Configuring the Transparent Bridge

Use the Bridging General window to set global bridge settings.  These parameters allow you to configure the Spanning Tree Protocol and to enable IPX translation.

Configuring the Spanning Tree Protocol

Influencing the Selection of the Root Bridge

The root bridge is selected based on a value that is the combination of a bridge's address and a number you can assign that is the bridge's priority.  A lower number for bridge priority makes it likely that the bridge is selected as the root.

Influencing the Selection of the Root Port

The root port on a bridge is selected based on which port provides access to the root bridge at the lowest path cost and the relative priority of each port.

You can choose either to set the path cost for a port manually through the CLI or allow the Spanning Tree Protocol (STP) to do so automatically.  If the spanning tree sets the path cost, the value it assigns is based on the line speed associated with the port.  The higher the line speed, the lower the assigned cost.

Using Rate Limiting to Minimize the Effects of Broadcast Storms

Rate limiting is used to minimize the effects of broadcast storms. A broadcast storm is typically caused when a host system responds to multicast packets that are circulating continuously on the network, or when it tries to respond to another system that never replies.  The generation of such traffic at an uncontrolled rate can severely affect the available bandwidth on a network, perhaps making communications impossible.

You can limit broadcast storms to that segment of the network from which the packets are generated.  You do so by setting the maximum number of multicast packets per second that the line card is to forward.  The rate you set is applied only if rate limiting is enabled.  If the maximum number of packets per second is reached, the line card forwards packets at the specified maximum rate, thereby limiting the effect of the broadcast storm on the other side of the line card.

Detecting Changes in Network Topology

Caution: Do not alter the Spanning Tree Protocol default settings associated with the following tasks until you fully understand the effect the change will have on the bridge network.

Detecting a Failed Bridge or Link

Use the Max Age parameter to fine-tune the Spanning Tree Protocol's ability to detect a change in topology that is the result of a failed bridge or link.    If the time since a bridge last received a Hello Message on a port exceeds the Max Age setting, perhaps the result of a failed link, the bridge recalculates the root, path cost, and root port.

Note: Although you can configure each bridge in a spanning tree with a different value for Max Age, the value configured on the root bridge is used by all bridges in the spanning tree.  The values configured on non-root bridges are used only if a non-root bridge subsequently becomes root.

The lower the Max Age value, the earlier a failed bridge or link may be detected.  However, if the Max Age time is exceeded due to a normal drop in network activity and not because of a link or bridge failure, the result may be failure to compute a correct spanning tree.  This may cause forwarding loops and severe network congestion.

Preventing Forwarding Loops Between Bridges

Use the Forward Delay parameter to prevent temporary forwarding loops between bridges. If a temporary forwarding loop occurs, it can cause severe network congestion. The forward delay must be at least twice the maximum amount of time it takes for data to traverse the network.

Setting Hello Time, Max Age, and Forward Delay

There is a functional relationship among Hello Time, Bridge Max Age, and Bridge Forward Delay.  That relationship requires that the values set for the three parameters conform to the following algorithm:

(2) * (Bridge Forward Delay - 1 second)  greater_equal.gif (64 bytes) Bridge Max Age

and

Bridge Max Age greater_equal.gif (64 bytes) (2) * (Bridge Hello Time + 1 second)

Determining Whether to Enable or Disable IPX Translation

IPX translation is a peer-to-peer networking protocol for Novell NetWare. You may need to enable IPX translation on a line card under the following circumstances:

  • Some nodes on your network are set to generate and receive IPX datagrams in Raw Ethernet frame format.

  • Some nodes in the same NetWare network number as above are directly connect to an FDDI LAN.

Translation is necessary in the above conditions because the FDDI-attached NetWare node cannot recognize raw frames, while the Ethernet-attached nodes are configured to recognize only raw frames.  For all these nodes to communicate, the line cards can be configured to translate between raw format (on the Ethernet LAN) and SNAP format (on the FDDI LAN). See IPX translation rules for more information.

Field Descriptions

IPX Translation Sets the state of IPX translation: either enabled or disabled. IPX translation is a peer-to-peer networking protocol for Novell NetWare. The default is disabled.
Priority Sets a number, used in combination with the bridge's address, that determines whether a bridge is a root bridge. The lower the number, the more likely the bridge is selected as the root. The number can be from 0 to 65535.
Rate Limiting Determines the state of rate limiting: either enabled or disabled. The default is disabled.
Rate Sets the maximum number of frames per second the  line card is to forward destined for a particular MAC address, or that are of a particular protocol type. The default is 400 frames per second.
Bridge Hello Time Sets the frequency with which you want the bridge port to transmit Hello Messages to check for changes in network topology. The value can be from 1 to 10 seconds.  The default is 2 seconds.

The greater the frequency ( the lower the number of seconds), the sooner changes in topology are detected. The lower the frequency (the greater the number of seconds), the lower the overhead associated with detecting changes.

Bridge Max Age Sets the maximum amount of time a bridge port waits to receive a Hello Message before the bridge recalculates the root, path cost, and root port. The value can be from 6 to 40. The default is 20.
Forward Delay Sets the number of seconds you want a bridge to wait before allowing disabled bridge ports to transition to the forwarding state. The value can be from 4 through 30 seconds. The default is 15 seconds.

The forward delay parameter is used to prevent temporary forwarding loops between bridges. If a temporary forwarding loop occurs it an cause severe network congestion.  The forward delay must be at least twice the maximum amount of time it takes for data to traverse the network.