Port Trunking
Trunk Group Operation Using LACP
â– If there are ports that you do not want on the default VLAN, ensure that
they cannot become dynamic LACP trunk members. Otherwise a traffic
loop can unexpectedly occur. For example:
VLAN-1
(Default
VLAN)
VLAN-2
VLAN-1
(Default
VLAN)
VLAN-2
VLAN-1
(Default
VLAN)
VLAN-2
VLAN-1
(Default
VLAN)
VLAN-2
If the ports in VLAN 2 are configured to allow a dynamic trunk (and GVRP is disabled), adding a
second link in VLAN 2 automatically forms a dynamic LACP trunk and moves the trunk to VLAN-1
(the default VLAN), which creates a traffic loop in VLAN 1 between the two switches and
eliminates the link in VLAN 2 between the two switches.
Figure 12-12. A Dynamic LACP Trunk Forming in a VLAN Can Cause a Traffic Loop
Easy control methods include either disabling LACP on the selected ports or
configuring them to operate in static LACP trunks.
Spanning Tree and IGMP. If Spanning Tree and/or IGMP is enabled in the
switch, a dynamic LACP trunk operates only with the default settings for these
features and does not appear in the port listings for these features.
Half-Duplex and/or Different Port Speeds Not Allowed in LACP
Trunks. The ports on both sides of an LACP trunk must be configured for
the same speed and for full-duplex (FDx). The 802.3ad LACP standard speci-
fies a full-duplex (FDx) requirement for LACP trunking. (10-gigabit ports
operate only at FDx.)
A port configured as LACP passive and not assigned to a port trunk can be
configured to half-duplex (HDx). However, in any of the following cases, a
port cannot be reconfigured to an HDx setting:
â– If the port is a 10-gigabit port.
â– If a port is set to LACP Active, you cannot configure it to HDx.
â– If a port is already a member of a static or dynamic LACP trunk, you cannot
configure it to HDx.
â– If a port is already set to HDx, the switch does not allow you to configure
it for a static or dynamic LACP trunk.
12-25
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