Cisco MDS SanOS Troubleshooting Guide -- Troubleshooting PortChannels and Trunking

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PortChannels refer to the aggregation of multiple physical interfaces into one logical interface to provide higher aggregated bandwidth, load balancing, and link redundancy.


PortChannel Overview

A PortChannel has the following functionality:

  • Provides a point-to-point connection over ISL (E ports) or EISL (TE ports). Multiple links can be combined into a PortChannel.
  • Increases the aggregate bandwidth on an ISL by distributing traffic among all functional links in the channel.
  • Load balances across multiple links and maintains optimum bandwidth utilization. Load balancing is based on the source ID, destination ID, and exchange ID (OX ID).
  • Provides high availability on an ISL. If one link fails, traffic previously carried on this link is switched to the remaining links. If a link goes down in a PortChannel, the upper protocol is not aware of it. To the upper protocol, the link is still there, although the bandwidth is diminished. The routing tables are not affected by link failure. PortChannels may contain up to 16 physical links and may span multiple modules for added high availability.

On switches with Generation 1 switching modules, or a combination of Generation 1 and Generation 2 switching modules, you can configure a maximum of 128 PortChannels. On switches with Generation 2 switching modules only, you can configure a maximum of 265 PortChannels.

A PortChannel number refers to the unique (to each switch) identifier associated with each channel group. This number ranges from of 1 to 256.

Trunking Overview

Trunking, also known as VSAN trunking, is a feature specific to switches in the Cisco MDS 9000 Family. Trunking enables interconnected ports to transmit and receive frames in more than one VSAN, over the same physical link using extended ISL (EISL) frame format.

Trunking configurations are only applicable to E ports. If trunk mode is enabled in an E port and that port becomes operational as a trunking E port, it is referred to as a TE port. The trunk-allowed VSANs configured for TE ports are used by the trunking protocol to determine the allowed-active VSANs in which frames can be received or transmitted.

Trunking is a commonly used storage industry term. However, the Cisco SAN-OS software and switches in the Cisco MDS 9000 Family implement trunking and PortChannels as follows:

  • PortChannel enables several physical links to be combined into one aggregated logical link.
  • Trunking enables a link transmitting frames in the EISL format to carry (trunk) multiple VSAN traffic. When trunking is operational on an E port, that E port becomes a TE port. A TE port is specific to switches in the Cisco MDS 9000 Family. An industry standard E port can link to other vendor switches and is referred to as a nontrunking interface.

Initial Troubleshooting Checklist

Begin troubleshooting Portchannel and trunking issues by verifying that you have completed following actions first:


Check off

Use the show port-channel compatibility-parameters CLI command to determine PortChannel requirements.

Ensure that one PortChannel is not connected to different sets of switches. PortChannels require point-to-point connections between the same set of switches.

Verify that either side of a PortChannel is connected to the same number of interfaces.

Verify that each interface is connected to the same type of interface on the other side.

Verify that all required VSANS on a TE port are in the allowed-active VSAN list.

Note Note: Use the show running interface CLI command to view the interface configuration in Cisco SAN-OS Release 3.0(1) or later. The interface configuration as seen in the show running-config CLI command is no longer consolidated.

Common Troubleshooting Tools in Fabric Manager

The following Fabric Manager navigation paths may be useful in troubleshooting any issues with PortChannel and trunking:

  • Choose ISLs > PortChannel to access the PortChannel configuration.
  • Choose Switches > Interfaces > FC Logical and select the Trunk Config tab to access the trunking configuration.

Common Troubleshooting Commands in the CLI

The following commands may be useful in troubleshooting PortChannel and trunking:

  • show port-channel compatibility-parameters
  • show port-channel summary
  • show port-channel database
  • show port-channel consistency detail
  • show port-channel usage
  • show interface
  • show interface trunk
  • show trunk protocol

PortChannel Issues

This section describes common PortChannel issues.

Cannot Configure a PortChannel

Symptom Cannot configure a PortChannel

Table 10-1 Cannot Configure a PortChannel


Possible Cause


Cannot configure a PortChannel.

PortChannel autocreation is enabled.

Disable autocreation if you want to manually configure PortChannels. In Device Manager, select Interfaces > FC ALL..., select the Other tab, uncheck the AutoChannelCreate check box, and click Apply.

Use the no channel-group auto CLI command.

Newly Added Interface Does Not Come Online In a PortChannel

Configuring Port Channel Modes Using Fabric Manager

Trunking Issues

Cannot Configure Trunking

VSAN Traffic Does Not Traverse Trunk

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