Lab: three cables, one logical link
Bundling links so spanning tree sees one. Easy to configure and easy to get silently wrong.
Three cables between two switches is a loop, and spanning tree will block two of them. EtherChannel makes all three into one logical link, so STP sees one port and all three carry traffic.
Do this
- Put all three ports into a channel group with LACP active on both ends.
- Confirm the bundle with
show etherchannel summary. - Look at the spanning-tree output and notice there is now one port, not
SW1(config)# interface range GigabitEthernet0/1 - 3
SW1(config-if-range)# channel-group 1 mode active
Reading the flags
show etherchannel summary puts a letter after every port and the letters
are the diagnosis:
- (P) — bundled. This is what you want.
- (I) — independent. The port is up and refused to join.
- (D) — down. A layer-one problem, not a channel problem.
An (I) means something differs between the members: speed, duplex, switchport mode, allowed VLAN list, native VLAN, or access VLAN. One mismatch on one member is enough.
Try breaking it
- Change the allowed VLAN list on one member only and watch it go (I).
- Set both ends to
passiveand watch nothing happen at all — somebody has - Put one end on PAgP (
desirable) and the other on LACP (active). They do
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