−tree path through the bridge is used.

If the network is configured with parallel full−duplex backbones, this detected path may be very undesirable. If the spanning−tree path is used, then only one of the backbones will carry the traffic.

Source Route Translational Bridging

Source Route Translational Bridging (SR/TLB) has a Token Ring attached to at least one port of the bridge and another media−type topology (such as FDDI or Ethernet) attached to another port. SR/TLB’s main function is to make the two media types transparent to one another. The bridge receives the token, converts the data to a readable format for the Ethernet segment, and then forwards the data out the Ethernet to the receiving host address. All this takes place transparently to both hosts on the network—the Ethernet host believes that the Token Ring host is on Ethernet, and vice versa.

Transparent Bridging

Transparent bridges get their name because they are invisible to all the network nodes for which they provide services. Transparent bridges and switches acquire knowledge of the network by looking at the source address of all frames coming into their interfaces. The bridge then creates a table based on the information from the frames it received.

If a host sends a frame to a single host on another port, then if the bridge or switch has learned the port the destination resides on and it is on the local switch, the switch forwards the frame out the destination interface. If the bridge or switch does not know the port the destination host resides on, it will flood the frame received out all the ports except for the port the frame was received on. Broadcasts and multicasts are also flooded in the same way out all the ports.

Source Route Switching

Source Route Switching (SRS) was created to overcome the disadvantages of standard TB. TB does not support source−routing information. SRS forwards frames that do not contain routing information based on the MAC address the same way that TB does. All the rings that are source−route switched have the same ring number, and the switch learns the MAC addresses of adapters on these rings.

SRS also learns the route descriptors. A route descriptor is a portion of a RIF that indicates a single hop. It defines a ring number, a bridge number, and the hop closest to the switch. Future frames received from other ports with the same next−hop route descriptor are forwarded to that port.

If you have a Token Ring switch that has reached the limitation of ring stations on the current ring, SRS is your best choice for bridging. Unlike SRB, SRS looks at the RIF; it never makes the changes to the RIF. Using SRS, the switch does not need to obtain the MAC addresses of the devices. This method reduces the number of MAC addresses that the switch must learn and maintain.

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