Port-Channel Auto
This post is based on Cisco IOSv2 15.2 firmware running in EVE-NG. The behavior may differ on other firmware versions.
When configuring aggregated links - or bundles - using the Cisco "port-channel auto" option, the most popular etherchannel verification commands may not always tell the whole story.
Auto LAG can be enabled either globally or directly on member interfaces. It automatically uses LACP to bring up bundles between the devices. In this post, we'll just enable it globallly on all three participating switches in the topology below.
TL;DR: Using the command "show etherchannel auto" displays the flag A indicating the bundles were formed using auto LAG. In contrast, the commonly used etherchannel verification oommands "show etherchannel summary" or "show etherchannel 1 detail" do not provide indication that auto LAG is in use.
Topology:
All spanning-tree configuration on the switches is left on default. SW1 is selected as the root for VLAN 1 due to it having the lowest bridge ID. (i.e., 5000.001f.0000 vs the other two switches SW2 and SW3)
SW1#sh spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address 5000.001f.0000
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 5000.001f.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Desg FWD 4 128.1 Shr
Gi0/1 Desg FWD 4 128.2 Shr
Gi0/2 Desg FWD 4 128.5 Shr
Gi0/3 Desg FWD 4 128.6 ShrSW2 has selected its Gi0/2 as root port towards the root SW1. SW2 has won the per-segment designated port election versus SW3. Gi0/3 on SW2 is placed in a blocked state.
SW2#sh spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address 5000.001f.0000
Cost 4
Port 5 (GigabitEthernet0/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 5000.0020.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Desg FWD 4 128.1 Shr
Gi0/1 Desg FWD 4 128.2 Shr
Gi0/2 Root FWD 4 128.5 Shr
Gi0/3 Altn BLK 4 128.6 ShrSW3 has selected its Gi0/0 as root port towards the root SW1 and blocked all its other ports.
SW3#sh spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address 5000.001f.0000
Cost 4
Port 1 (GigabitEthernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 5000.0021.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Root FWD 4 128.1 Shr
Gi0/1 Altn BLK 4 128.2 Shr
Gi0/2 Altn BLK 4 128.3 Shr
Gi0/3 Altn BLK 4 128.4 ShrAll connected ports between the switches have been configured to trunk with 802.1Q as the encapsulation method.
Now we enable port-channel auto in global mode on all three swiches
SW1, SW2, & SW3
(config)# port-channel autoThe switches automatically form bundles with each other.
SW1#show etherch summa
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 N - not in use, no aggregation
f - failed to allocate aggregator
M - not in use, minimum links not met
m - not in use, port not aggregated due to minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port
A - formed by Auto LAG
Number of channel-groups in use: 2
Number of aggregators: 2
Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
1 Po1(SU) LACP Gi0/2(P) Gi0/3(P)
2 Po2(SU) LACP Gi0/0(P) Gi0/1(P)
SW2
SW2#sh etherch summa
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 N - not in use, no aggregation
f - failed to allocate aggregator
M - not in use, minimum links not met
m - not in use, port not aggregated due to minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port
A - formed by Auto LAG
Number of channel-groups in use: 2
Number of aggregators: 2
Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
1 Po1(SU) LACP Gi0/2(P) Gi0/3(P)
2 Po2(SU) LACP Gi0/0(P) Gi0/1(P)
SW3
SW3#sh etherch summa
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 N - not in use, no aggregation
f - failed to allocate aggregator
M - not in use, minimum links not met
m - not in use, port not aggregated due to minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port
A - formed by Auto LAG
Number of channel-groups in use: 2
Number of aggregators: 2
Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
1 Po1(SU) LACP Gi0/0(P) Gi0/1(P)
2 Po2(SU) LACP Gi0/2(P) Gi0/3(P)Checking under the port-channels shows no configuration at all. On SW1:
SW1#sh run int po1
Building configuration...
Current configuration : 5 bytes
end
SW1#sh run int po2
Building configuration...
Current configuration : 5 bytes
end
SW1#The individual interfaces of Po1 on SW1 show the configuration below. But this still doesn't tell us that these interfaces are bundled; let alone using Auto LAG.
SW1#show run int gi0/2
Building configuration...
Current configuration : 132 bytes
!
interface GigabitEthernet0/2
switchport trunk encapsulation dot1q
switchport mode trunk
media-type rj45
negotiation auto
end
SW1#show run int gi0/3
Building configuration...
Current configuration : 132 bytes
!
interface GigabitEthernet0/3
switchport trunk encapsulation dot1q
switchport mode trunk
media-type rj45
negotiation auto
end
SW1#There's still no indication in the outputs that these bundles were formed using port-channel auto. The "show etherch summa" flags show that A stands for formed by Auto LAG. But we're not seeing that flag in the outputs either.
Let's give the detailed output a try and see if there's any indicator.
SW1#show etherch 1 detail
Group state = L2
Ports: 2 Maxports = 4
Port-channels: 1 Max Port-channels = 4
Protocol: LACP
Minimum Links: 0
Ports in the group:
-------------------
Port: Gi0/2
------------
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
Gi0/2 SA bndl 32768 0x1 0x1 0x101 0x3D
Partner's information:
LACP port Admin Oper Port Port
Port Flags Priority Dev ID Age key Key Number State
Gi0/2 SA 32768 5000.0020.0000 15s 0x0 0x1 0x101 0x3D
Age of the port in the current state: 0d:00h:10m:14s
Port: Gi0/3
------------
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
Gi0/3 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
Gi0/3 SA 32768 5000.0020.0000 13s 0x0 0x1 0x102 0x3D
Age of the port in the current state: 0d:00h:10m:14s
Port-channels in the group:
---------------------------
Port-channel: Po1 (Primary Aggregator)
------------
Age of the Port-channel = 0d:00h:10m:16s
Logical slot/port = 16/0 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 Gi0/2 Active 0
0 00 Gi0/3 Active 0
Time since last port bundled: 0d:00h:10m:14s Gi0/3Still no luck with an output indicator telling us that this port channel was formed using auto method.
But the command show etherchannel auto on SW1 displays what we're expecting to see.
SW1#show etherch auto
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 N - not in use, no aggregation
f - failed to allocate aggregator
M - not in use, minimum links not met
m - not in use, port not aggregated due to minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port
A - formed by Auto LAG
Number of channel-groups in use: 2
Number of aggregators: 2
Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
1 Po1(SUA) LACP Gi0/2(P) Gi0/3(P)
2 Po2(SUA) LACP Gi0/0(P) Gi0/1(P)This time the flag A is visible in the output telling us that the two port channels were formed using auto LAG.
This is one of those cosmetic output issues that makes it appear one thing is happening when in fact another is the real operational state.
Tags: switching, lacp
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