CCNA Routing and Switching Practice and Study Guide: Link Aggregation
Date: May 16, 2014
Link aggregation is the ability to create one logical link using multiple physical links between two devices. This allows load sharing among the physical links, rather than having a STP block one or more of the links.
Link Aggregation Concepts
One of the best ways to reduce the time it takes for STP convergence is to simply avoid STP. EtherChannel is a form of link aggregation used in switched networks.
EtherChannel Advantages
EtherChannel technology was originally developed by Cisco as a technique of grouping several Fast Ethernet or Gigabit Ethernet switch ports into one logical channel.
List at least three advantages to using EtherChannel:
EtherChannel Operation
You can configure EtherChannel as static or unconditional. However, there are also two protocols that can be used to configure the negotiation process: Port Aggregation Protocol (PAgP—Cisco proprietary) and Link Aggregation Control Protocol (LACP—IEEE 802.3ad).
These two protocols ensure that both sides of the link have compatible configurations—same speed, duplex setting, and VLAN information. The modes for each differ slightly.
For PAgP, briefly describe each of the following modes:
- On:
- Desirable:
- Auto:
For LACP, briefly describe each of the following modes:
- On:
- Active:
- Passive:
In Table 3-1, indicate the mode that is described.
Table 3-1 PAgP and LACP Modes
Mode |
PAgP and/or LACP Mode Description |
Initiates LACP negotiations with other interfaces. |
|
Forces EtherChannel state without PAgP or LACP initiated negotiations. |
|
Places an interface in a passive, responding state. Does not initiate PAgP negotiations. |
|
Actively initiates PAgP negotiations with other interfaces. |
|
Places an interface in a passive, responding state. Does not initiate LACP negotiations. |
The mode that is configured on each side of the EtherChannel link determines whether EtherChannel will be operational.
In Table 3-2, two switches are using PAgP. Indicate with “yes” or “no” whether EtherChannel is established.
Table 3-2 EtherChannel Negotiation Using PAgP
Switch 1 Mode |
Switch 2 Mode |
EtherChannel Established? |
Auto |
Auto |
|
Auto |
Desirable |
|
On |
Desirable |
|
On |
Off |
|
Desirable |
Desirable |
In Table 3-3, two switches are using LACP. Indicate with “yes” or “no” whether EtherChannel is established.
Table 3-3 EtherChannel Negotiation Using LACP
Switch 1 Mode |
Switch 2 Mode |
EtherChannel Established? |
Passive |
On |
|
Passive |
Active |
|
On |
On |
|
Passive |
Passive |
|
On |
Active |
Link Aggregation Configuration
EtherChannel configuration is rather straightforward once you decide on which protocol you will use. In fact, the easiest method is to just force both sides to be on.
Configuring EtherChannel
To configure EtherChannel, complete the following steps:
Step 1. Specify the interfaces that, participate in the EtherChannel group using the interface range interface command.
What are the requirements for each interface before they can form an EtherChannel?
Step 2. Create the port channel interface with the channel-group identifier mode {on | auto | desirable | active | passive} command in interface range configuration mode. The keyword _______________ forces the port to channel without PAgP or LACP. The keywords _______________ and _______________ enable PAgP. The keywords _______________ and _______________ enable LACP.
Step 3. The channel-group command automatically creates a port channel interface using the identifier as the number. Use the interface port-channel identifier command to configure channel-wide settings like trunking, native VLANs, or allowed VLANs.
As you can see from the configuration steps, the way you specify whether to use PAgP, LACP, or no negotiations is by configuring one keyword in the channel-group command.
So, with those steps in mind, consider Figure 3-1 in each of the following configuration scenarios.
Figure 3-1 EtherChannel Topology
EtherChannel Configuration Scenario 1
Record the commands, including the switch prompt, to configure the S1 Fa0/1 and Fa0/2 into an EtherChannel without negotiations. Then force the channel to trunking using native VLAN 99.
S1(config)#
EtherChannel Configuration Scenario 1
Record the commands, including the switch prompt, to configure the S1 Fa0/1 and Fa0/2 into an EtherChannel using PAgP. S1 should initiate the negotiations. The channel should trunk, allowing only VLANs 1, 10, and 20.
S1(config)#
EtherChannel Configuration Scenario 1
Record the commands, including the switch prompt, to configure the S1 Fa0/1 and Fa0/2 into an EtherChannel using LACP. S1 should not initiate the negotiations. The channel should trunk, allowing all VLANs.
S1(config)#
Lab - Configuring EtherChannel (SN 3.2.1.4/SwN 5.2.1.4)
Packet Tracer - Configuring EtherChannel (SN 3.2.1.3/SwN 5.2.1.3)
Verifying and Troubleshooting EtherChannel
Record the commands used to display the output in Example 3-1.
Example 3-1 EtherChannel Verification Commands
S1# __________________________________________________________________________________________
Port-channel1 is up, line protocol is up (connected)
Hardware is EtherChannel, address is 0cd9.96e8.8a01 (bia 0cd9.96e8.8a01)
MTU 1500 bytes, BW 200000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
<output omitted>
S1# __________________________________________________________________________________________
Flags: D - down P - bundled 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
M - not in use, minimum links not met
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) LACP Fa0/1(P) Fa0/2(P)
S1# __________________________________________________________________________________________
Channel-group listing:
----------------------
Group: 1
----------
Port-channels in the group:
---------------------------
Port-channel: Po1 (Primary Aggregator)
------------
Age of the Port-channel = 0d:00h:25m:17s
Logical slot/port = 2/1 Number of ports = 2
HotStandBy port = null
Port state = Port-channel Ag-Inuse
Protocol = LACP
Port security = Disabled
Ports in the Port-channel:
Index Load Port EC state No of bits
------+------+------+------------------+-----------
0 00 Fa0/1 Active 0
0 00 Fa0/2 Active 0
Time since last port bundled: 0d:00h:05m:41s Fa0/2
Time since last port Un-bundled: 0d:00h:05m:48s Fa0/2
S1# __________________________________________________________________________________________
Port state = Up Mstr Assoc In-Bndl
Channel group = 1 Mode = Active Gcchange = -
Port-channel = Po1 GC = - Pseudo port-channel = Po1
Port index = 0 Load = 0x00 Protocol = LACP
Flags: S - Device is sending Slow LACPDUs F - Device is sending fast LACPDUs.
A - Device is in active mode. P - Device is in passive mode.
Local information:
LACP port Admin Oper Port Port
Port Flags State Priority Key Key Number State
Fa0/1 SA bndl 32768 0x1 0x1 0x102 0x3D
Partner's information:
LACP port Admin Oper Port Port
Port Flags Priority Dev ID Age key Key Number State
Fa0/1 SA 32768 0cd9.96d2.4000 4s 0x0 0x1 0x102 0x3D
Age of the port in the current state: 0d:00h:24m:59s
S1#
When troubleshooting an EtherChannel issue, keep in mind the configuration restrictions for interfaces that participate in the channel. List at least four restrictions.
Refer to the output for S1 and S2 in Example 3-2. Record the command that generated the output.
Example 3-2 Troubleshooting an EtherChannel Issue
S1# __________________________________________________________________________________________ Flags:D - downP - bundled 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 M - not in use, minimum links not met 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(SD) - Fa0/1(D) Fa0/2(D) S1# show run | begin interface Port-channel interface Port-channel1 switchport mode trunk ! interface FastEthernet0/1 switchport mode trunk channel-group 1 mode auto ! interface FastEthernet0/2 switchport mode trunk channel-group 1 mode auto ! <output omitted> S 1# _________________________________________________________________________________ S2# show run | begin interface Port-channel interface Port-channel1 switchport mode trunk ! interface FastEthernet0/1 switchport mode trunk channel-group 1 mode auto ! interface FastEthernet0/2 switchport mode trunk channel-group 1 mode auto ! <output omitted> S2#
Explain why the EtherChannel between S1 and S2 is down.
EtherChannel and spanning tree must interoperate. For this reason, the order in which EtherChannel-related commands are entered is important. To correct this issue, you must first remove the port channel. Otherwise, spanning-tree errors cause the associated ports to go into blocking or errdisabled state. With that in mind, what would you suggest to correct the issue shown in Example 3-2 if the requirement is to use PAgP? What commands would be required?
Lab - Troubleshooting EtherChannel (SN 3.2.2.4/SwN 5.2.2.4)
Packet Tracer - Troubleshooting EtherChannel (SN 3.2.2.3/SwN 5.2.2.3)
Packet Tracer - Skills Integration Challenge (SN 3.3.1.2/SwN 5.3.1.2)