EtherChannels

EtherChannels allow you to bundle multiple ports for redundancy and increased bandwidth.
Figure 12-9 shows how you connect two switches with a channel. Each switch considers
the four ports together as a single port with four times the throughput capacity. (If these
were gigabit ports, you’d end up with a 4 Gb port.)


The benefits of channeling are
• Subsecond convergence for link failures—If you lose any of the links in the
channel, the switch detects the failure and distributes the traffic on the remaining
links.
• Increased bandwidth—Each port-channel link has as much bandwidth as the sum of
the bundled links.

TIP 
For maximum availability, you should bundle ports that belong to different line cards. In
case one line card breaks down, you can still carry the traffic on the remaining links.
Before doing so, make sure that the switch you are using supports channeling across line
cards.

It is apparent by now that the key feature which determines the effectiveness of channeling
is the load-balancing algorithm that distributes the traffic on the available links. You need
to read the specific product documentation to understand the options in distributing the traffic.
Typically, they consist of a hashing algorithm that involves the source and destination MAC
addresses or a similar algorithm applied on the source and destination IP addresses.

Creating a Channel
This section describes the configuration using LACP as the negotiation protocol, which
provides equivalent capabilities to the PAgP protocol. LACP allows the port to successfully
form a channel only if the trunking configuration and the port speed match on both sides of
the links.

Example 12-15 shows how to configure one of the two switches: You assign the ports that
need to form the bundle to the same group (in Example 12-15, group 2). The command
channel-protocol lacp enables the use of LACP as the protocol for the negotiation.
You configure the ports on the local switch to behave as active; you configure the ports on
the remote switch to behave either as active or as passive.


Example 12-15 EtherChannel Configuration
interface GigabitEthernet4/1
switchport
channel-group 2 mode active
channel-protocol lacp
end
!
interface GigabitEthernet5/1
switchport
channel-group 2 mode active
channel-protocol lacp
end

Once the channel forms, you can configure its switching parameters from the interface Portchannel
, which in Example 12-15 is group 2. For example, if you need to configure
trunking on top of the channel, you can use the interface Port-channel 2 and the configurations
are replicated on the bundled ports.