Understanding How EtherChannels Work
This section contains the following topics:
EtherChannel Feature Overview
An EtherChannel bundles individual Ethernet links into a single logical link that provides the aggregate bandwidth of up to eight physical links.
The Cisco ASR 901 router supports a maximum of eight EtherChannels with a maximum eight member links in each EtherChannel.
You can form an EtherChannel with up to eight compatibly configured LAN ports in a Cisco ASR 901 . All LAN ports in each EtherChannel must be of the same speed and must all be configured as Layer 2 LAN ports.
Note |
The network device to which a Cisco ASR 901 is connected may impose its own limits on the number of ports in an EtherChannel. |
If a segment within an EtherChannel fails, traffic previously carried over the failed link switches to the remaining segments within the EtherChannel. When a failure occurs, the EtherChannel feature sends a trap that identifies the router, the EtherChannel, and the failed link. Inbound broadcast packets on one segment in an EtherChannel are blocked from returning on any other segment of the EtherChannel.
Understanding How EtherChannels Are Configured
This section contains the following topics:
EtherChannel Configuration Overview
You can configure EtherChannels manually or use the Link Aggregation Control Protocol (LACP) to form EtherChannels. The EtherChannel protocols allow ports with similar characteristics to form an EtherChannel through dynamic negotiation with connected network devices. LACP is defined in IEEE 802.3ad.
Table 1 lists the user-configurable EtherChannel modes.
Mode |
Description |
---|---|
on |
This is the mode that forces the LAN port to channel unconditionally. In the on mode, a usable EtherChannel exists only when a LAN port group in the on mode is connected to another LAN port group in the on mode. Because ports configured in the on mode do not negotiate, there is no negotiation traffic between the ports. You cannot configure the on mode with an EtherChannel protocol. |
passive |
(Default for LACP) LACP mode that places a port into a passive negotiating state, in which the port responds to LACP packets it receives but does not initiate LACP negotiation. |
active |
LACP mode that places a port into an active negotiating state, in which the port initiates negotiations with other ports by sending LACP packets. |
Understanding Manual EtherChannel Configuration
Manually configured EtherChannel ports do not exchange EtherChannel protocol packets. A manually configured EtherChannel forms only when you enter configure all ports in the EtherChannel compatibly.
Understanding IEEE 802.3ad LACP EtherChannel Configuration
LACP supports the automatic creation of EtherChannels by exchanging LACP packets between LAN ports. LACP packets are exchanged only between ports in passive and active modes.
The protocol learns the capabilities of LAN port groups dynamically and informs the other LAN ports. Once LACP identifies correctly matched Ethernet links, it facilitates grouping the links into an EtherChannel. The EtherChannel is then added to the spanning tree as a single bridge port.
Both the passive and active modes allow LACP to negotiate between LAN ports to determine if they can form an EtherChannel, based on criteria such as port speed and trunking state. Layer 2 EtherChannels also use VLAN numbers.
LAN ports can form an EtherChannel when they are in different LACP modes as long as the modes are compatible. For example:
- A LAN port in active mode can form an EtherChannel successfully with another LAN port that is in active mode.
- A LAN port in active mode can form an EtherChannel with another LAN port in passive mode.
- A LAN port in passive mode cannot form an EtherChannel with another LAN port that is also in passive mode, because neither port will initiate negotiation.
Table 1 provides a summary of these combinations.
Router A |
Router B |
Result |
---|---|---|
passive mode |
passive mode |
No EtherChannel group is created. |
passive mode |
active mode |
EtherChannel group is created. |
active mode |
passive mode |
EtherChannel group is created. |
active mode |
active mode |
EtherChannel group is created. |
LACP uses the following parameters:
Note |
The LACP system ID is the combination of the LACP system priority value and the MAC address of the router. |
Note |
Port priority is only effective when it is configured on a device with an LACP system priority higher than the peer. |
-
LACP administrative key—LACP automatically configures an administrative key value equal to the channel group identification
number on each port configured to use LACP. The administrative key defines the ability of a port to aggregate with other ports.
A port’s ability to aggregate with other ports is determined by these factors:
- Port physical characteristics, such as data rate, duplex capability, and point-to-point or shared medium
- Configuration restrictions that you establish
On ports configured to use LACP, LACP tries to configure the maximum number of compatible ports in an EtherChannel, up to the maximum allowed by the hardware (eight ports). If LACP cannot aggregate all the ports that are compatible (for example, the remote system might have more restrictive hardware limitations), then all the ports that cannot be actively included in the channel are put in hot standby state and are used only if one of the channeled ports fails. You can configure an additional 8 standby ports (total of 16 ports associated with the EtherChannel).
Understanding Port-Channel Interfaces
Each EtherChannel has a numbered port-channel interface. The configuration that you apply to the port-channel interface affects all LAN ports assigned to the port-channel interface.
After you configure an EtherChannel, the configuration that you apply to the port-channel interface affects the EtherChannel; the configuration that you apply to the LAN ports affects only the LAN port to which you apply the configuration. To change the parameters of all ports in an EtherChannel, apply the configuration commands to the port-channel interface, for example, Spanning Tree Protocol (STP) commands or commands to configure a Layer 2 EtherChannel as a trunk.
Understanding Load Balancing
An EtherChannel balances the traffic load across the links in an EtherChannel by reducing part of the binary pattern formed from the addresses in the frame to a numerical value that selects one of the links in the channel.
EtherChannel load balancing can use MAC addresses or IP addresses. EtherChannel load balancing can use either source or destination or both source and destination addresses or ports. The selected mode applies to all EtherChannels configured on the router. EtherChannel load balancing can use MPLS Layer 2 information.
Use the option that provides the balance criteria with the greatest variety in your configuration. For example, if the traffic on an EtherChannel is going only to a single MAC address and you use the destination MAC address as the basis of EtherChannel load balancing, the EtherChannel always chooses the same link in the EtherChannel; using source addresses or IP addresses might result in better load balancing.