Operating port trunks

Media:

For proper trunk operation, all ports on both ends of a trunk group must have the same media type and mode (speed and duplex.) (For the switches, Switch recommends leaving the port Mode setting at Auto or, in networks using Cat 3 cabling, Auto-10.)

Port Configuration

The default port configuration is Auto, which enables a port to sense speed and negotiate duplex with an auto-enabled port on another device. Switch recommends that you use the Auto setting for all ports you plan to use for trunking. Otherwise, you must manually ensure that the mode setting for each port in a trunk is compatible with the other ports in the trunk.

All of the following operate on a per-port basis, regardless of trunk membership:

• Enable/Disable

• Flow control (Flow Ctrl)

LACP is a full-duplex protocol.

Trunk configuration:

All ports in the same trunk group must be the same trunk type (LACP or trunk.) All LACP ports in the same trunk group must be either all static LACP or all dynamic LACP.

A trunk appears as a single port labeledDyn1(for an LACP dynamic trunk) or Trk1 (for a static trunk of type LACP, Trunk) on various menu and CLI screens.

For spanning-tree or VLAN operation, configuration for all ports in a trunk is done at the trunk level. (You cannot separately configure individual ports within a trunk for spanning-tree or VLAN operation.)

Traffic distribution:

All of the switch trunk protocols use the SA/DA (source address/destination address) method of distributing traffic across the trunked links.

Spanning Tree:

802.1D (STP) and 802.1w (RSTP) Spanning Tree operate as a global setting on the switch (with one instance of Spanning Tree per switch.) 802.1s (MSTP) Spanning Tree operates on a per-instance basis (with multiple instances allowed per switch.) For each Spanning Tree instance, you can adjust Spanning Tree parameters on a per-port basis.

A static trunk of any type appears in the Spanning Tree configuration display, and you can configure Spanning Tree parameters for a static trunk in the same way that you would configure Spanning Tree parameters on a non-trunked port. (Note that the switch lists the trunk by name—such as Trk1—and does not list the individual ports in the trunk.) For example, if ports C1 and C2 are configured as a static trunk named Trk1, they are listed in the Spanning Tree display as Trk1 and do not appear as individual ports in the Spanning Tree displays.

When Spanning Tree forwards on a trunk, all ports in the trunk will be forwarding. Conversely, when Spanning Tree blocks a trunk, all ports in the trunk are blocked.

NOTE:

A dynamic LACP trunk operates only with the default Spanning Tree settings. Also, this type of trunk appears in the CLI show spanning-tree display, but not in the Spanning Tree Operation display of the Menu interface.

If you remove a port from a static trunk, the port retains the same Spanning Tree settings that were configured for the trunk.

Example of a port trunk in a Spanning Tree listing

IP multicast protocol (IGMP):

A static trunk of any type appears in the IGMP configuration display, and you can configure IGMP for a static trunk in the same way that you would configure IGMP on a non-trunked port. (Note that the switch lists the trunk by name—such as Trk1—and does not list the individual ports in the trunk.) Also, creating a new trunk automatically places the trunk in IGMP Auto status if IGMP is enabled for the default VLAN.

A dynamic LACP trunk operates only with the default IGMP settings and does not appear in the IGMP configuration display or show ip igmp listing.

VLANs:

Creating a new trunk automatically places the trunk in the DEFAULT_VLAN, regardless of whether the ports in the trunk were in another VLAN. Similarly, removing a port from a trunk group automatically places the port in the default VLAN. You can configure a static trunk in the same way that you configure a port for membership in any VLAN.

NOTE:

For a dynamic LACP trunk to operate in a VLAN other than the default VLAN (DEFAULT_VLAN), GVRP must be enabled.

Port security

Trunk groups (and their individual ports) cannot be configured for port security, and the switch excludes trunked ports from the show port-security listing. If you configure non-default port security settings for a port, then subsequently try to place the port in a trunk, you see the following message and the command is not executed:

<PORT-LIST> Command cannot operate over a logical port.

Monitor port

NOTE:

A trunk cannot be a monitor port. A monitor port can monitor a static trunk but cannot monitor a dynamic LACP trunk.