What is EtherChannel, and how do LACP, PAgP and static mode differ?
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EtherChannel bundles 2 to 8 parallel links into one logical link, so spanning tree sees one port and all
links carry traffic. LACP (IEEE 802.3ad, now 802.1AX) negotiates with active and passive modes and works
with any vendor. PAgP is Cisco only, with desirable and auto. Static on negotiates nothing. LACP forms
with active plus active or active plus passive, never passive plus passive. I use LACP active on both ends.
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Mode combinations
| Side A | Side B | Result |
|---|---|---|
| active | active or passive | LACP bundle |
| passive | passive | No bundle |
| desirable | desirable or auto | PAgP bundle |
| auto | auto | No bundle |
| on | on | Static bundle |
| on | anything else | Broken: one side bundled, the other not |
Why not static
on cannot see the far end. If the neighbor is not bundled or is cabled to the wrong switch, on still
bundles, and you get MAC flapping, blackholed traffic or a loop. LACP only bundles a port after
exchanging LACPDUs with the same partner.
LACP basics
LACPDUs go to 0180.c200.0002 (EtherType 0x8809), every 30 seconds by default or every second with
fast rate. Up to 8 members are active; many platforms allow up to 8 more as hot-standby.
interface range GigabitEthernet1/0/47 - 48
switchport mode trunk
channel-group 1 mode active
interface Port-channel1
switchport mode trunk
switchport trunk allowed vlan 10,20,30They will ask next
What does spanning tree see?
One logical port, Po1, with one cost. A member failing is handled by EtherChannel and does not cause a spanning-tree topology change as long as the bundle stays up.
Can one side run LACP and the other PAgP?
No. Both ends must run the same protocol.
Traps
- Choosing
onbecause it is simpler. It is the one mode that cannot detect a far end that is not bundled.