4-4. EtherChannel You can aggregate several individual switch ports into a single logical port or EtherChannel. Fast Ethernet ports, when bundled together, form a Fast EtherChannel (FEC). Gigabit ports form a Gigabit EtherChannel (GEC). You can manually configure EtherChannels or aggregate them through the use of dynamic protocols. PAgP is a Cisco proprietary protocol, whereas Link Aggregation Control Protocol (LACP) is a standards-based protocol defined in IEEE 802.3ad (also known as IEEE 802.3 Clause 43, "Link Aggregation"). Frames are distributed onto the individual ports that make up an EtherChannel by using a hashing algorithm. The algorithm can use source, destination, or a combination of source and destination IP addresses, source and destination MAC addresses, or TCP/UDP port numbers, depending on the hardware platform and configuration. Frame distribution is deterministic; that is, the same combination of addresses or port numbers always points to the same port within the EtherChannel. The frame distribution hashing algorithm performs an exclusive-OR (XOR) operation on one or more low-order bits of the addresses or TCP/UDP port numbers to select on which link a frame will be forwarded. For a two-port bundle, the last bit is used; a four-port bundle uses the last two bits; an eight-port bundle uses the last three bits. (With XOR, if two bits are identical, a 0 bit results; if two bits are different, a 1 bit results.) If a link within an EtherChannel fails, the traffic that normally crosses the failed link is moved to the remaining links. EtherChannel links can be static access ports or trunk ports. However, all links to be bundled must have consistent configurations before an EtherChannel can form.
NOTE PAgP sends frames to destination address 01:00:0C:CC:CC:CC, as an 802.2 Subnetwork Access Protocol (SNAP) protocol 0x000C0104. LACP sends frames to destination address 01-80-c2-00-00-02 using protocol 0x8809.
Configuration 1. | (Optional; Catalyst 6000 COS only) Select an EtherChannel protocol for a module:
COS | set channelprotocol {pagp | lacp} mod
| IOS | N/A |
By default, each module uses the PAgP protocol (pagp) for dynamic EtherChannel control. You can use the lacp keyword to select LACP for the module number mod instead, if desired.
TIP PAgP and LACP are not interoperable. Therefore, use the same protocol on the modules and ports at both ends of a potential EtherChannel. | 2. | (Optional) Adjust the STP costs for an EtherChannel.
- a. Set the STP port cost:
COS | set spantree channelcost {channel-id | all} cost
| IOS | N/A |
By default, the STP port cost for an EtherChannel is based on the port cost of the aggregate bandwidth. For example, a single 100 mbps port has a port cost of 19. When two 100 mbps ports are bundled as an FEC, the port cost for 200 mbps is 12. A bundle of four 100 mbps ports gives a port cost of 8 for the 400 mbps bandwidth. Refer to Table 7-1 in Chapter 7, "Spanning Tree Protocol (STP)," for STP port cost values.
You can change the port cost for all EtherChannels by using the all keyword, or a single EtherChannel by giving its channel-id number. To find this index, use the show channel group (PAgP) or show lacp-channel group (LACP) command. The channel-id is a unique number that is automatically assigned to the EtherChannel.
The STP port cost is given as cost (1-65535 in 16-bit "short mode" or 1-4294967296 in 32-bit "long mode"). Refer to section "7-1: STP Operation" for more cost information.
- b. Set the STP port cost per VLAN:
COS | [View full width] set spantree channelvlancost channel_id cost set spantree portvlancost mod/port [cost cost]  [vlan-list]
| IOS | N/A |
Use the set spantree channelvlancost command to allow the port cost per VLAN to be configured for the EtherChannel with channel-id. The STP port cost is set to cost for all VLANs that will be carried over the EtherChannel. Then you should adjust the port cost for specific VLANs by using the set spantree portvlancost command. Refer to section "7-1: STP Operation" for more cost information.
| 3. | (Optional) Use PAgP on an EtherChannel:
TIP When you make configuration changes to add or remove ports from an EtherChannel, be aware of the effects this has on the STP. This is especially important in a live production network where you might cause an interruption of service. STP operates on an EtherChannel as if it were a normal switch port. After ports have been assigned to an EtherChannel, STP moves through its various states to guarantee a loop-free topology. Switch ports within the EtherChannel administrative group can be enabled and disabled without triggering an STP topology change. As a result, the other links in the EtherChannel remain in the STP "forwarding" state. If you attempt to add a new port into an active EtherChannel administrative group, however, an STP topology change is triggered. The same result occurs if you change the administrative group number on an active EtherChannel. You have now reconfigured the logical link, so STP moves the EtherChannel (and all of its ports) back through the "listening" and "learning" states. This interrupts traffic on the EtherChannel for up to 50 seconds. - a. Assign ports to the EtherChannel:
COS | set port channel mod/port [admin-group]
| IOS | N/A |
One or more ports, given by mod/port, are assigned as an EtherChannel. A specific administrative group number admin-group can be given if desired. If this is omitted, the switch automatically assigns these ports a new unique group number. If you do specify a group number and that number is already in use, the new EtherChannel receives the group number and the ports that were previously assigned are moved to a different unique group number.
On an IOS switch, ports are assigned to an EtherChannel group at the same time as the PAgP mode is set. This is done in Step 3b.
TIP You must list all the ports that will belong to the EtherChannel in this one command. To add or delete individual ports from the bundle, reissue this command with an updated list of all the desired ports. - b. Set the PAgP mode:
COS | [View full width] set port channel mod/port mode {on | off | desirable | auto} [silent | non-silent]
| IOS | [View full width] (interface) channel-group number mode {on | auto [non-silent] | desirable [non-silent]}
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The channel is referenced by one of its ports as mod/port (COS) or by selecting the interface and the group number (IOS). You can configure PAgP in one of these modes: on (EtherChannel is used, but no PAgP packets are sent), off (EtherChannel is disabled), desirable (switch is actively willing to form an EtherChannel; PAgP packets are sent), or auto (switch is passively willing to form an EtherChannel; no PAgP packets are sent; the default).
When in auto or desirable mode, PAgP packets are required before an EtherChannel can be negotiated and brought up. However, there might be times when one end of the EtherChannel (a server or network analyzer) doesn't generate PAgP packets or is "silent." You can use the silent keyword (the default) to allow a port to become an EtherChannel with a silent partner after a 15-second delay. Use the non-silent keyword to require PAgP negotiation before bringing the EtherChannel active.
TIP PAgP is not available on the Catalyst 2900 or 3500XL platforms. An EtherChannel on these switches is either "on" or "off" with no negotiation. If you connect an EtherChannel from another switch platform to one of these, make sure that PAgP is not used for negotiation. In other words, use the on mode to force the use of an EtherChannel. - c. (Optional) Choose a load-balancing algorithm:
COS | [View full width] set port channel all distribution {ip | mac | session} [source | destination | both]
| IOS | (global) port-channel load-balance method
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Choose a load-balancing method:
- - Source IP address ip source (COS) or src-ip (IOS)
- - Destination IP address ip destination (COS) or dst-ip (IOS)
- - Source and destination IP ip both (COS) or src-dst-ip (IOS)
- - Source MAC address mac source (COS) or src-mac (IOS)
- - Destination MAC address mac destination (COS) or dst-mac (IOS)
- - Source and destination MAC mac both (COS) or src-dst-mac (IOS)
- - Source port number session source (COS) or src-port (IOS)
- - Destination port number session destination (COS) or dst-port (IOS)
- - Source and destination port session both (COS) or src-dst-port (IOS)
TIP Load balancing based on session or port is available only on the Catalyst 6000 Supervisor Engine 2 with a Policy Feature Card 2 (PFC2) (both COS and IOS).
| 4. | (Optional; COS only) Use LACP on an EtherChannel.
- a. Set the system priority:
COS | set lacp-channel system-priority value
| IOS | N/A |
The system ID is an 8-byte quantity formed from a 2-byte priority of value (1 to 65535; default 32768) followed by the switch MAC address. A lower value means a higher priority. The switch with the lowest overall system ID value can modify which ports are actively participating in an EtherChannel at any given time.
- b. Set the port priority for individual ports:
COS | set port lacp-channel mod/ports port-priority value
| IOS | N/A |
The switch with the lowest system ID value selects ports with the lowest port ID value for use in an EtherChannel. The port ID is made up of a 2-byte priority value (1 to 255, default 128) followed by a 2-byte port number. Ports that cannot be used are placed in a "standby" state; they are held inactive until another link in the EtherChannel fails.
- c. Group ports by setting their administrative keys:
COS | set port lacp-channel mod/ports [admin-key]
| IOS | N/A |
Ports that have the potential to become an EtherChannel should have their administrative key, admin-key (1 to 65535), set to the same value. Up to eight ports can be assigned the same administrative key value. Ports that have a unique value are considered to be individual ports and do not become part of an EtherChannel.
By default, each group of four consecutive ports in a module has the same unique key value. Key values are only locally significant. However, ports with the same key value on one switch can potentially form an EtherChannel with ports sharing another common key value on another switch.
If the admin-key value is not specified, the switch selects an unused, unique value for the ports listed. If you do specify a key value and that value is already in use, the ports already assigned are moved to another unique key value.
- d. Set the EtherChannel mode:
COS | [View full width] set port lacp-channel mod/ports mode {on | off | active | passive}
| IOS | N/A |
LACP can be configured in one of these modes: on (EtherChannel is used, but no LACP packets are sent), off (EtherChannel is disabled), active (switch is actively willing to form an EtherChannel; LACP packets are sent), or passive (switch is passively willing to form an EtherChannel; no LACP packets are sent; the default).
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TIP Although PAgP and LACP are not compatible or interoperable, you can form an EtherChannel between one switch that uses PAgP on a module and another switch that uses LACP on its module. In this case, set the PAgP switch to the on mode and the LACP switch to the on mode. Neither protocol will be used to negotiate an EtherChannel, but the EtherChannel will be formed.
EtherChannel Example Figure 4-1 shows a network diagram for this example. A switch has three linecards with Ethernet ports. Modules 4 and 5 use PAgP to aggregate ports, whereas module 6 uses LACP. One EtherChannel is made up of ports 4/1, 4/2, 5/1, and 5/2, demonstrating that an EtherChannel can be split across multiple linecards. This EtherChannel uses PAgP in the desirable mode to dynamically bundle the ports together. The nonsilent mode requires a PAgP speaker on the far end before the EtherChannel will be built. Both source and destination IP addresses distribute traffic across the bundled ports. Figure 4-1. Network Diagram for the EtherChannel Example 
A second EtherChannel is configured to use LACP. The LACP system priority is set to 8192 so that this switch will become the higher-priority decision maker. Ports 6/1, 6/2, 6/7, and 6/8 all belong to LACP administrative key 101, forming a common aggregate link. Ports 6/1 and 6/2 are given a port priority of 100, which is less than the default 128. These ports are used in the LACP bundle first. If for some reason ports 6/7 or 6/8 are not able to be used in the EtherChannel, they are placed in a "standby" state and used if another port fails. Each of the bundled ports is put into the active LACP mode and is willing to initiate an EtherChannel with the far-end switch: COS | set channelprotocol pagp 4 set channelprotocol pagp 5 set channelprotocol lacp 6 set port channel 4/1-2,5/1-2 set port channel 4/1 mode desirable non-silent set port channel all distribution ip both set lacp-channel system-priority 8192 set port lacp-channel 6/1-2 port-priority 100 set port lacp-channel 6/1-2,6/7-8 101 set port lacp-channel 6/1-2,6/7-8 mode active
| IOS | [View full width] (global) interface fastethernet 4/1 (interface) channel-group 100 mode desirable non-silent (global) interface fastethernet 4/2 (interface) channel-group 100 mode desirable non-silent (global) interface fastethernet 5/1 (interface) channel-group 100 mode desirable non-silent (global) interface fastethernet 5/2 (interface) channel-group 100 mode desirable non-silent (global) port-channel load-balance src-dst-ip
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Displaying Information About EtherChannels Table 4-3 lists some switch commands that you can use to display helpful information about EtherChannel links. Table 4-3. Switch Commands to Display EtherChannel Link InformationDisplay Function | Switch OS | Command |
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EtherChannel protocol used on each module | COS | show channelprotocol
| IOS | N/A | EtherChannel capabilities on a module | COS | show port capabilities [mod[/port]]
| IOS | N/A | EtherChannel ID numbers | COS | show channel group admin-group
-OR- show lacp-channel group admin-key
| IOS | N/A | EtherChannel load balancing | COS | show channel [channel-id] info
| IOS | (exec) show etherchannel [channel-group] load-balance
| EtherChannel traffic utilization | COS | show channel traffic [channel-id]
-OR- show lacp-channel traffic [channel-id]
| IOS | (exec) show pagp [group-number] counters
| Outgoing port for an address or port number | COS | show channel hash channel-id {src | dest | src dest}
-OR- [View full width] show lacp-channel hash channel-id {src | dest | src dest}
| IOS | N/A |
TIP If you are debugging an EtherChannel that will not form, remember that all ports with the bundle must have the same attributes. For example, all the ports should have the same speed, duplex, VLAN (or native VLAN for a trunk), trunk mode, trunk encapsulation, allowed VLAN range, and so on. The commands listed in Table 4-3 provide a great deal of information about EtherChannels that are already formed. To be sure that the ports are all configured consistently, use other show commands that display the port attributes . Beyond that, you sometimes have to resort to looking through the switch configuration to spot port configurations that are not identical.
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