LABORATORY WORK NO.

10
LINK AGGREGATION AND ETHERCHANNEL

1. Objectives

The objective of this work is link aggregation description, operation, configuration,

verification and troubleshooting.

2. Theoretical considerations

2.1 EtherChannel

Link aggregation is the ability to create one logical link using multiple physical links between
two devices. EtherChannel is a form of link aggregation used in switched networks (figure
10.1). This allows for redundancy and higher bandwidth through load sharing among the
physical links. EtherChannel creates a one-to-one relationship; that is, one EtherChannel link
connects only two devices. An EtherChannel link can be created between two switches or an
EtherChannel link can be created between an EtherChannel-enabled server and a switch.

Figure 10.1 EtherChannel

EtherChannel technology was originally developed by Cisco as a LAN switch-to-switch

technique of grouping several physical ports into one logical channel. When an EtherChannel
is configured, the resulting virtual interface is called a port channel. The physical interfaces
are bundled together into a port channel interface (figure 10.2). Most configuration tasks can
be done on the EtherChannel interface instead of on each individual port, ensuring
configuration consistency throughout the links.
 COMPUTER NETWORKS

Figure 10.2 Port channel interface

EtherChannel relies on existing switch ports. There is no need to upgrade the link to a faster
and more expensive connection to have more bandwidth.

Load balancing takes place between links that are part of the same EtherChannel. Depending
on the hardware platform, one or more load-balancing methods can be implemented. These
methods include source MAC to destination MAC load balancing, or source IP to destination
IP load balancing, across the physical links.

EtherChannel creates an aggregation that is seen as one logical link. Where there is only one
EtherChannel link, all physical links in the EtherChannel are active because STP sees only
one logical link.

EtherChannel provides redundancy because the overall link is seen as one logical connection.
Additionally, the loss of one physical link within the channel does not create a change in the
topology; therefore a spanning-tree recalculation is not required. Assuming at least one
physical link is present, the EtherChannel remains functional, even if its overall throughput
decreases because of a lost link within the EtherChannel. The spanning-tree cost is calculated
based on the number of ports assigned to the port-channel and it does not dynamically change
when links go down or are brought back up within the port-channel. Spanning-Tree Protocol
calculates the shortest path network based on cumulative link costs. Link costs are based on
the speed of the link. Some of the link costs for links specified in the IEEE 802.1d
specification are presented in table 10.1.

2
 LINK AGGREGATION AND ETHERCHANNEL

Table 10.1

Link Speed Cost (Short mode – 16bit)

10Mbps 100

100Mbps 19

Two-port * 100Mbps EtherChannel 9

Three-port * 100Mbps EtherChannel 8

Four-port * 100Mbps EtherChannel 7

Five-port * 100Mbps EtherChannel 6

Six-port * 100Mbps EtherChannel 5

Seven-port * 100Mbps EtherChannel 5

Eight-port * 100Mbps EtherChannel 5

1Gbps 4

Two-port * 1Gbps EtherChannel 3

Three-port * 1Gbps EtherChannel 2

Four-port * 1Gbps EtherChannel 2

Five-port * 1Gbps EtherChannel 2

Six-port * 1Gbps EtherChannel 2

Seven-port * 1Gbps EtherChannel 2

Eight-port * 1Gbps EtherChannel 1

10Gbps 2

Two-port * 10Gbps EtherChannel 1

Interface types cannot be mixed; they must be compatibly-configured Ethernet ports. The
individual EtherChannel group member port configuration must be consistent on both
devices. If the physical ports of one side are configured as trunks, the physical ports of the
other side must also be configured as trunks within the same native VLAN. Additionally, all

3
 COMPUTER NETWORKS

ports in each EtherChannel link must be configured as Layer 2 ports. Each EtherChannel has
a logical port channel interface. A configuration applied to the port channel interface affects
all physical interfaces that are assigned to that interface. Layer 3 EtherChannels can be
configured on Cisco Catalyst multilayer switches. A Layer 3 EtherChannel has a single IP
address associated with the logical aggregation of switch ports in the EtherChannel.

The maximum number of physical ports in an EtherChannel link depends on the switch
hardware platform and IOS version. Usually each EtherChannel can consist of up to 8
compatibly-configured Ethernet ports.

The maximum number of EtherChannels supported by a switch depends on the hardware

platform and IOS version. The Cisco IOS switch can usually supports 6 EtherChannels.

EtherChannel can be configured static, unconditional or it can be formed through negotiation

using one of two protocols: Port Aggregation Protocol (PAgP) or Link Aggregation Control
Protocol (LACP). These protocols allow ports with similar characteristics to form a channel
through dynamic negotiation with adjoining switches.

PAgP is a Cisco-proprietary protocol that aids in the automatic creation and management of
EtherChannel links. There are three modes for PAgP: on, desirable and auto. The on mode
forces the interface to channel without PAgP. The desirable mode places an interface in an
active negotiating state in which the interface initiates negotiations with other interfaces by
sending PAgP packets. The auto mode places an interface in a passive negotiating state in
which the interface responds to the PAgP packets that it receives, but does not initiate PAgP
negotiation. Figure 10.3 presents the channel establishment when ports of switches S1 and S2
are in the different modes for PAgP.

S1 S2 Channel Establishment

On On Yes

Auto/Desirable Desirable Yes

On/Auto/Desirable Not Configured No

On Desirable No

Auto/On Auto No

Figure 10.3 Channel establishment with PAgP

LACP is part of an IEEE specification (802.3ad) that allows several physical ports to be
bundled to form a single logical channel. LACP is also defined in IEEE 802.1AX standard
for local and metropolitan area networks. LACP allows a switch to negotiate an automatic
bundle by sending LACP packets to the peer. It performs a function similar to PAgP. Because

4
 LINK AGGREGATION AND ETHERCHANNEL

LACP is an IEEE standard, it can be used to facilitate EtherChannels in multivendor

environments. There are three modes for LACP: on, active and passive. The on mode forces
the interface to channel without LACP. The active mode places a port in an active negotiating
state in which the port initiates negotiations with other ports by sending LACP packets. The
passive mode places a port in a passive negotiating state in which the port responds to the
LACP packets that it receives, but does not initiate LACP packet negotiation. Figure 10.4
presents the channel establishment when ports of switches S1 and S2 are in the different
modes for LACP.

S1 S2 Channel Establishment

On On Yes

Active/Pasive Active Yes

On/Active/Passive Not Configured No

On Active No

Passive/On Passive No

Figure 10.4 Channel establishment with LACP

3. Lab activity

3.1 EtherChannel

Cable the network presented in the figure 10.5.

Figure 10.5 Etherchannel test network

5
 COMPUTER NETWORKS

1. Verify the connectivity between the laptops and Server0 with the ping command.
2. Connect to the Switch0 and enter the Privileged EXEC mode. View the Spanning
Tree information with the show spanning-tree command. Examine and explain
the output of this command.
Switch0#show spanning-tree
3. Repeat the previous step for Switch1.
4. Connect to Switch0 and specify the interfaces that compose the EtherChannel group
using the interface range interface global configuration mode command. Create
the port channel interface with the channel-group identifier mode on
command in interface range configuration mode. The identifier specifies a channel
group number.

Switch0(config)#interface range fastEthernet 0/2-3

Switch0(config-if-range)#channel-group 1 mode on
5. Repeat the previous step for Switch1.
6. Connect to the Switch0 and enter the Privileged EXEC mode. View the running-
config file with the show running-config command. Examine and explain the
output of this command. View the Etherchannel information with the show
etherchannel summary command. Examine and explain the output of this
command. View the Spanning Tree information with the show spanning-tree
command. Examine and explain the output of this command.
Switch0#show running-config
Switch0#show etherchannel summary
Switch0#show spanning-tree
7. Repeat the previous step for Switch1.
8. Connect to Switch0 and enter port channel interface configuration mode using the
interface port-channel command, followed by the interface identifier.
Configure the EtherChannel as a trunk interface using the switchport mode
trunk command.
Switch0(config)#interface port-channel 1
Switch0(config-if)#switchport mode trunk
9. Repeat the previous step for Switch1.
10. Connect to the Switch0 and enter the Privileged EXEC mode. View the running-
config file with the show running-config command. Examine and explain the
output of this command. View the trunking information with the show interfaces
trunk command. Examine and explain the output of this command.
Switch0#show running-config
Switch0#show interfaces trunk
11. Repeat the previous step for Switch1.
12. Connect to Switch0 and configure EtherChannel load balancing method using the
port-channel load-balance global configuration mode command. Select the
load-distribution method based on the destination-host MAC address of the incoming
packet (dst-mac). Enter the Privileged EXEC mode and view the EtherChannel load
balancing method information with the show etherchannel load-balance
command. Examine and explain the output of this command.

Switch0(config)#port-channel load-balance dst-mac

Switch0(config)#end

6
 LINK AGGREGATION AND ETHERCHANNEL

Switch0#show etherchannel load-balance

Notes