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pairs are different. The more ports you have in the trunk, the more likely it is that the traffic will be

distributed among the links.

When a new port is added to the trunk, the switch begins sending traffic, either new traffic or

existing traffic, through the new link. As links are added or deleted, the switch redistributes traffic

across the trunk group. For example, in Figure 63 (page 159) showing a three-port trunk, traffic

could be assigned as shown in Table 13 (page 159).

Figure 63 Example of port-trunked network

Table 13 Example of link assignments in a trunk group (SA/DA distribution)

LinkDestinationSource

1Node WNode A

2Node XNode B

3Node YNode C

1Node ZNode D

2Node YNode A

3Node WNode B

Because the amount of traffic coming from or going to various nodes in a network can vary widely,

it is possible for one link in a trunk group to be fully utilized while other links in the same trunk

have unused bandwidth capacity, even if the assignments were evenly distributed across the links

in a trunk.

Trunk load balancing using Layer 4 ports

Trunk load balancing using Layer 4 ports allows the use of TCP/UDP source and destination port

number for trunk load balancing. This is in addition to the current use of source and destination IP

address and MAC addresses. Configuration of Layer 4 load balancing would apply to all trunks

on the switch. Only non-fragmented packets will have their TCP/UDP port number used by load

balancing. This ensures that all frames associated with a fragmented IP packet are sent through

the same trunk on the same physical link.

The priority for using Layer 4 packets when this feature is enabled is as follows:

1. If the packet protocol is an IP packet and has Layer 4 port information, use Layer 4.

2. If the packet protocol is an IP packet and does not have Layer 4 information, use Layer 3

information.

3. If the packet is not an IP packet, use Layer 2 information.

Distributed trunking overview

The IEEE standard 802.3ad requires that all links in a trunk group originate from the same switch.

Distributed trunking uses a proprietary protocol that allows two or more port trunk links distributed

across two switches to create a trunk group. The grouped links appear to the downstream device

as if they are from a single device. This allows third party devices such as switches, servers, or

Trunk load balancing using Layer 4 ports 159