● Configure Layer 2 EtherChannel between SW1 and all other switches as follows:
● Use Cisco's proprietary protocol for negotiation.
● Only SW1 should actively initiate the EtherChannel negotiation.
● SW1 should initiate 802.1q trucking negotiation.
● Use port-channel numbers in the format of 1Y, where Y is the switch number for SW2, SW3, and SW4.
Explanation
Port Aggregation Protocol (PAgP) is a Cisco proprietary negotiation protocol for EtherChannel links. The desirable mode of PAgP, like DTP, is used to initiate negotiation, whereas the auto mode is used to listen for negotiation.
This implies that one side running desirable with the other side running desirable or auto will result in a channel being formed, but both sides running auto will not. Verify that EtherChannel has been successfully negotiated and the protocol used is PAgP.
SW1
show ether channel summary
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: 0
Number of aggregators: 0
Group Port-channel Protocol Ports
show ether channel protocol
Channel-group listing:
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Verify that SW1 is configured for desirable mode, and all other switches for auto mode.
Verify the EtherChannel state on all switches. However, because switches use a negotiation protocol, if EtherChannel shows as functional on one side, it has to be functional on the other side as well.
With no negotiation protocol being used, EtherChannel may not actually be functional from the perspective of both switches, and you must verify that.
SW2
show ether channel summary
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
If configuration is done on one side of the EtherChannel only, for example, only on SW1, because PAgP negotiation will fail, the following messages will be logged:
%EC-5-L3DONTBNDL1: eth3/1 suspended: PAgP not enabled on the remote port.
%EC-5-L3DONTBNDL1: eth1/3 suspended: PAgP not enabled on the remote port.
To avoid any problems in the network, physical interfaces will be logically disabled at Layer 2 and will appear in the suspended state for the EtherChannel.
SW1
show ether channel summary
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
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