EtherChannel 101
An etherchannel is a logical port that consists of multiple links bundled into a single logical link. To have a working etherchannel you must use static config or a negotiation protocol (LACP or PAgP). All ports in an EtherChannel must operate at the same speed and duplex. When an EtherChannel group is first created, it will follow the configuration of the first port that was added to the group. The following configurations must match between all ports in a group:
Allowed VLAN List and Native VLAN
Spanning-tree path cost and priority for each VLAN
Spanning-tree PortFast setting
Static Configuration
This configuration will work only if all ports in the channel-groups on both sides as configured like this. No negotiation takes place.
PAgP
PAgP is a Cisco Proprietary protocol. Up to 8 ports of the same type can be configured for the same EtherChannel.
auto – enables PAgP passively – It will respond to PAgP packets, but will not start a negotiation.
desirable – enables PAgP actively – it responds and starts a PAgP negotiation.
silent – by default, PAgP assumes the silent mode, when it considers that the other end doesn’t use PAgP so it will bring the portchannel Up, if the other end doesn’t respond to PAgP packets.
non-silent – If set, then PAgP will wait for a response from the other before bringing up the PortChannel.
LACP
LACP is a IEEE protocol. Up to 16 ports of the same type can be configured in an EtherChannel, but only 8 will be active, while the other 8 will be in standby. The software will determine which ports to use based on the lowest priority, which consists of:
LACP System Priority
System ID (Switch MAC)
LACP Port Priority
Port Number
The side with the lowest System Priority will decide what ports to use. To modify and verify the System Priority, use:
To modify and veridy the LACP Port Priority, use:
To set up an EtherChannel using LACP, use:
passive – enables LACP passively – It will respond to LACP packets, but will not start a negotiation.
active – enables LACP actively – it responds and starts a LACP negotiation.
Load Balancing
In order to decide on which link should traffic be forwarded out on a channel group, the switch uses a load balancing algorithm which takes the source or destination MAC or IP address of each packet and computes a hash value which is mapped to the port it should be forwarded on. As long as the input data is the same, the same port will be chosen for output. To set the load-balance method, use:
To verify, use:
The hash is fixed length and most platforms use a 3 bit size. This creates some unbalance in utilization as the ports are mapped to the possible hash values:
| Ports in EtherChannel | Hash assignment | Ratio |
|---|---|---|
2 | 1|2|1|2|1|2|1|2 | 4:4 |
3 | 1|2|3|1|2|3|1|2 | 3:3:2 |
4 | 1|2|3|4|1|2|3|4 | 2:2:2:2 |
5 | 1|2|3|4|5|1|2|3 | 2:2:2:1:1 |
6 | 1|2|3|4|5|6|1|2 | 2:2:1:1:1:1 |
7 | 1|2|3|4|5|6|7|1 | 2:1:1:1:1:1:1 |
8 | 1|2|3|4|5|6|7|8 | 1:1:1:1:1:1:1:1 |
Other platforms use 8 bits for hashing which smooth out some of the unbalance
Monitor
EtherChannel Guard
This feature can detect misconfigurations between connected devices. Normally it is configured by default and it will put the physical interfaces in err-disabled state.
To check if Etherchannel Guard is enabled, use:
To enable the feature, use the global config command:
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