Fast Ether Channel

Fast EtherChannel (FEC) is a Cisco method that bundles 100-Mbps Fast Ethernet ports into a logical link. The existence of any redundant paths between two switches results in some ports being in a blocking state, thus reducing available bandwidth.

Figure 1-4 displays a switched network with two 100-Mbps connections between them. Because of STP, one of the links (Switch A, in this case) will be in a blocking state after the election of a root bridge. Switch B will block one of the paths to ensure that only one path exists to the root bridge (Switch A). To purchase and enable a Fast Ethernet port is expensive, and to have it sitting in an idle position means wasted resources, so Cisco developed a method that enables Fast Ethernet ports to be bundled together and used concurrently (in other words, cheating spanning tree into believing that the two ports are one to send data from Switch A to Switch B with two 100-Mbps links instead of one).

Figure 1-4 Spanning Tree Loop Avoidance

Switch A Switch B

Set spantree priority 0 Default priority 32768

Forwarding Forwarding 1/1 A

Forwarding Blocking 1/2 A

One port will block on Switch B to avoid loop to root bridge.

To enable Fast EtherChannel, follow these steps:

Step 1 Set to the same speed all ports that are part of FEC. Step 2 Configure all ports so that they belong to the same VLAN.

Step 3 Set duplex to be the same, either half or full, not a mixture. Step 4 Bundle up to eight ports together.

Step 5 To set Fast EtherChannel on a switch, use the CatOS syntax set port channel.

Step 6 To set Fast EtherChannel on a router, use the Cisco IOS syntax channel-group under the Fast Ethernet interface.

Step 7 Configure up to four FEC groups per switch. This limit could change with future Catalyst releases.

For Cisco IOS-based switches, fewer steps are required.

Step 1 Create the port-channel interface. The number can be from 1 to 64 (256 with Release 12.1(2)E and earlier).

Router(config)# interface port-channel port_channel_number Step 2 Assign an IP address and subnet mask to the EtherChannel. Router(config-if)# ip address ip_address mask

NOTE A group of bundled ports running FEC is commonly known as a trunk. In switching terms, a trunk is a physical and logical connection between two switches. A trunk, for example, can carry multiple VLANs.

Inter-Switch Link (ISL) is a Cisco proprietary protocol that maintains VLAN information as traffic flows between switches and routers. ISL allows members of one VLAN to be located on any given switch. 802.1Q is an IEEE standard for trunking. You can use IEEE 802.1q in a multivendor environment. Be aware that not all Cisco switches support ISL encapsulation. For example, the 2850 only supports 802.1q.

Figure 1-5 displays the logical link when FEC is enabled between Switch A and Switch B.

Figure 1-5 FEC: Logical Link or Trunk-Enabled

Set port channel 1/1 on Set port channel 1/2 on

Set port channel 1/1 on Set port channel 1/2 on

Figure 1-5 FEC: Logical Link or Trunk-Enabled

Set port channel 1/1 on Set port channel 1/2 on

Set port channel 1/1 on Set port channel 1/2 on

Ports are bundled together; effective bandwidth now up to 400 Mbps at full duplex instead of 200 Mbps.

Both ports forwarding now when FEC is configured.

Ports are bundled together; effective bandwidth now up to 400 Mbps at full duplex instead of 200 Mbps.

Both ports forwarding now when FEC is configured.

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