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Network Engineer — 03. EtherChannel Troubleshooting

2025-08-28 Networking EtherChannel Troubleshooting Cisco

Lab: Troubleshooting EtherChannel

Topology

Troubleshooting topology

Addressing table

DeviceInterfaceIP addressSubnet Mask
S1VLAN 99192.168.1.11255.255.255.0
S2VLAN 99192.168.1.12255.255.255.0
S3VLAN 99192.168.1.13255.255.255.0
PC-ANIC192.168.0.2255.255.255.0
PC-CNIC192.168.0.3255.255.255.0

VLAN assignments

VLANName
10Users
99Management

Goals

  • Build the network and apply the starting configuration
  • Identify and fix all EtherChannel issues

Starting configuration (before troubleshooting)

S1

enable
conf t
hostname S1
interface range f0/1-24, g0/1-2
shutdown
exit
enable secret class
no ip domain lookup
line vty 0 15
password cisco
login
line con 0
 exec-t 0 0
 password cisco
 logging synchronous
 login
 exit
vlan 10
 name User
vlan 99
 Name Management
interface range f0/1-2
 switchport mode trunk
 channel-group 1 mode active
 switchport trunk native vlan 99
 no shutdown
interface range f0/3-4
 channel-group 2 mode desirable
 switchport trunk native vlan 99
 no shutdown
interface f0/6
 switchport mode access
 switchport access vlan 10
 no shutdown
interface vlan 99
 ip address 192.168.1.11 255.255.255.0
interface port-channel 1
 switchport trunk native vlan 99
 switchport mode trunk
interface port-channel 2
 switchport trunk native vlan 99
 switchport mode access
do wr
end
copy running-config startup-config
S2

enable
conf t
hostname S2
interface range f0/1-24, g0/1-2
 shutdown
 exit
enable secret class
no ip domain lookup
line vty 0 15
 password cisco
 login
line con 0
 exec-t 0 0
 password cisco
 logging synchronous
 login
 exit
vlan 10
 name User
vlan 99
 name Management
spanning-tree vlan 1,10,99 root primary
interface range f0/1-2
 switchport mode trunk
 channel-group 1 mode desirable
 switchport trunk native vlan 99
 no shutdown
interface range f0/3-4
 switchport mode trunk
 channel-group 3 mode desirable
 switchport trunk native vlan 99
interface vlan 99
 ip address 192.168.1.12 255.255.255.0
interface port-channel 1
 switchport trunk native vlan 99
 switchport trunk allowed vlan 1,99
interface port-channel 3
 switchport trunk native vlan 99
 switchport trunk allowed vlan 1,10,99
 switchport mode trunk
do wr
end
copy running-config startup-config
S3

enable
configure terminal
enab
conf t
hostname S3
interface range f0/1-24, g0/1-2
 shutdown
 exit
enable secret class
no ip domain lookup
line vty 0 15
 password cisco
 login
line con 0
 exec-t 0 0
 password cisco
 logging synchronous
 login
 exit
vlan 10
 name User
vlan 99
 name Management
interface range f0/1-2
interface range f0/3-4
 switchport mode trunk
 channel-group 3 mode desirable
 switchport trunk native vlan 99
 no shutdown
interface f0/18
 switchport mode access
 switchport access vlan 10
 no shutdown
interface vlan 99
 ip address 192.168.1.13 255.255.255.0
interface port-channel 3
 switchport trunk native vlan 99
 switchport mode trunk
do wr
end
copy running-config startup-config

Troubleshooting

Step 1 — Check EtherChannel summary

show etherchannel summary
S1

enable
configure terminal
Group  Port-channel  Protocol    Ports
------+-------------+-----------+----------------------------------------------
1      Po1(SD)           LACP   Fa0/1(I) Fa0/2(I)
2      Po2(SU)           PAgP   Fa0/3(P) Fa0/4(P)
end
copy running-config startup-config
S2

enable
configure terminal
Group  Port-channel  Protocol    Ports
------+-------------+-----------+----------------------------------------------
1      Po1(SD)           PAgP   Fa0/1(I) Fa0/2(I)
3      Po3(SD)           PAgP   Fa0/3(D) Fa0/4(D)
end
copy running-config startup-config
S3

enable
configure terminal
Group  Port-channel  Protocol    Ports
------+-------------+-----------+----------------------------------------------
3      Po3(SU)           PAgP   Fa0/3(P) Fa0/4(P)
end
copy running-config startup-config

Issue 1: S3 is missing channel-group 2. Fix:

S3(config)# int r fa0/3-4
S3(config-if-range)# channel-group 2 mode auto

Check S2’s running config to identify all issues at once:

show running-config | begin Port-channel
S2 output

enable
configure terminal
interface Port-channel1
 switchport trunk native vlan 99
 switchport trunk allowed vlan 1,99
 switchport mode trunk
!
interface Port-channel3
 switchport trunk native vlan 99
 switchport trunk allowed vlan 1,10,99
 switchport mode trunk
!
interface FastEthernet0/1
 switchport trunk native vlan 99
 switchport trunk allowed vlan 1,99
 switchport mode trunk
 channel-group 1 mode desirable
!
interface FastEthernet0/2
 switchport trunk native vlan 99
 switchport trunk allowed vlan 1,99
 switchport mode trunk
 channel-group 1 mode desirable
!
interface FastEthernet0/3
 switchport trunk native vlan 99
 switchport trunk allowed vlan 1,10,99
 switchport mode trunk
 channel-group 3 mode desirable
 shutdown
!
interface FastEthernet0/4
 switchport trunk native vlan 99
 switchport trunk allowed vlan 1,10,99
 switchport mode trunk
 channel-group 3 mode desirable
 shutdown
end
copy running-config startup-config

Issues visible in S2 config:

  • Po1 allows only VLAN 1,99 (missing VLAN 10)
  • Fa0/1-2 use mode desirable (PAgP) but S1 uses LACP active — protocol mismatch
  • Fa0/3-4 are shutdown

Step 2 — Check VLAN 10 access port

Po2 on S1 is in access mode — verify with show run interface:

S1 — interface Ethernet1/2

enable
configure terminal
interface Ethernet1/2
 switchport access vlan 10
 switchport mode access
 channel-group 1 mode desirable
end

EtherChannel requires all member ports to be in trunk mode. Fix:

S1(config)# interface Port-channel2
S1(config-if)# switchport mode trunk

Step 3 — Check allowed VLANs on Po1 (S1–S2)

S2 allows only VLAN 1,99 on Po1, blocking VLAN 10. Fix:

S2(config)# int r fa0/1-2
S2(config-if-range)# switchport trunk allowed vlan add 10

Step 4 — Fix protocol mismatch on Po1

S1 uses LACP active, S2 uses PAgP desirable — protocols don’t match. Fix S2 to LACP:

S2(config)# no int po1
S2(config)# int r fa0/1-2
S2(config-if-range)# no shutdown
S2(config-if-range)# channel-group 1 mode passive
S2(config-if-range)# exit
S2(config)# int po1
S2(config-if)# switchport trunk native vlan 99
S2(config-if)# switchport trunk allowed vlan 1,10,99
S2(config-if)# switchport mode trunk

Step 5 — Fix shutdown ports on S2

Fa0/3-4 on S2 are shutdown. Fix:

S2(config)# int r FastEthernet0/3-4
S2(config-if-range)# no shutdown

Bring up S3 Fa0/1-2 and add to channel-group 3:

S3(config)# int r fa0/1-2
S3(config-if-range)# no shutdown
S3(config-if-range)# switchport trunk native vlan 99
S3(config-if-range)# switchport mode trunk
S3(config-if-range)# channel-group 3 mode auto

Final verification

show etherchannel summary
S1

enable
configure terminal
1      Po1(SU)           LACP   Fa0/1(P) Fa0/2(P)
2      Po2(SU)           PAgP   Fa0/3(P) Fa0/4(P)
end
copy running-config startup-config
S2

enable
configure terminal
1      Po1(SU)           LACP   Fa0/1(P) Fa0/2(P)
3      Po3(SU)           PAgP   Fa0/3(P) Fa0/4(P)
end
copy running-config startup-config
S3

enable
configure terminal
2      Po2(SU)           PAgP   Fa0/3(P) Fa0/4(P)
3      Po3(SU)           PAgP   Fa0/1(P) Fa0/2(P)
end
copy running-config startup-config

Connectivity check — ping Management VLAN from S3:

S3

enable
configure terminal
S3(config-if)#do ping 192.168.1.13
Sending 5, 100-byte ICMP Echos to 192.168.1.13, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms

S3(config-if)#do ping 192.168.1.12
Sending 5, 100-byte ICMP Echos to 192.168.1.12, timeout is 2 seconds:
.!!!!
Success rate is 60 percent (3/5), round-trip min/avg/max = 0/0/1 ms


S3(config-if)#do ping 192.168.1.11
Sending 5, 100-byte ICMP Echos to 192.168.1.11, timeout is 2 seconds:
..!!!
Success rate is 60 percent (3/5), round-trip min/avg/max = 0/1/4 ms

Connectivity check — ping between PCs:

PC-A

enable
configure terminal
C:\>ping 192.168.0.2

Pinging 192.168.0.2 with 32 bytes of data:
Reply from 192.168.0.2: bytes=32 time=3ms TTL=128
Reply from 192.168.0.2: bytes=32 time=4ms TTL=128
Reply from 192.168.0.2: bytes=32 time=1ms TTL=128
Reply from 192.168.0.2: bytes=32 time=4ms TTL=128


Ping statistics for 192.168.0.2:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)


C:>ping 192.168.0.3


Pinging 192.168.0.3 with 32 bytes of data:
Reply from 192.168.0.3: bytes=32 time<1ms TTL=128
Reply from 192.168.0.3: bytes=32 time<1ms TTL=128
Reply from 192.168.0.3: bytes=32 time<1ms TTL=128
Reply from 192.168.0.3: bytes=32 time<1ms TTL=128


Ping statistics for 192.168.0.3:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)

Network Engineer Course | Lab 03

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