6.2.4 Packet Tracer – Configure EtherChannel Answers

6.2.4 Packet Tracer – Configure EtherChannel Answers

Packet Tracer – Configure EtherChannel (Answers Version)

Answers Note: Red font color or gray highlights indicate text that appears in the Answers copy only.

Objectives

Part 1: Configure Basic Switch Settings

Part 2: Configure an EtherChannel with Cisco PAgP

Part 3: Configure an 802.3ad LACP EtherChannel

Part 4: Configure a Redundant EtherChannel Link

Background

Three switches have just been installed. There are redundant uplinks between the switches. As configured, only one of these links can be used; otherwise, a bridging loop might occur. However, using only one link utilizes only half of the available bandwidth. EtherChannel allows up to eight redundant links to be bundled together into one logical link. In this lab, you will configure Port Aggregation Protocol (PAgP), a Cisco EtherChannel protocol, and Link Aggregation Control Protocol (LACP), an IEEE 802.3ad open standard version of EtherChannel.

Before beginning the configuration, review the EtherChannel Configuration Guidelines and Restrictions listed at the end of this activity.

Port Channel Table

Channel Group

Ports

Protocol

1

S1 F0/21. F0/22

S3 F0/21, F0/22

PAgP

2

S1 G0/1, G0/2

S2 G0/1, G0/2

LACP

3

S2 F0/23, F0/24

S3 F0/23, F0/24

Negotiated LACP

Instructions

Part 1:  Configure Basic Switch Settings

  1. Assign each switch a hostname according to the topology diagram.
  2. Before beginning the link aggregation between switches, verify the existing configuration of the ports that connect the switches to ensure that the ports will successfully join the EtherChannels. Commands that provide information about the state of the switch ports include:

Open configuration window

S1# show interfaces | include Ethernet

S1# show interface status

S1# show interfaces trunk

  1. Configure all ports that are required for the EtherChannels as static trunk ports.

Close configuration window

Note: If the ports are configured with DTP dynamic auto mode, and you do not set the mode of the ports to trunk, the links do not form trunks and remain access ports. The default mode on a 2960 switch is for DTP to be enabled and set to dynamic auto. DTP can be disabled on interfaces with the switchport nonegotiate command.

Part 2:  Configure an EtherChannel with Cisco PAgP

Note: When configuring EtherChannels, it is recommended to shut down the physical ports being grouped on both devices before configuring them into channel groups. Otherwise, EtherChannel Misconfig Guard may place these ports into err-disabled state. The ports and port channels can be re-enabled after EtherChannel is configured.

Step 1:  Configure Port Channel 1.

  1. The first EtherChannel that is created for this activity aggregates ports F0/21 and F0/22 between S1 and S3. Configure the ports on both switches as static trunk ports.

Open configuration window

  1. Use the show interfaces trunk command to ensure that you have an active trunk link for those two links, and the native VLAN on both links is the same.

S1# show interfaces trunk

 

Port        Mode         Encapsulation  Status        Native vlan

F0/21      on           802.1q         trunking      1

F0/22      on           802.1q         trunking      1

G0/1       on           802.1q         trunking      1

G0/2       on           802.1q         trunking      1

 

<output omitted>

  1. On S1 and S3, add ports F0/21 and F0/22 to Port Channel 1 with the channel-group 1 mode desirable command. The mode desirable option enables the switch to actively negotiate to form a PAgP link. Note: Interfaces must be shutdown before adding them to the channel group.

S1(config)# interface range f0/21 – 22

S1(config-if-range)# shutdown

S1(config-if-range)# channel-group 1 mode desirable

S1(config-if-range)# no shutdown

 

S3(config)# interface range f0/21 – 22

S3(config-if-range)# shutdown

S3(config-if-range)# channel-group 1 mode desirable

S3(config-if-range)# no shutdown

The message “Creating a port-channel interface Port-channel 1” should appear on both switches when the channel-group is configured. This interface designation will appear as Po1 in command output.

  1. Configure the logical interface to become a trunk by first entering the interface port-channel number command and then the switchport mode trunk command. Add this configuration to both switches.

Answers Note: Packet Tracer versions 7.2.1 and earlier do not grade the switchport mode trunk command on port-channel interfaces.

S1(config)# interface port-channel 1

S1(config-if)# switchport mode trunk

 

S3(config)# interface port-channel 1

S3(config-if)# switchport mode trunk

Step 2:  Verify Port Channel 1 status.

  1. Issue the show etherchannel summary command on S1 and S3 to verify that EtherChannel is working on both switches. This command displays the type of EtherChannel, the ports utilized, and the port states. Command output is shown for S1.

S1# show etherchannel summary

Flags:  D – down        P – in port-channel

        I – stand-alone s – suspended

        H – Hot-standby (LACP only)

        R – Layer3      S – Layer2

        U – in use      f – failed to allocate aggregator

        u – unsuitable for bundling

        w – waiting to be aggregated

        d – default port

 

Number of channel-groups in use: 1

Number of aggregators:           1

 

Group  Port-channel  Protocol    Ports

——+————-+———–+—————————————-

1      Po1(SU)         PAgP      F0/21(P)   F0/22(P)  

  1. If the EtherChannel does not come up, shut down the physical interfaces on both ends of the EtherChannel and then bring them back up again. The show interfaces trunk and show spanning-tree commands should show the port channel as one logical link.

Close configuration window

Part 3:  Configure an 802.3ad LACP EtherChannel

Step 1:  Configure Port Channel 2.

  1. In 2000, the IEEE released 802.3ad, which is an open standard version of EtherChannel. It is commonly referred to as LACP. Using the previous commands, configure the link between S1 and S2, using ports G0/1 and G0/2, as an LACP EtherChannel. You must use a different port channel number on S1 than 1, because you already used that in the previous step. To configure port channel 2 as LACP, use the interface configuration mode channel-group 2 mode active command. Active mode indicates that the switch actively tries to negotiate that link as LACP, as opposed to PAgP. The configuration of S1 is shown below.

Open configuration window

S1(config)# interface range g0/1 – 2

S1(config-if-range)# shutdown

S1(config-if-range)# channel-group 2 mode active

S1(config-if-range)# no shutdown

S1(config-if-range)# interface port-channel 2

S1(config-if)# switchport mode trunk

 

S2(config)# interface range g0/1 – 2

S2(config-if-range)# shutdown

S2(config-if-range)# channel-group 2 mode active

S2(config-if-range)# no shutdown

S2(config-if-range)# interface port-channel 2

S2(config-if)# switchport mode trunk

Step 2:  Verify Port Channel 2 status.

Use the show commands from Part 1 Step 2 to verify the status of Port Channel 2. Look for the protocol used by each port.

S1# show etherchannel summary

Flags:  D – down        P – in port-channel

        I – stand-alone s – suspended

        H – Hot-standby (LACP only)

        R – Layer3      S – Layer2

        U – in use      f – failed to allocate aggregator

        u – unsuitable for bundling

        w – waiting to be aggregated

        d – default port

 

 

Number of channel-groups in use: 2

Number of aggregators:           2

 

Group  Port-channel  Protocol    Ports

——+————-+———–+———————————————-

 

1      Po1(SU)           PAgP   Fa0/21(P) Fa0/22(P)

2      Po2(SU)           LACP   Gig0/1(P) Gig0/2(P)

Part 4:  Configure a Redundant EtherChannel Link

Step 1:  Configure Port Channel 3.

There are various options for the channel-group number mode command:

S2(config)# interface range f0/23 – 24

S2(config-if-range)# channel-group 3 mode ?

  active     Enable LACP unconditionally

  auto       Enable PAgP only if a PAgP device is detected

  desirable  Enable PAgP unconditionally

  on         Enable Etherchannel only

  passive    Enable LACP only if a LACP device is detected

  1. On switch S2, add ports F0/23 and F0/24 to Port Channel 3 with the channel-group 3 mode passive command. The passive option indicates that you want the switch to use LACP only if another LACP device is detected. Statically configure Port Channel 3 as a trunk interface.

S2(config)# interface range f0/23 – 24

S2(config-if-range)# shutdown

S2(config-if-range)# channel-group 3 mode passive

S2(config-if-range)# no shutdown

S2(config-if-range)# interface port-channel 3

S2(config-if)# switchport mode trunk

  1. On S3, add ports F0/23 and F0/24 to Port Channel 3 with the channel-group 3 mode active command. The active option indicates that you want the switch to use LACP unconditionally. Statically configure Port Channel 3 as a trunk interface.

Step 2:  Verify Port Channel 3 status.

  1. Use the show commands from Part 1 Step 2 to verify the status of Port Channel 3. Look for the protocol used by each port.

S2# show etherchannel summary

<output omitted>

Number of channel-groups in use: 2

Number of aggregators:           2

Group  Port-channel  Protocol    Ports

——+————-+———–+—————————————-

2      Po2(SU)           LACP   Gig0/1(P) Gig0/2(P)

3      Po3(SU)           LACP   Fa0/23(P) Fa0/24(P)

  1. Creating EtherChannel links does not prevent Spanning Tree from detecting switching loops. View the spanning tree status of the active ports on S1.

S1# show spanning-tree active

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     0001.436E.8494

             Cost        9

             Port        27(Port-channel1)

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

 

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     000A.F313.2395

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time  20

 

Interface        Role Sts Cost      Prio.Nbr Type

—————- —- — ——— ——– ——————————–

Po1              Root FWD 9         128.27   Shr

Po2              Altn BLK 3         128.28   Shr

Port Channel 2 is not operative because Spanning Tree Protocol placed some ports into blocking mode. Unfortunately, those ports were the Gigabit ports. In this topology, you can restore these ports by configuring S1 to be primary root for VLAN 1. You could also set the priority to 24576.

S1(config)# spanning-tree vlan 1 root primary

or

S1(config)# spanning-tree vlan 1 priority 24576

Close configuration window

You may have to wait for STP to recalculate the tree topology. Press fast-forward if necessary. Use the show spanning-tree active command to verify that the Gigabit ports are now in the forwarding state.

EtherChannel Configuration Guidelines and Restrictions

EtherChannel has some specific guidelines that must be followed in order to avoid configuration problems.

1)      All Ethernet interfaces support EtherChannel up to a maximum of eight interfaces with no requirement that the interfaces be on the same interface module.

2)      All interfaces within an EtherChannel must operate at the same speed and duplex.

3)      EtherChannel links can function as either single VLAN access ports or as trunk links between switches.

4)      All interfaces in a Layer 2 EtherChannel must be members of the same VLAN or be configured as trunks.

5)      If configured as trunk links, Layer 2 EtherChannel must have the same native VLAN and have the same VLANs allowed on both switches connected to the trunk.

6)      When configuring EtherChannel links, all interfaces should be shutdown prior to beginning the EtherChannel configuration.  When configuration is complete, the links can be re-enabled.

7)      After configuring the EtherChannel, verify that all interfaces are in the up/up state.

8)      It is possible to configure an EtherChannel as static, or for it to use either PAgP or LACP to negotiate the EtherChannel connection.  The determination of how an EtherChannel is setup is the value of the channel-group number mode command.  Valid values are:

active  LACP is enabled unconditionally

passive LACP is enabled only if another LACP-capable device is connected.

desirable PAgP is enabled unconditionally

auto  PAgP is enabled only if another PAgP-capable device is connected.

on  EtherChannel is enabled, but without either LACP or PAgP.

9)      LAN ports can form an EtherChannel using PAgP if the modes are compatible.  Compatible PAgP modes are:

desirable => desirable

desirable => auto

If both interfaces are in auto mode, an Etherchannel cannot form.

10)  LAN ports can form an EtherChannel using LACP if the modes are compatible.  Compatible LACP modes are:

active => active

active => passive

If both interfaces are in passive mode, an EtherChannel cannot form using LACP.

11)  Channel-group numbers are local to the individual switch.  Although this activity uses the same Channel-group number on either end of the EtherChannel connection, it is not a requirement.  Channel-group 1 (interface po1) on one switch can form an EtherChannel with Channel-group 5 (interface po5) on another switch.

End of document

Answer Script

Switch S1

enable

config terminal

hostname S1

interface range f0/21 – 22

 switchport mode trunk

 shutdown

 channel-group 1 mode desirable

 no shutdown

interface port-channel 1

 switchport mode trunk

interface range g0/1 – 2

 switchport mode trunk

 shutdown

 channel-group 2 mode active

 no shutdown

interface port-channel 2

 switchport mode trunk

spanning-tree vlan 1 root primary

end

Switch S2

enable

config terminal

hostname S2

interface range g0/1 – 2

 switchport mode trunk

 shutdown

 channel-group 2 mode active

 no shutdown

interface port-channel 2

 switchport mode trunk

interface range f0/23 – 24

 switchport mode trunk

 shutdown

 channel-group 3 mode passive

 no shutdown

interface port-channel 3

 switchport mode trunk

 end

Switch S3

enable

config terminal

hostname S3

interface range f0/21 – 22

 switchport mode trunk

 shutdown

 channel-group 1 mode desirable

 no shutdown

interface port-channel 1

 switchport mode trunk

interface range f0/23 – 24

 switchport mode trunk

 shutdown

 channel-group 3 mode active

 no shutdown

interface port-channel 3

 switchport mode trunk

end

 

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