Port Trunks

Port trunks are connections that allow devices to communicate using up to four links in parallel.

This chapter explains more about port trunks and how to set them up on your network. It covers the following topics:

• What are Port Trunks?

• Port Trunks and Your Switch

• Placing Ports in a Port Trunk

• Port Trunk Example

Port trunks are connections that allow devices to communicate using up to four links in parallel. These parallel links provide two benefits:

• They can potentially double, triple or quadruple the bandwidth of a connection.

• They can provide a redundancy - if one link is broken, the other links share the traffic for that link.

Figure 30 shows a Switch 3300 and a Switch 630 connected using a port trunk with four links. If all ports on both Switch units are configured as 100BASE-TX and they are operating in full duplex, the potential bandwidth of the connection is 800Mbps.

Figure 30 Switch units connected using a port trunk

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Each unit in the Switch 1100/3300 family supports two port trunks, and if you have a stack of units, the stack can support up to eight port trunks.

If you install a SuperStack^® II Switch 1000BASE-SX Module (3C16975) into a Switch, only one port trunk is supported by that unit.

When setting up a port trunk, note that:

• The ports at both ends of a connection must be configured as trunk ports.

• The trunk ports can only belong to one port trunk.

• The trunk ports must be fiber or twisted pair ports.

• The trunk ports must be from the same Switch in the stack.

• The trunk ports must have an identical configuration. Port speed is the only exception - if ports of a different speed are trunked together, the higher speed links carry the traffic. The lower speed links only carry the traffic if the higher speed links fail.

• The ports in a trunk do not support security, VLT tagging, resilient links, or roving analysis. For information about security, see "Configuring a Port". For information about VLT tagging, see "Placing a Port in Multiple VLANs". For information about resilient links, see "Setting Up Resilient Links". For information about roving analysis, see "Setting Up Roving Analysis Ports".

• Port trunks cannot have a permanent entry placed against them in the Switch Database.

When using a port trunk, note that:

• To gather statistics about a port trunk, you must add together the statistics for each port in the trunk.

• To disable a link in a port trunk, you must remove the connection and then disable both trunk ports in the link separately. If you do this, the traffic destined for that link is distributed to the other links in the port trunk. If you do not remove the connection and only disable one trunk port in the link, traffic is still forwarded to that port by the trunk port at the other end. This means that a significant amount of traffic may be lost.

• Before removing a port trunk, you must disable all the trunk ports or disconnect all the links - if you do not, a loop may be created.

To place ports into a port trunk, use the Port Trunk Setup page of the web interface; for more information, see "Setting Up Port Trunks".

The example shown in Figure 31 illustrates an 800Mbps port trunk between two Switch units.

Figure 31 An 800Mbps port trunk between two Switch units

To set up this configuration:

1 .   Prepare ports 13, 15, 17 and 19 on the higher Switch for port trunking:

a .   Use the web interface to ensure that the ports have an identical configuration:

b .   Use the Port Trunk Setup page of the web interface to specify that ports 13, 15, 17 and 19 are ports of the port trunk.

2 .   Prepare ports 1, 3, 5 and 7 on the lower Switch for port trunking:

a .   Use the web interface to ensure that the ports have an identical configuration:

b .   Use the Port Trunk Setup page of the web interface to specify that ports 1, 3, 5 and 7 are ports of the port trunk.

3 .   Connect port 13 on the higher Switch to port 1 on the lower Switch.

4 .   Connect port 15 on the higher Switch to port 3 on the lower Switch.

5 .   Connect port 17 on the higher Switch to port 5 on the lower Switch.

6 .   Connect port 19 on the higher Switch to port 7 on the lower Switch.