LACP and PAgP: The Negotiation Protocols

You're connecting a Cisco switch to a Juniper switch in a mixed-vendor data center. The design calls for four-link EtherChannel between them. You configure PAgP on the Cisco side, and nothing works. Time to learn why the protocol choice matters.

EtherChannel has two negotiation protocols — LACP and PAgP. They're not interchangeable. Picking wrong means the bundle silently never forms, and you'll spend an hour wondering why the links are all blocked.

#LACP vs. PAgP#link

LACP wins on interoperability and on multi-bundle flexibility. PAgP is fine in all-Cisco environments. For anything new, use LACP — it's the standard, and the modes are clearer once you internalize them.

#The Negotiation Handshake#link

#The Mode Rules#link

• ▪LACP active + active — both initiate, bundle forms (preferred)
• ▪LACP active + passive — active initiates, passive responds, bundle forms
• ▪LACP passive + passive — neither initiates, bundle never forms
• ▪PAgP desirable + desirable — both initiate, bundle forms
• ▪PAgP desirable + auto — desirable initiates, auto responds, bundle forms
• ▪PAgP auto + auto — neither initiates, bundle never forms
• ▪Static (on) + on — no negotiation, works but no failure detection
• ▪Any protocol mix (LACP + PAgP) — no bundle, silent failure

#Which Protocol for Which Scenario#link

#Check Your Understanding#link

#Common Mistakes#link

• ▪Mixing LACP and PAgP on opposite ends — silent failure, no bundle forms
• ▪Using passive/passive — bundle never initiates
• ▪Configuring static mode on switches that don't support it — some platforms don't allow `mode on`
• ▪Forgetting that LACP is required for some line cards or platforms — check the hardware docs
• ▪Assuming the bundle is up without verification — always check `show etherchannel summary`