Ethernet link speeds are standardized in factors of 10 (Ethernet, FastEthernet, GigabitEthernet, and Ten Gigabit Ethernet). When switch-to-switch links within TelePresence campus networks become congested, however, it might be costly to upgrade by a full factor of 10. It is generally more cost-effective to add another parallel link at the same speed; however, as more parallel links are added, these might become operationally complex to administer. Therefore, administration of multiple redundant links can be simplified through the use of EtherChannels.
EtherChannel technologies create a single logical link by bundling multiple physical Ethernet-based links (such as Gigabit Ethernet or Ten Gigabit Ethernet links) together, as shown in Figure 1. As such, EtherChannel links can provide for increased redundancy, capacity, and load-balancing. To optimize the load balancing of traffic over multiple links, it is recommended to deploy EtherChannels in powers of two (two, four, or eight) physical links. EtherChannel links can operate at either L2 or L3.
EtherChannel links can be created using Cisco Port Aggregation Protocol (PAgP), which performs a negotiation prior to forming a channel, to ensure compatibility and administrative policies.
You can configure PAgP in four channeling modes:
- On: 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. Ports configured in the on mode do not negotiate to form EtherChannels: They just do or do not, depending on the other port’s configuration.
- Off: Precludes the LAN port from channeling unconditionally.
- Desirable: Places a LAN port into an active negotiating state in which the port initiates negotiations with other LAN ports to form an EtherChannel by sending PAgP packets. A port in this mode forms an EtherChannel with a peer port that is in either auto or desirable PAgP mode.
- Auto: (Default) Places a LAN port into a passive negotiating state in which the port responds to PAgP packets it receives but does not initiate PAgP negotiation. A port in this mode forms an EtherChannel with a peer port that is in desirable PAgP mode (only).
PAgP, when enabled as an L2 link, is enabled on the physical interface (only). Optionally, you can change the PAgP mode from the default “auto” negotiation mode, as follows:
Switch(config)# interface GigabitEthernet8/1 Switch(config-if)# channel-protocol pagp Switch(config-if)# channel-group 15 mode desirable
Alternatively, EtherChannels can be negotiated with the IEEE 802.3ad Link Aggregation Control Protocol (LACP), which similarly allows a switch to negotiate an automatic bundle by sending LACP packets to the peer. LACP supports two channel negotiation modes:
- Active: Places a port into an active negotiating state in which the port initiates negotiations with other ports by sending LACP packets. A port in this mode forms a bundle with a peer port that is in either active or passive LACP mode.
- Passive: (Default) Places a port into a passive negotiating state in which the port responds to LACP packets it receives but does not initiate LACP negotiation. A port in this mode forms a bundle with a peer port that is in active LACP mode (only).
Similar to PAgP, LACP requires only a single command on the physical interface when configured as a L2 link. Optionally, you can change the LACP mode from the default “passive” negotiation mode, as follows:
Switch(config)# interface GigabitEthernet8/2 Switch(config-if)# channel-protocol lacp Switch(config-if)# channel-group 16 mode active
Note that PAgP and LACP do not interoperate with each other; ports configured to use PAgP cannot form EtherChannels with ports configured to use LACP, and can ports configured to use LACP cannot form EtherChannels with ports configured to use PAgP.
EtherChannel plays a critical role in provisioning network link redundancy, especially at the campus distribution and core layers. Furthermore, an evolution of EtherChannel technology plays a key role the Cisco Virtual Switching System
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