Catalyst 4500/4000 Series
Catalyst OS
In the Catalyst 4500/4000 series switches with CatOS (Supervisor Engine I and II), you can form an EtherChannel with up to eight compatibly configured Fast Ethernet or Gigabit Ethernet ports on the switch. The exact EtherChannel formation depends on the hardware. Because the spanning tree feature handles the port ID, the maximum number of channels is 126 for a six-slot chassis. In addition, you can configure an EtherChannel with the use of ports from multiple modules in CatOS release 5.x and later. All ports in an EtherChannel must be the same speed.
Catalyst OS for Catalyst 4500/4000 uses MAC address based load balancing. EtherChannel distributes frames across the links in a channel based on the low-order bits of the source and destination MAC addresses of each frame. The frame distribution method is not configurable.
Cisco IOS
A Catalyst 4500/4000 series switch with Cisco IOS Software (Supervisor Engine II+ and later) supports a maximum of 64 EtherChannels. You can form an EtherChannel with up to eight compatibly configured Ethernet interfaces on any module and across modules. All interfaces in each EtherChannel must be the same speed, and you must configure all the interfaces as either Layer 2 or Layer 3 interfaces.
EtherChannel reduces part of the binary pattern that is formed from the addresses in the frame to a numerical value that selects one of the links in the channel in order to balance the traffic load across the links in a channel. EtherChannel load balancing can use MAC addresses, IP addresses, or Layer 4 port numbers and either source mode, destination mode, or both. Use the option that provides the greatest variety in your configuration. For example, if the traffic on a channel only goes to a single MAC address, use of the destination MAC address results in the choice of the same link in the channel each time. Use of source or IP addresses can result in a better load balance. Issue the port-channel load-balance {src-mac | dst-mac | src-dst-mac | src-ip | dst-ip | src-dst-ip | src-port | dst-port | src-dst-port} global configuration command in order to configure load balancing. Load Balance must be configured globally and the load balancing option cannot be changed on a per port basis.
Note: The switch uses the lower order bits of source MAC address and destination MAC address in order to determine which links must be used to transmit the data. So if the data is received from the same source, then same link of the EtherChannel is used in order to forward the data.
Catalyst 2900XL/3500XL Series
A Catalyst 2900XL that runs a Cisco IOS software release that is earlier than Cisco IOS Software Release 11.2(8)SA3 chooses a link in the channel based on the link on which the destination MAC address was last heard. The software dynamically reallocates this address to another link in the channel if the link on which the address was learned is busier than the others. You can configure a Catalyst 2900XL that runs Cisco IOS Software Release 11.2(8)SA3 or later and a Catalyst 3500XL that runs Cisco IOS Software Release 11.2(8)SA6 or later in order to choose a link to be sent across the Fast EtherChannel. The switch chooses the link on the basis of the destination or source MAC address of the frame. The default is to use the source MAC address. This default means that all packets that the switch receives on a non-Fast EtherChannel port with the same MAC source address that have a destination of the MAC addresses on the other side of the channel take the same link in the channel. Use source-based forwarding when many stations that are attached to the Catalyst 2900XL/3500XL send to a few stations, such as a single router, on the other side of the Fast EtherChannel. The use of source-based forwarding in this situation evenly distributes traffic across all links in the channel. Also, the Catalyst 2900XL/3500XL switches maintain a notion of a default port on which to transmit traffic, such as Spanning Tree Protocol (STP), multicasts, and unknown unicasts.
Catalyst 3750/3560
The Catalyst 3750/3560 series switch can support up to eight compatibly configured Ethernet interfaces in an EtherChannel. The EtherChannel provides full-duplex bandwidth up to 800 Mbps (Fast EtherChannel) or 8 Gbps (Gigabit EtherChannel) between your switch and another switch or host. With Cisco IOS Software Release 12.2(20)SE and earlier, the number of EtherChannels has a limit of 12. With Cisco IOS Software Release 12.2(25)SE and later, the number of EtherChannels has a limit of 48.
EtherChannel balances the traffic load across the links in a channel through the reduction of part of the binary pattern that the addresses in the frame form to a numerical value that selects one of the links in the channel. EtherChannel load balancing can use MAC addresses or IP addresses, source or destination addresses, or both source and destination addresses. The mode applies to all EtherChannels that are configured on the switch. You configure the load balancing and forwarding method with use of the port-channel load-balance {dst-ip | dst-mac | src-dst-ip | src-dst-mac | src-ip | src-mac} global configuration command.
You can find out which interface is used in the EtherChannel to forward traffic based on the load balancing method. The command for this determination is test etherchannel load-balance interface port-channel number {ip | mac} [source_ip_add | source_mac_add] [dest_ip_add | dest_mac_add].
Catalyst 2950/2955/3550
The Catalyst 2950/2955 series switch can support up to eight compatibly configured Ethernet interfaces in an EtherChannel. The EtherChannel can provide full-duplex bandwidth up to 800 Mbps (Fast EtherChannel) or 2 Gbps (Gigabit EtherChannel) between your switch and another switch or host. The number of EtherChannels has the limit of six with eight ports per EtherChannel.
The Catalyst 3550 series switches support both Layer 2 and Layer 3 EtherChannel, with up to eight compatibly configured Ethernet interfaces. The EtherChannel provides full-duplex bandwidth up to 800 Mbps (Fast EtherChannel) or 8 Gbps (Gigabit EtherChannel) between your switch and another switch or host. The limit of the number of EtherChannels is the number of ports of the same type.
For the 2950/2955/3550 series switch, EtherChannel balances the traffic load across the links in a channel by randomly associating a newly learned MAC address with one of the links in the channel. EtherChannel load balancing can use either source-MAC or destination-MAC address forwarding.
With source-MAC address forwarding, when packets are forwarded to an EtherChannel, the packets are distributed across the ports in the channel based on the source-MAC address of the incoming packet. Therefore, to provide load balancing, packets from different hosts use different ports in the channel, but packets from the same host use the same port in the channel. With destination-MAC address forwarding, when packets are forwarded to an EtherChannel, the packets are distributed across the ports in the channel based on the destination host MAC address of the incoming packet. Therefore, packets to the same destination are forwarded over the same port, and packets to a different destination are sent on a different port in the channel.
For the 3550 series switch, when source-MAC address forwarding is used, load distribution based on the source and destination IP address is also enabled for routed IP traffic. All routed IP traffic chooses a port based on the source and destination IP address. Packets between two IP hosts always use the same port in the channel, and traffic between any other pair of hosts can use a different port in the channel.
Issue the port-channel load-balance {dst-mac | src-mac} global configuration command in order to configure the load-balance and forward method.
Note: The default port is used to transmit traffic, such as Spanning Tree Protocol (STP), multicasts, and unknown unicasts. The default port can be identified from the output of the command show etherchannel summary by a notation of d.
Catalyst 1900/2820
With the enablement of PAgP, the two possible methods of link determination are preserve order and maximize load balancing between the links on the Fast EtherChannel. The What Is PAgP and Where Do You Use It? section of this document describes PAgP. The default is to maximize load balancing. PAgP is used to negotiate the configured method with the device at the other side of the channel. If preserve order is configured, the device at the other side is instructed in order to use source-based transmissions so that the Catalyst 1900/2820 always receives packets with the same source MAC address on the same link in the channel. This is the link that the Catalyst 1900/2820 always uses to send traffic to this MAC address. If maximize load balancing is configured, PAgP tells the other side that it can distribute traffic arbitrarily, and unicast traffic is transmitted by the Catalyst 1900/2820 on the link where the source address was last seen. This provides the maximum possible load-balancing configuration. When Fast EtherChannel is configured with PAgP disabled, the switch cannot negotiate with the partner about the switch learning capability. Whether the switch preserves frame ordering depends on whether the Fast EtherChannel partner performs source-based distribution. The Catalyst 1900/2820s also elect an active port. The active port is used for flooded traffic such as unknown unicast, unregistered multicast, and broadcast packets. If the port-channel mode is on (PAgP disabled), the active port is the link with the highest priority value. If the mode is desirable or auto (PAgP enabled), the active port is selected based on the priority of the links on the switch that has the higher Ethernet address. When two ports on the switch with the higher Ethernet address have the same priority, the port with the lower ifIndex is selected.
Catalyst 2948G-L3/4908G-L3 and Catalyst 8500
When one link fails, all traffic that previously used that link now uses the link next to it. For example, if Link 1 fails in a bundle, traffic that previously used Link 1 before the failure now uses Link 2.
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