PAGP (Port Aggregation Protocol)

• Cisco proprietary protocol for automatically negotiating EtherChannel links between switches
• Creates logical bundled interfaces from multiple physical links to increase bandwidth and provide redundancy
• Operates at Layer 2 and uses special frames to communicate channel formation capabilities between devices
• Only works between Cisco devices - cannot interoperate with other vendors’ equipment

PAgP Modes

• Auto: Passively waits for PAgP negotiation packets (responds but doesn’t initiate)
• Desirable: Actively sends PAgP negotiation packets to form channel
• Channel formation requires at least one side to be Desirable (Auto + Auto = no channel)
• On: Forces channel formation without negotiation (not recommended for production)

Technical Specifications

• Supports 2-8 physical interfaces per EtherChannel (practical limit often 2-4)
• All ports must have identical configuration (speed, duplex, VLAN assignment, STP settings)
• Uses destination MAC address for load balancing by default
• PAgP frames sent every 30 seconds to maintain channel health
• Automatically removes failed links from bundle without bringing down logical interface

Configuration Example

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interface range fastethernet 0/1-2
 channel-group 1 mode desirable
 switchport mode trunk

Vocabulary

EtherChannel: Logical interface created by bundling multiple physical links Channel Group: Number identifying the logical channel (1-64 typically) Load Balancing: Method for distributing traffic across bundled links Negotiation: Process where switches agree to form channel using control frames

Notes

• PAgP is Cisco proprietary - use LACP (802.3ad) for multi-vendor environments
• Always verify port configurations match before enabling PAgP (mismatched configs prevent channel formation)
• Use show etherchannel summary to verify channel status and member ports
• For small networks with all-Cisco infrastructure when automatic negotiation is preferred over static configuration
• PAgP provides better error detection than manually configured “on” mode
• Desirable mode recommended for most implementations as it actively establishes channels
• Load balancing algorithm affects traffic distribution - default MAC-based may not suit all traffic patterns
• Failed negotiation often caused by: mismatched port configs, different switch models, or connectivity issues