Cisco documentation states the following on forming channels:
SW1 SW2 ---------------------------------------- On On = Yes Auto/Desirable Desirable = Yes On/auto/desirable Not configured = No On Desirable = No Auto/On Auto = No
It is to my understanding that Auto only responds to PAgP, but does not initiate the PAgP negotiation. But for some reason when testing both sides with Auto, the port channel comes up anyways.
SW1#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 Fa0/1(P) Fa0/2(P)
My question is, shouldn't an Auto – Auto configuration fail? Why does this work?
Edit
Both switch configs
Switch1
Building configuration... Current configuration : 1131 bytes ! version 12.1 no service timestamps log datetime msec no service timestamps debug datetime msec no service password-encryption ! hostname SW1 ! ! spanning-tree mode pvst ! interface FastEthernet0/1 channel-group 1 mode auto switchport mode trunk ! interface FastEthernet0/2 channel-group 1 mode auto switchport mode trunk ! interface FastEthernet0/3 ! interface FastEthernet0/4 ! interface FastEthernet0/5 ! interface FastEthernet0/6 ! interface FastEthernet0/7 ! interface FastEthernet0/8 ! interface FastEthernet0/9 ! interface FastEthernet0/10 ! interface FastEthernet0/11 ! interface FastEthernet0/12 ! interface FastEthernet0/13 ! interface FastEthernet0/14 ! interface FastEthernet0/15 ! interface FastEthernet0/16 ! interface FastEthernet0/17 ! interface FastEthernet0/18 ! interface FastEthernet0/19 ! interface FastEthernet0/20 ! interface FastEthernet0/21 ! interface FastEthernet0/22 ! interface FastEthernet0/23 ! interface FastEthernet0/24 ! interface Port-channel 1 switchport mode trunk ! interface Vlan1 ip address 192.168.1.1 255.255.255.0 ! ! line con 0 ! line vty 0 4 login line vty 5 15 login ! ! end
Switch 2
Current configuration : 1134 bytes ! version 12.1 no service timestamps log datetime msec no service timestamps debug datetime msec no service password-encryption ! hostname SW2 ! ! spanning-tree mode pvst ! interface FastEthernet0/1 channel-group 1 mode auto switchport mode trunk ! interface FastEthernet0/2 channel-group 1 mode auto switchport mode trunk ! interface FastEthernet0/3 ! interface FastEthernet0/4 ! interface FastEthernet0/5 ! interface FastEthernet0/6 ! interface FastEthernet0/7 ! interface FastEthernet0/8 ! interface FastEthernet0/9 ! interface FastEthernet0/10 ! interface FastEthernet0/11 ! interface FastEthernet0/12 ! interface FastEthernet0/13 ! interface FastEthernet0/14 ! interface FastEthernet0/15 ! interface FastEthernet0/16 ! interface FastEthernet0/17 ! interface FastEthernet0/18 ! interface FastEthernet0/19 ! interface FastEthernet0/20 ! interface FastEthernet0/21 ! interface FastEthernet0/22 ! interface FastEthernet0/23 ! interface FastEthernet0/24 ! interface Port-channel 1 switchport mode trunk ! interface Vlan1 ip address 192.168.1.2 255.255.255.0 ! ! line con 0 ! line vty 0 4 login line vty 5 15 login ! ! end
Best Answer
As per default, etherchannel is running in "silent" mode. So ethch goes up at the first time, but if you try to disable/enable on side maybe ethch doesn't goes up again.
From CISCO CCO : non-silent —(Optional) If your switch is connected to a partner that is PAgP capable, configure the switch port for nonsilent operation when the port is in the auto or desirable mode. If you do not specify non-silent , silent is assumed. The silent setting is for connections to file servers or packet analyzers. This setting allows PAgP to operate, to attach the port to a channel group, and to use the port for transmission.