Today’s lab: Check your static EtherChannel config. It’s pretty simple, but worth a quick review. Answers below the fold as usual. And check the original lab post for the lab exercise.
Figure 1: Three EtherChannels, with Two Links Each
Answers
Example 5: SW1 Config
interface GigabitEthernet0/1 channel-group 20 mode on ! interface GigabitEthernet0/2 channel-group 20 mode on ! interface GigabitEthernet0/3 channel-group 21 mode desirable ! interface GigabitEthernet0/2 channel-group 21 mode desirable
Example 6: SW2 Config
interface GigabitEthernet0/1 channel-group 20 mode on ! interface GigabitEthernet0/2 channel-group 20 mode on ! ! interface GigabitEthernet0/3 channel-group 21 mode active ! interface GigabitEthernet0/2 channel-group 21 mode active
Example 7: SW3 Config
interface GigabitEthernet0/1 channel-group 21 mode auto ! interface GigabitEthernet0/2 channel-group 21 mode auto
Example 8: SW4 Config
interface GigabitEthernet0/1 channel-group 22 mode passive ! interface GigabitEthernet0/2 channel-group 22 mode passive
Commentary
Cisco uses the term EtherChannel to refer to their link aggregation technology. Link aggregation overcomes some of the shortcomings of the Spanning Tree Protocol (STP). For example, if EtherChannel was not being run on the links between SW1 and SW2, then one of the links would automatically be blocked by STP to ensure that a switching loop does not form; this will lose half of the available throughput. EtherChannel combines both links into a single logical link; because of this, STP considers this a single connection between the devices and does not block any of the ports.
Static EtherChannel configuration uses on mode per the channel-group mode interface subcommand; however, using this mode has the potential for causing problems if only a single side is configured. Cisco supports two protocols to negotiate the use of EtherChannel: Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP). PAgP is a Cisco proprietary version of link aggregation that was introduced before LACP. LACP is a standard-based solution and is defined in 802.3ad.
For EtherChannel 20, between SW1 and SW2, both interfaces (G0/1 and G0/2) on both switches need to be configured in on mode with the channel-group 20 mode on command.
For EtherChannel 21, between SW1 and SW3, SW1’s G0/3 and G0/4 use the channel-group 20 mode desirable command, which uses PAgP, and initiates the negotiation. SW3’s G0/1 and G0/2 use the channel-group 20 mode auto command, which uses PAgP, but passively waits for the other switch to initiate negotiations.
For EtherChannel 22, between SW2 and SW4, SW2’s G0/3 and G0/4 use the channel-group 20 mode active command, which uses LACP, and initiates the negotiation. SW4’s G0/1 and G0/2 use the channel-group 20 mode passive command, which uses LACP, but passively waits for the other switch to initiate negotiations.
Note that all the interfaces used default configuration, so there should be no issues with mismatched interface settings preventing any interfaces from joining a Layer 2 EtherChannel. See the ICND2 Official Cert Guide for more details on how to check for those dependencies, and for a broader look at EtherChannels.