CCNA RnS Article #37: Layer 2 Dynamic EtherChannel

In this article, we are going to talk about Dynamic EtherChannel configuration. If you have missed a previous article on how to configure static EtherChannel, please read it here.

In the field, we see two protocols that are used to configure dynamic EtherChannel. Port Aggregation Protocol (PAgP) was developed by Cisco and only supported on Cisco Switches. Another protocol is the Link Aggregation Control Protocol (LACP) based on the IEEE 802.3ad standard. LACP is supported by all vendor switches.

Fig 1.1- Etherchannel

⭐ LACP Vs PAgP 👇

LACP is an open standard protocol supported by all vendor switches. PAgP is Cisco proprietary and only supported by Cisco Switches.

LACP supports Auto and desirable configuration modes. PAgP supports Active and Passive configuration modes.

LACP can support more than 8 physical interfaces in a bundle (however, active are only 8). The PAgP can support up to 8 physical interfaces in a bundle.

Dynamic EtherChannel depends on some of the conditions to be met to work. Both LACP & PAgP communicate over the port between switches and share these configuration settings. Once these settings are complied an EtherChannel is formed. If the conditions are not met, the physical interface is put in a block state and does not forward any traffic.

There is not much difference in the configuration command to configure but the parameters are different. If you want to configure LACP EtherChannel then the command is

channel-group <number> <active | passive>.

In the case of PAgP command is channel-group <number> <auto | desirable>.

When we use dynamic EtherChannel configurations, there can be one switch sending the protocol (active [LACP Setting] / desirable [PAgP setting]) and other switch listening (passive [LACPSetting] / auto [PAgPsetting]).

Figure 1: Switch 1 is talking & 2 is listening.

⭐ When LACP or PAgP is used for form EtherChannel? 👇

If you are using auto or desirable it means that you want to use PAgP to form the dynamic EtherChannel. If an active or passive keyword is used, it means that you want to use LACP to form the EtherChannel.

Figure 2: LACP or PAgP

Let’s start with multiple configuration examples to see the behavior of the EtherChannel.

⭐ What if switches are configured not to initiate negotiation? 👇

Active and desirable keywords in channel-group command, initiate the negotiation to form the EtherChannel dynamically. If both ends are configured to Passive or Auto, negotiation will not start and no EtherChannel will form.

Figure 3: Switch 0 - Physical Interface with Auto keyword

Figure 4: Switch 1 - Physical Interface with Auto Keyword

As stated above if both sides are configured as auto, neither of them will start negotiation to form the EtherChannel and nothing will happen. Port-Channel will be in Downstate as per Figure 5 -

Figure 5: No EtherChannel Configured as both switches are configured as auto keyword

Solution:

As soon as the either end is configured with desirable keyword, our EtherChannel gets configured and transitions to an up state.

Figure 6: Switch 0 is configured as desirable

Figure 7: EtherChannel is in use now.

⭐ What happens when either end is configured with static EtherChannel configuration and other end is configured with LACP or PAgP? 👇

This action will bring down the EtherChannel. In our previous example, we configured switch 0 to use the PAgP with desirable configuration and Switch 1 was configured with auto keyword. Now we have changed the Switch 1 configuration to use “on” keyword to form a static EtherChannel. As a result, EtherChannel goes down.

Figure 8: On and Desirable configuration = No EtherChannel

The below table lists various keyword combinations and the result whether EtherChannel is formed or not:

⭐ What happens if a new link is added to the EtherChannel with different settings like speed, duplex, mode [access/trunk]? 👇

As we have already stated new physical link should follow the same settings that are configured on the existing member’s physical interface else new port will not come up. Let’s see with configuration examples –

Figure 9: All interfaces are negotiated to 10Mbps Speed

Port fa0/1 – 2 are already part of EtherChannel 1 and negotiated to speed 10Mbps. Now lets configure interface fa0/3 but with different speeds.

Speed changed to 100Mbps –

Interface configured with EtherChannel Configuration –

Interface doesn’t get included in EtherChannel –

All other members of the EtherChannel are configured with 10Mbps and the new interface fa0/3 is configured with 100Mbps speed, this inconsistency stops the new interface from becoming the part of the EtherChannel.

In addition to speed, duplex, and interface mode (access/trunk). In the case of trunk, allowed VLANs and Native VLANs should match otherwise this will not be included in the EtherChannel.

With this, I conclude this long article. Hope you find this informative.

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CCNA RnS Article #37: Layer 2 Dynamic EtherChannel