View figure.
This chapter describes the methods of bridging supported by the adaptive source routing transparent (ASRT) bridge. Each section gives an overview of a specific technology and is followed by a description of the data frames supported by that technology. The chapter includes the following sections:
The transparent bridge is also commonly known as a spanning tree bridge (STB). The term transparent refers to the fact that the bridge silently forwards non-local traffic to attached LANs in a way that is transparent or unseen to the user. End station applications do not know about the presence of the bridge. The bridge learns about the presence of end stations by listening to traffic passing by. From this listening process it builds a database of end station addresses attached to its LANs.
For each frame it receives, the bridge checks the frame's destination address against the ones in its database. If the frame's destination is an end station on the same LAN, the frame is not forwarded. If the destination is on another LAN, the frame is forwarded. If the destination address is not present in the database, the frame is forwarded to all the LANs that are connected to the bridge except the LAN from which it originated.
All transparent bridges use the spanning tree protocol and algorithm. The spanning tree algorithm produces and maintains a loop-free topology in a bridged network that might contain loops in its physical design. In a mesh topology where more than one bridge is connected between two LANs, looping occurs. In such cases, data packets bounce back and forth between two LANs on parallel bridges. This creates a redundancy in data traffic and produces the phenomenon known as looping.
When looping occurs, you must configure the local and/or remote LAN to remove the physical loop. With spanning tree, a self-configuring algorithm allows a bridge to be added anywhere in the LAN without creating loops. When the new bridge is added, the spanning tree protocol automatically reconfigures all bridges on the LAN into a single loop-free spanning tree.
A spanning tree never has more than one active data route between two end stations, thus eliminating data loops. For each bridge, the algorithm determines which bridge ports can forward data and which ones must be blocked to form a loop-free topology. The features that spanning tree provides include:
Transparent Bridge implements a spanning tree bridge that conforms to the IEEE 802.1D standard. All transparent bridges on the network must be 802.1D spanning tree bridges. This spanning tree protocol is not compatible with bridges implementing the proprietary Digital Equipment Corporation spanning tree protocol used in some older bridges.
In a mesh topology where more than one bridge is connected between two LANs, a looping phenomenon can occur where two LANs bounce packets back and forth over parallel bridges. A loop is a condition where multiple data paths exist between two LANs. The spanning tree protocol operating automatically eliminates loops by blocking redundant paths.
During startup, all participating bridges in the network exchange Hello bridge protocol data units (BPDUs) which provide configuration information about each bridge. BPDUs include information such as the bridge ID, root ID, and root path cost. This information helps the bridges to unanimously determine which bridge is the root bridge and which bridges are the designated bridges for LANs to which they are connected.
Of all the information exchanged in the HELLO messages, the following parameters are the most important for computing the spanning tree:
With this information available, the spanning tree begins to determine its shape and direction and then creates a logical path configuration. This process can be summarized as follows:
Through this process, the spanning tree algorithm reduces a bridged LAN network of arbitrary topology into a single spanning tree. With the spanning tree, there is never more than one active data path between any two end stations, thus eliminating data loops. For each bridge on the network, the spanning tree determines which bridge ports to block from forming loops.
This new configuration is bounded by a time factor. If a designated bridge fails or is physically removed, other bridges on the LAN detect the situation when they do not receive Hello BPDUs within the time period set by the bridge maximum age time. This event triggers a new configuration process where another bridge is selected as the designated bridge. A new configuration is also created if the root bridge fails.
When the spanning tree uses its default settings the spanning tree algorithm generally provides acceptable results. The algorithm, however, may sometimes produce a spanning tree with poor network performance. In this case you can adjust the bridge priority, port priority, and path cost to shape the spanning tree to meet your network performance expectations. The following examples explain how this is done.
Figure 20 shows three LANs networked using three bridges. Each bridge is using default bridge priority settings for its spanning tree configuration. In this case, the bridge with the lowest physical address is chosen as the root bridge because the bridge priority of each bridge is the same. In this example, this is Bridge 2.
The newly configured spanning tree stays intact due to the repeated
transmissions of Hello BPDUs from the root bridge at a preset interval (bridge
hello time). Through this process, designated bridges are updated with
all configuration information. The designated bridges then regenerate
the information from the Hello BPDUs and distribute it to the LANs for which
they are designated bridges.
Table 51. Spanning Tree Default Values
| Bridge 1 | Bridge 2 | Bridge 3 |
|---|---|---|
| Bridge Priority: 32768 Address: 00:00:90:00:00:10 | Bridge Priority: 32768 Address: 00:00:90:00:00:01 | Bridge Priority: 32768 Address: 00:00:90:00:00:05 |
Port 1
| Port 1
| Port 1
|
Port 2
| Port 2
| Port 2
|
Port 3
| Port 3
| Port 3
|
Figure 20. Networked LANs Before Spanning Tree
| View figure. |
The spanning tree algorithm designates the port connecting Bridge 1 to Bridge 3 (port 2) as a backup port and blocks it from forwarding frames that would cause a loop condition. The spanning tree created by the algorithm using the default values in Table 51 is shown in Figure 21 as the heavy lines connecting Bridge 1 to Bridge 2, and then Bridge 2 to Bridge 3. The root bridge is Bridge 2.
This spanning tree results in poor network performance because the workstations on LAN C can get to the file server on LAN A only indirectly through Bridge 2 rather than using the direct connection between Bridge 1 and Bridge 3.
Figure 21. Spanning Tree Created With Default Values
 View figure. |
Normally, this network uses the port between Bridge 2 and Bridge 3 infrequently. Therefore you can improve network performance by making Bridge 1 the root bridge of the spanning tree. You can do this by configuring Bridge 1 with the highest priority of 1000. The spanning tree that results from this modification is shown in Figure 22 as the heavy lines connecting Bridge 1 to Bridge 3 and Bridge 1 to Bridge 2. The root bridge is now Bridge 1. The connection between Bridge 2 and Bridge 3 is now blocked and serves as a backup data path.
Figure 22. User-Adjusted Spanning Tree
 View figure. |
The ATM interface forwards transparent frames from Ethernet networks, provided bridging is enabled on the virtual channel connection (VCC).
Hello BPDUs are generated and transmitted for each LEC configured for transparent bridging. The spanning tree protocol causes ATM LECs that have not been designated as part of the active data path to be BLOCKED, thereby eliminating loops.
This section reviews the terms and concepts commonly used in transparent bridging.
The length of time (age) before a dynamic entry is removed from the filtering database when the port with the entry is in the forwarding state. If dynamic entries are not referenced by the aging time, they are deleted.
A protocol-independent device that connects local area networks (LANs). These devices operate at the data link layer, storing and forwarding data packets between LANs.
The least significant 6-octet part of the bridge identifier used by the spanning tree algorithm to identify a bridge on the network. The bridge address is set to the MAC address of the lowest-numbered port by default. You can override the default address by using the set bridge configuration command.
The bridge hello time specifies how often a bridge sends out Hello BPDUs (containing bridge configuration information) when it becomes the root bridge in the spanning tree. This value is useful only for the root bridge because it controls the hello time for all bridges in the spanning tree. Use the set protocol bridge command to set the bridge hello time.
The amount of time a bridge port spends in the listening state as well as the learning state. The forward delay is the amount of time the bridge port listens in order to adjust the spanning tree topology. It is also the amount of time the bridge spends learning the source address of every packet that it receives while the spanning tree is configuring. This value is useful only for the root bridge because it controls the forward delay for all bridges in the spanning tree.
The root bridge conveys this value to all bridges. This time is set with the set protocol bridge command. The procedure for setting this parameter is discussed in the next chapter.
A unique identifier that the spanning tree algorithm uses to determine the spanning tree. Each bridge in the network must have a unique bridge identifier.
The bridge identifier consists of two parts: a least-significant 6-octet bridge address and a most-significant 2-octet bridge priority. By default, the bridge address is set to the MAC address of the lowest-numbered port. You can override the default address with the set bridge configuration command.
The amount of time that spanning tree protocol information is considered valid before the protocol discards the information and a topology changes. All the bridges in the spanning tree use this age to time out the received configuration information in their databases. This can cause a uniform timeout for every bridge in the spanning tree. Use the set protocol bridge command to set the bridge maximum age.
The most significant 2-octet part of the bridge identifier set by the set protocol bridge command. This value indicates the chances of each bridge becoming the root bridge of the network. In setting the bridge priority, the spanning tree algorithm chooses the bridge with the highest priority value to be the root bridge of the spanning tree. A bridge with the lowest numerical value has the highest priority value.
The bridge that claims to be the closest to the root bridge on a specific LAN. This closeness is measured according to the accumulated path cost to the root bridge.
The port ID of the designated bridge attached to the LAN.
Databases that contain information about station addresses that belong to specific port numbers of ports connected to the LAN.
The filtering database is initialized with entries from the permanent database. These entries are permanent and survive power on/off or system resets. You can add or delete these entries through the spanning tree configuration commands. Entries in the permanent database are stored as static random access memory (SRAM) records, and the number of entries is limited by the size of SRAM.
| Note: | You can also add entries (static) by using the monitoring commands but these do not survive power on/off and system resets. |
The filtering database also accumulates entries learned by the bridge (dynamic entries) which have an aging time associated with them. When entries are not referenced over a certain time period (age time), they are deleted. Static entries are ageless, so dynamic entries cannot overwrite them.
Entries in the filtering and permanent databases contain the following information:
Make changes to the permanent database through the spanning tree configuration commands and make changes to the filtering database through the GWCON monitoring process.
Two or more bridges connecting the same LANs.
Each port interface has an associated path cost which is the relative value of using this port to reach the root bridge in a bridged network. The spanning tree algorithm uses the path cost to compute a path that minimizes the cost from the root bridge to all other bridges in the network topology. The sum total of all the designated costs and the path cost of the root port is called the root path cost.
The bridge's connection to each attached LAN or WAN. A bridge must have at least two ports to function as a bridge.
A 2-octet port identifier. The most-significant octet represents the port priority and the least-significant octet represents the port number. Both port number and port priority are user-assignable. The port ID must be unique within the bridge.
A user-assigned 1-octet part of the port ID whose value represents the attachment to the physical medium. A port number of zero is not allowed.
The second 1-octet part of the port ID. This value represents the priority of the port that the spanning tree algorithm uses in making comparisons for port selection and blocking decisions.
The time factor by which dynamic entries are ticked down as they age within the database. The range is 1 to 60 seconds.
The bridge selected as the root of the spanning tree because it possesses the highest priority bridge ID. This bridge is responsible for keeping the spanning tree intact by regularly emitting Hello BPDUs (containing bridge configuration information). The root bridge is the designated bridge for all the LANs to which it is connected.
The port ID of a bridge's port that offers the lowest cost path to the root bridge.
A topology of bridges such that there is one and only one data route between any two end stations.
This type of bridging involves a mechanism that is transparent to end stations applications. Transparent bridging interconnects local area network segments by bridges designated to forward data frames through a spanning tree algorithm.