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2012-03-01T03:23:00.000-08:00

Understanding the EtherChannel

2012-02-01T01:26:00.000-08:00

The EtherChannel is composed of individual Fast Ethernet or Gigabit Ethernet links bundled into a
single logical link as shown in Figure 10-1. The EtherChannel provides full-duplex bandwidth up to
800 Mbps (Fast EtherChannel) or 2 Gbps (Gigabit EtherChannel) between your switch and another
switch or host.

Note The network device to which your switch is connected can impose its own limits on the number of
interfaces in the EtherChannel. For Catalyst 2950 switches, the number of EtherChannels is limited to
six with eight ports per EtherChannel.

Each EtherChannel can consist of up to eight compatibly configured Ethernet interfaces. All interfaces
in each EtherChannel must be the same speed, and all must be configured as Layer 2 interfaces.
If a link within an EtherChannel fails, traffic previously carried over that failed link changes to the
remaining links within the EtherChannel. A trap is sent for a failure, identifying the switch, the
EtherChannel, and the failed link. Inbound broadcast and multicast packets on one link in an
EtherChannel are blocked from returning on any other link of the EtherChannel.

Understanding Port-Channel Interfaces

When you create an EtherChannel for Layer 2 interfaces, a logical interface is dynamically created. You
then manually assign an interface to the EtherChannel by using the channel-group interface
configuration command as shown in Figure 10-2.
Each EtherChannel has a logical port-channel interface numbered from 1 to 6.

After you configure an EtherChannel, configuration changes applied to the port-channel interface apply
to all the physical interfaces assigned to the port-channel interface. Configuration changes applied to the
physical interface affect only the interface where you apply the configuration. To change the parameters
of all ports in an EtherChannel, apply configuration commands to the port-channel interface, for
example, Spanning Tree Protocol (STP) commands or commands to configure a Layer 2 EtherChannel
as a trunk.

Understanding the Port Aggregation Protocol
The Port Aggregation Protocol (PAgP) facilitates the automatic creation of EtherChannels by
exchanging packets between Ethernet interfaces. By using PAgP, the switch learns the identity of
partners capable of supporting PAgP and learns the capabilities of each interface. It then dynamically
groups similarly configured interfaces into a single logical link (channel or aggregate port); these
interfaces are grouped based on hardware, administrative, and port parameter constraints. For example,
PAgP groups the interfaces with the same speed, duplex mode, native VLAN, VLAN range, and trunking
status and type. After grouping the links into an EtherChannel, PAgP adds the group to the spanning tree
as a single switch port.

PAgP Modes
Table 10-1 shows the user-configurable EtherChannel modes for the channel-group interface
configuration command: on, auto, and desirable. Switch interfaces exchange PAgP packets only with
partner interfaces configured in the auto or desirable modes; interfaces configured in the on mode do
not exchange PAgP packets.

Table 10-1 EtherChannel Modes
Mode Description
auto
Places an interface into a passive negotiating state, in which the interface responds to PAgP packets it receives but does not initiate PAgP packet negotiation. This setting minimizes
the transmission of PAgP packets.

desirable
Places an interface into an active negotiating state, in which the interface initiates
negotiations with other interfaces by sending PAgP packets.

on
Forces the interface to channel without PAgP. With the on mode, a usable EtherChannel
exists only when an interface group in the on mode is connected to another interface group
in the on mode.

Both the auto and desirable modes allow interfaces to negotiate with partner interfaces to determine if
they can form an EtherChannel based on criteria such as interface speed and, for Layer 2 EtherChannels,
trunking state and VLAN numbers.
Interfaces can form an EtherChannel when they are in different PAgP modes as long as the modes are
compatible. For example:
• An interface in desirable mode can form an EtherChannel with another interface that is in desirable
or auto mode.
• An interface in auto mode can form an EtherChannel with another interface in desirable mode.
• An interface in auto mode cannot form an EtherChannel with another interface that is also in auto
mode because neither interface initiates PAgP negotiation.
An interface in the on mode that is added to a port channel is forced to have the same characteristics as
the already existing on mode interfaces in the channel.

Caution:You should exercise care when setting the mode to on (manual configuration). All ports configured in
the on mode are bundled in the same group and are forced to have similar characteristics. If the group is
misconfigured, packet loss or STP loops might occur.

If your switch is connected to a partner that is PAgP-capable, you can configure the switch interface for
nonsilent operation by using the non-silent keyword. If you do not specify non-silent with the auto or
desirable mode, silent mode is assumed.
The silent mode is used when the switch is connected to a device that is not PAgP-capable and seldom,
if ever, transmits packets. An example of a silent partner is a file server or a packet analyzer that is not
generating traffic. In this case, running PAgP on a physical port connected to a silent partner prevents
that switch port from ever becoming operational; however, the silent setting allows PAgP to operate, to
attach the interface to a channel group, and to use the interface for transmission.
Physical Learners and Aggregate-Port Learners
Network devices are classified as PAgP physical learners or aggregate-port learners. A device is a
physical learner if it learns addresses by physical ports and directs transmissions based on that learning.
A device is an aggregate-port learner if it learns addresses by aggregate (logical) ports.

When a device and its partner are both aggregate-port learners, they learn the address on the logical
port-channel. The device transmits packets to the source by using any of the interfaces in the
EtherChannel. With aggregate-port learning, it is not important on which physical port the packet
arrives.
The Catalyst 2950 switch uses source-MAC address distribution for a channel if it is connected to a
physical learner even if the user configures destination-MAC address distribution.
These frame distribution mechanisms are possible for frame transmission:
• Port selection based on the source-MAC address of the packet
• Port selection based on the destination- MAC address of the packet
Catalyst 2950 switches support a maximum of eight ports to a PAgP group.

PAgP Interaction with Other Features
The Dynamic Trunking Protocol (DTP) and Cisco Discovery Protocol (CDP) send and receive packets
over the physical interfaces in the EtherChannel. Trunk ports send and receive PAgP protocol data units
(PDUs) on the lowest numbered VLAN.
STP sends packets over a single physical interface in the EtherChannel. Spanning tree regards the
EtherChannel as one port.
PAgP sends and receives PAgP PDUs only from interfaces that are up and have PAgP enabled for auto
or desirable modes.

Understanding Load Balancing and Forwarding Methods
EtherChannel balances the traffic load across the links in a channel by reducing part of the binary pattern
formed from the addresses in the frame to a numerical value that selects 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, they 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 (and the MAC address learned by the switch
does not change).
With destination-MAC address forwarding, when packets are forwarded to an EtherChannel, they are
distributed across the ports in the channel based on the destination host’s 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. You configure the load balancing and
forwarding method by using the port-channel load-balance global configuration command.
In Figure 10-3, an EtherChannel of four workstations communicates with a router. Because the router is
a single-MAC-address device, source-based forwarding on the switch EtherChannel ensures that the
switch uses all available bandwidth to the router. The router is configured for destination-based
forwarding because the large number of workstations ensures that the traffic is evenly distributed from
the router EtherChannel.
Use the option that provides the greatest variety in your configuration. For example, if the traffic on a
channel is going only to a single MAC address, using the destination-MAC address always chooses the
same link in the channel; using source addresses might result in better load balancing.

Table 10-2 Default EtherChannel Configuration

Feature Default Setting

Channel groups None assigned.
PAgP mode No default.
PAgP learn method Aggregate-port learning on all interfaces.
PAgP priority 128 on all interfaces. (Changing this value on Catalyst 2950 switches has no effect.)
Load balancing Load distribution on the switch is based on the source-MAC address of the incoming packet.

EtherChannel Configuration Guidelines
If improperly configured, some EtherChannel interfaces are automatically disabled to avoid network
loops and other problems. Follow these guidelines to avoid configuration problems:
• Each EtherChannel can have up to eight compatibly configured Ethernet interfaces.
• Configure all interfaces in an EtherChannel to operate at the same speeds and duplex modes.
• Enable all interfaces in an EtherChannel. If you shut down an interface in an EtherChannel, it is
treated as a link failure, and its traffic is transferred to one of the remaining interfaces in the
EtherChannel.
• When a group is first created, all ports follow the parameters set for the first port to be added to the
group. If you change the configuration of one of these parameters, you must also make the changes
to all ports in the group:
– Allowed-VLAN list
– STP path cost for each VLAN
– STP port priority for each VLAN
– STP Port Fast setting
• If you configure SPAN on a port that is a member of the EtherChannel, it leaves the EtherChannel.
• For EtherChannels:
– Assign all interfaces in the EtherChannel to the same VLAN, or configure them as trunks.
– If you configure an EtherChannel from trunk interfaces, verify that the trunking mode (802.1Q)
is the same on all the trunks. Inconsistent trunk modes on EtherChannel interfaces can have
unexpected results.
– An EtherChannel supports the same allowed range of VLANs on all the interfaces in a trunking
Layer 2 EtherChannel. If the allowed range of VLANs is not the same, the interfaces do not
form an EtherChannel even when PAgP is set to the auto or desirable mode.
– Interfaces with different STP path costs can form an EtherChannel as long they are otherwise
compatibly configured. Setting different STP path costs does not, by itself, make interfaces
incompatible for the formation of an EtherChannel.

Configuring EtherChannels
You configure Layer 2 EtherChannels by configuring the Ethernet interfaces with the channel-group
interface configuration command, which creates the port-channel logical interface.
Note Layer 2 interfaces must be connected and functioning for IOS to create port-channel interfaces.

Step 1 configure terminal Enter global configuration mode.

Step 2 interface interface-id Enter interface configuration mode, and specify a physical
interface to configure.Valid interfaces include physical interfaces.Up to eight interfaces of the same type and speed can be configured for the same group.

Step 3 channel-group channel-group-number mode {auto [non-silent] | desirable [non-silent] | on}
Assign the interface to a channel group, and specify the PAgP mode. The default mode is auto silent.For channel-group-number, the range is 1 to 6. Each EtherChannel can have up to eight compatibly configured Ethernet interfaces. For mode, select one of these keywords:

• auto—Enables PAgP only if a PAgP device is detected. It

places an interface into a passive negotiating state, in which

the interface responds to PAgP packets it receives but does

not initiate PAgP packet negotiation.

• desirable—Unconditionally enables PAgP. It places an

interface into an active negotiating state, in which the

interface initiates negotiations with other interfaces by

sending PAgP packets.

• on—Forces the interface to channel without PAgP. With the

on mode, a usable EtherChannel exists only when an

interface group in the on mode is connected to another

interface group in the on mode.

• non-silent—If your switch is connected to a partner that is

PAgP-capable, you can configure the switch interface for

non-silent operation. You can configure an interface with the

non-silent keyword for use with the auto or desirable mode.

If you do not specify non-silent with the auto or desirable

mode, silent is assumed. The silent setting is for connections

to file servers or packet analyzers; this setting allows PAgP

to operate, to attach the interface to a channel group, and to

use the interface for transmission.

For information on compatible PAgP modes for the switch and its

partner, see the “PAgP Modes” section on page 10-10.

Step 4 end Return to privileged EXEC mode.
Step 5 show running-config Verify your entries.
Step 6 copy running-config startup-config (Optional) Save your entries in the configuration file.

If you delete the EtherChannel is by using the no interface port-channel global configuration command
without removing the physical interfaces, the physical interfaces are shutdown. If you do not want the
member physical interfaces to shut down, remove the physical interfaces before deleting the
EtherChannel.
To remove an interface from the EtherChannel group, use the no channel-group interface configuration
command.
This example shows how to assign Gigabit Ethernet interfaces 0/1 and 0/2 with PAgP mode desirable:
Switch# configure terminal
Switch(config)# interface range gigabitethernet0/1 -2
Switch(config-if)# channel-group 5 mode desirable
Switch(config-if)# end

Configuring EtherChannel Load Balancing
This section describes how to configure EtherChannel load balancing by using source-based or
destination-based forwarding methods. For more information, see the “Understanding Load Balancing
and Forwarding Methods” section on page 10-12.
Beginning in privileged EXEC mode, follow these steps to configure EtherChannel load balancing:
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 port-channel load-balance {dst-mac | src-mac} Configure an EtherChannel load-balancing method.
The default is src-mac.
Select one of these keywords to determine the load-distribution
method:

• dst-mac—Load distribution is based on the destination-host

MAC address of the incoming packet. Packets to the same

destination are sent on the same port, but packets to different

destinations are sent on different ports in the channel.

• src-mac—Load distribution is based on the source-MAC

address of the incoming packet. Packets from different hosts

use different ports in the channel, but packets from the same

host use the same port in the channel.

If the link partner to the switch is a physical learner, set the

load-distribution method to one of these ways:

• If the channel-group interface configuration command is

set to auto or desirable, the switch automatically uses the

load distribution method based on the source-MAC address,

regardless of the configured load-distribution method.

• If the channel-group interface configuration command is

set to on, set the load-distribution method based on the

source-MAC address by using the port-channel

load-balance src-mac global configuration command.

Step 3 end Return to privileged EXEC mode.
Step 4 show etherchannel load-balance Verify your entries.
Step 5 copy running-config startup-config (Optional) Save your entries in the configuration file.

To return EtherChannel load balancing to the default configuration, use the no port-channel
load-balance global configuration command.

Configuring the PAgP Learn Method and Priority
Network devices are classified as PAgP physical learners or aggregate-port learners. A device is a
physical learner if it learns addresses by physical ports and directs transmissions based on that
knowledge. A device is an aggregate-port learner if it learns addresses by aggregate ports.
For compatibility with Catalyst 1900 series switches, configure the Catalyst 2950 switches for
souce-MAC load distribution.
The Catalyst 2950 supports address learning only on aggregate ports even though the physical-port
keyword is provided in the command-line interface (CLI). The pagp learn-method and the pagp
port-priority interface configuration command have no effect on the switch hardware.
Note You should not set the learn method to physical-port because the switch is an aggregate-learning device.
If the link partner to the switch is a physical learner that has the channel-group interface configuration
command set to auto or desirable, the switch automatically uses the load-distribution method based on
the source MAC address, regardless of the configured load distribution method.
If the link partner to the Catalyst 2950 switch is a physical learner that has the channel-group interface
configuration command set to on, set the load-distribution method based on the source MAC address by
using the port-channel load-balance src-mac global configuration command.

Basic Port Security

2012-02-01T00:42:00.000-08:00

Beginning in privileged EXEC mode, follow these steps to enable port security: Command Purpose

Step 1 configure terminal Enter global configuration mode.

Step 2 interface interface Enter interface configuration mode for the port you want to secure.

Step 3 switchport port-security Enable basic port security on the interface.

Step 4 switchport port-security maximum max_addrsSet the maximum number of MAC addresses that is allowed on this interface.

Step 5 switchport port-security violation {shutdown | restrict | protect}Set the security violation mode for the interface.The default is shutdown.For mode, select one of these keywords:

• shutdown—The interface is shut down immediately following

a security violation.

• restrict—A security violation sends a trap to the network

management station.

• protect—When the port secure addresses reach the allowed

limit on the port, all packets with unknown addresses are

dropped.

Step 6 end Return to privileged EXEC mode.

Step 7 show port security [interface interface-id | address] Verify the entry.

Disabling Port Security: no switchport port-security

Note: If the port link goes down, all the dynamically learned addresses are removed.

Enabling Port Security

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