Source-Route Bridging (SRB)

Source-route bridging is implemented by IBM and compatible bridge products for use over token-ring LAN segments.

Source-routing requires a sending device to specify the path that should be taken by a frame across an internetwork, rather than allowing the decision to be made by individual bridges. To do this a sending device must determine the best path to a destination and include it in all frames to that destination. The best path to a destination is found using a discovery process, one implementation of which is described in this section.

A sending device sends a discovery frame to the intended destination device marked single-route broadcast. Bridges in a token-ring internetwork should be configured using the token-ring spanning tree algorithm to permit only one path for single-route broadcast frames between devices. The destination device should therefore receive only a single copy of the discovery frame.

The destination device responds to the discovery frame with a discovery response frame, marked all-routes broadcast. This will contain the most significant bit (the route information indicator, also called RII) set in the source MAC address field, and an entry in the routing information field (RIF). This will initially contain zero in the bridge number field, and the number of the networks to which the destination device is attached in the segment number field.

The discovery response frame, because it is marked all-routes broadcast, will pass through all bridges on its way back to the original sending device. Each bridge that the frame passes through must insert its bridge number and LAN segment, and hence the frames that return to the original sending device contain the routes they have taken through the bridged internetwork.

The routing information field can currently only hold data for about seven bridges and eight LAN segments. If a frame is received by a bridge with this field full, it is discarded. This limits the number of bridge hops in the network to seven, and consequently the maximum size of source-route bridged internetworks.

The original sending device therefore receives one or more discovery response frames. These frames contain routing control and bridge and LAN segment numbers in their routing information fields. The routing control field indicates the number of bridge/LAN segments in the routing information field and also the maximum frame size that is supported by the route.

The sending device can now select the best route to use through the internetwork to reach the destination device. Current implementations select the route in the first received discovery response frame (the fastest path at the time of the discovery process), although the architecture allows route selection based on other criteria, for example, maximum frame size supported by the route.

Bridging Methods

There are two primary methods of bridging:
· Transparent bridging (mainly used with Ethernet LANs), also called spanning tree bridging (STB)
· Source-route bridging (SRB) (used in 802.5 LANs). Then, from these two primary methods of bridging, there are other methods listed as follows:
Source-route transparent bridging (SRT)
Source-route - translational bridging (SR-TB)
Tunnel bridge (IP encapsulation)
All of these bridging methods are supported by the IBM 2210.
In the following topics, we provide a summary description of these bridging
methods.

Transparent Bridging (STB)

A transparent bridge is also called a spanning tree bridge (STB).
Transparent bridging is normally used to connect LAN segments. It is specified in the ISO 8802-1 standard.

This form of bridging could also be used for connection of token-ring LAN segments, although this is not common.

Transparent bridging is based on the principle that a sending device can transmit a frame to a receiving device on a LAN network without having any knowledge of the location of, or the path to, that receiving device.

Transparent bridges within a network are responsible for forwarding the frame to the correct destination, making the determination of whether a frame should be forwarded based on MAC sub-layer destination address.

Transparent bridges achieve this by building and maintaining a filtering database that acts as a forwarding table for received frames. They build their database by copying all frames from the LANs to which they are attached and learning the location of devices by inspecting the MAC sub-layer source address in each received frame.

Figure: 1 Transparent Bridging

Figure 1 illustrates how a transparent bridge will build up its filtering database. When the bridge receives a frame from device D1 on port A, it learns that D1 is reached via the LAN on port A. Similarly, if a frame arrives from device D7 on port B, it learns that D7 is reached via the LAN on port B.

For each new source address the bridge sees on the LAN, it adds an additional entry in its database. In time a full picture is built up of all devices on the two LANs and via which port they are reached.

The bridge uses its filtering database to determine if an incoming frame should be forwarded or discarded. This is done by examining the MAC sub-layer destination address of each frame and comparing it to the list of addresses in the filtering database:
· If the destination address is not in the database, the frame is forwarded on each port except the receiving port.

. If the destination address is in the database and the frame was received on a port associated with the address, the frame is discarded.

· If the destination address is in the database and the frame was received on a port not associated with the address, the frame is forwarded to the associated port for this destination address in the database.

Transparent bridges require that there be only a single active path between any two LANs in an internetwork. This requirement is to ensure that frames do not loop in such a way that they are seen on both ports of a bridge. If this happens, the bridge will be unable to forward the frames correctly to their destination.

Transparent bridges support and use spanning tree protocol, which ensures a loop-free topology between all the transparent bridges within the network.

Bridges

Bridges act as data link layer relays between LANs.

A bridge participates as a device on the networks to which it is attached, exchanges information with devices on those networks, and forwards information between the networks selectively through the MAC address.
There are four types of bridges and they are classified by their hardware and
software capabilities.

Simple Bridges

Simple Bridges consist of two or more linked network interfaces connecting local area networks. Bridges interconnect separate local area networks (LANs) by relaying data frames between the separate MAC (media access control) entities of the bridged LANs.











Figure: Simple bridge connecting two homogeneous LANs.







The main functions of a simple bridge may be summarized as follows:

• The bridge reads all data frames transmitted on LAN A and receives those addressed to LAN B. Simple bridges make no changes to the content or format of the data frames that they receive. They also do not encapsulate frames with any additional headers. Most simple bridges contain routing addressing and routing intelligence. At a minimum, the bridge must know which addresses are on each connected network so that it can know which frames to pass on.
• The bridge retransmits the data frames addressed to LAN B using the MAC protocol for that LAN. Bridges should have enough buffer space to meet peak data traffic demands since data frames may arrive faster than the bridge can transmit them.
• The bridge does the same for LAN B-to-LAN A data frame traffic.

Complex Bridges

Complex Bridges carry out more sophisticated functions than simple bridges. These functions may include the bridge maintaining status information on the other bridges. This information includes the communication path cost as well as the number of hops required to reach each connected network. Periodic exchanges of information between bridges update all bridge information. These types of exchanges allow for dynamic routing between bridges.
Complex bridges can also modify frames and recognize and transmit packets from different LAN technologies (for example, token-ring and Ethernet). In this case the bridge is sometimes referred to as a translational bridge.
The Adaptive Source-Routing Transparent (ASRT) Bridge is the IBM 2210¢s
implementation of bridge technology. The ASRT Bridge is a collection of software components capable of several of the bridging options just described and more.

Local Bridges

Local Bridges provide connections between several LAN segments in the same geographical area. An example of this would be a bridge used to connect the various LANs located in your company's main headquarters.


Remote Bridges

Remote Bridges connect multiple LAN segments in different geographical areas. An example of this would be bridges used to connect LANs located in your company¢s main headquarters to LANs in other branch offices around the country. Because of the geographical differences, this configuration moves from a local area network configuration to a wide area network (WAN) configuration.
Remote bridges can differ from local bridges in several ways. One major
difference is found in the speed in which data is transmitted. WAN connections are generally slower than LAN connections. This difference in speed can make quite a difference when running time-sensitive applications. Another difference is found in the physical way that remote and local bridges are connected to LANs. In local bridges, the connections are made through local cabling media (for example, Ethernet). Remote bridge connections are made over the serial lines.