PAgP and LACP – Detailed Explanation

1. What is Link Aggregation?

• Purpose: Link aggregation combines multiple physical links into a single logical link (EtherChannel), enhancing:
• Bandwidth: Increases aggregate throughput (e.g., 4 × 1 Gbps = 4 Gbps logical bandwidth).
• Redundancy: If one link fails, traffic redistributes across remaining links for high availability.
• When to use: High-availability or high-bandwidth environments (core/distribution uplinks, switch-to-server, or between switches).

2. Overview of PAgP (Port Aggregation Protocol)

• Cisco proprietary link aggregation protocol.
• Negotiates the automatic formation of an EtherChannel.
• Modes:
• Auto: Passive; waits for PAgP packets, will not initiate.
• Desirable: Actively sends PAgP packets to form EtherChannel.
• EtherChannel forms if one side is desirable and the other is desirable or auto.

3. Overview of LACP (Link Aggregation Control Protocol)

• IEEE 802.3ad open standard protocol.
• Enables link aggregation across multi-vendor devices.
• Modes:
• Active: Actively sends LACP packets to negotiate.
• Passive: Waits for LACP packets, will not initiate.
• EtherChannel forms if at least one side is active.

4. Differences Between PAgP and LACP

5. EtherChannel Basics

• EtherChannel bundles 2–8 physical links into one logical interface (Port-Channel).
• Load balancing: Switches can distribute traffic based on source/destination MAC addresses, IP addresses, or Layer 4 ports.
• Presents a single MAC address to upstream devices.

Example: Four FastEthernet links combined into Port-Channel1 for redundancy and bandwidth.

6. Configuration of PAgP and LACP (Cisco IOS Example)

• PAgP Example:

interface range fa0/1 - 2
 channel-group 1 mode desirable   ! Actively forms EtherChannel
  

• Other end: channel-group 1 mode auto or desirable.
• LACP Example:

interface range gi0/1 - 2
 channel-group 2 mode active      ! Actively negotiates LACP
  

• Other end: channel-group 2 mode active or passive.

7. Operational States and Negotiation Process

• Ports must agree on protocol (PAgP or LACP) and mode (active/desirable vs passive/auto).
• EtherChannel only forms if compatible settings exist at both ends.
• If there is a protocol/mode mismatch, links remain independent and EtherChannel does not form.

8. Compatibility Considerations

• PAgP and LACP cannot interoperate—both ends must use the same protocol.
• Static EtherChannel: EtherChannel can be configured without PAgP/LACP (mode on), but negotiation is disabled—both sides must be forced "on".

9. Troubleshooting Link Aggregation

Useful Commands:

show etherchannel summary
show interfaces port-channel
show interfaces [interface] etherchannel

10. Load Balancing Algorithms

• Traffic can be distributed based on:
• Source/destination MAC addresses
• Source/destination IP addresses
• Layer 4 port numbers (TCP/UDP)
• Default on Cisco: Source and destination MAC addresses.
• The choice of algorithm affects how traffic is balanced across all links.

11. Limitations and Best Practices

• Maximum Links: LACP up to 16 (8 active, 8 standby), PAgP up to 8 links.
• All ports in the EtherChannel group must have identical speed, duplex, VLAN, and trunking status.
• Best Practice: Use LACP for interoperability unless in a Cisco-only environment. Always document your EtherChannel configuration and monitor for errors/mismatches.

12. Security Considerations

• Connect only trusted devices to EtherChannel ports.
• Use EtherChannel Guard to protect against misconfiguration or spoofing:

spanning-tree etherchannel guard misconfig

📘 Example Scenario

Switch1 (Cisco):
interface range gi0/1 - 2
 channel-group 1 mode active    ! LACP

Switch2 (HP/Juniper/Cisco):
interface range gi0/1 - 2
 channel-group 1 mode active    ! LACP

Four 1 Gbps links = 4 Gbps logical bandwidth. If any link goes down, traffic fails over to the remaining links with no disruption.

📑 Key Points and Exam Tips

• PAgP: Cisco only, auto/desirable modes.
• LACP: Open standard, active/passive modes, recommended for multi-vendor environments.
• Do not mix PAgP and LACP on the same EtherChannel.
• All physical links must have identical settings.
• Use show etherchannel summary to check status and troubleshoot.
• Load balancing method can affect throughput and redundancy.
• Always secure and document EtherChannel ports and protocols.

When and Where to Use PAgP/LACP

• PAgP: Use only in legacy, Cisco-only networks.
• LACP: Use for new deployments, multi-vendor, and server connections.
• Critical uplinks, high-traffic server links, and data center aggregation are typical applications.

PAgP and LACP Quiz

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