Troubleshoot static and dynamic EtherChannels

3.1 Layer 2

📘CCNP Encore (350-401-ENCORE-v1.1)

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EtherChannel is a Layer 2 (and sometimes Layer 3) technology that allows multiple physical links between switches or between a switch and a server/router to be combined into one logical link. This improves bandwidth and provides redundancy.

Troubleshooting EtherChannels is crucial because misconfigurations can break connectivity, cause loops, or prevent load balancing.

We’ll divide the explanation into:

1. EtherChannel Basics
2. Types of EtherChannels (Static vs. Dynamic)
3. Key Configuration Requirements
4. Common Problems & Troubleshooting Commands
5. Practical Troubleshooting Steps

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1. EtherChannel Basics

• Purpose: Combines multiple physical links into one logical link.
• Benefits:
  • Increased bandwidth: If you combine 4 x 1 Gbps links, your logical link has 4 Gbps.
  • Redundancy: If one physical link fails, traffic still passes through the others.
  • Simplified management: Logical link behaves like a single interface.
• Types of EtherChannel:
  • Layer 2: Between switches for VLAN traffic.
  • Layer 3: Between routers or switches for routing traffic.

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2. Types of EtherChannels

There are two ways to form EtherChannels:

A. Static EtherChannel (Manual)

• Configuration is manual. No negotiation protocol is used.
• Commands:
  • On a Cisco switch: interface range gi0/1 - 2 channel-group 1 mode on
  • mode on → forces the interface into EtherChannel.
• Pros: Simple, deterministic.
• Cons: If one side is not configured properly, the link stays down. No negotiation.

B. Dynamic EtherChannel (Negotiation)

Uses protocols to automatically form EtherChannels:

1. PAgP (Port Aggregation Protocol) – Cisco proprietary
   • mode auto → waits for the other side to initiate
   • mode desirable → actively tries to form a channel
   • Only works with Cisco devices.
2. LACP (Link Aggregation Control Protocol) – Industry standard (IEEE 802.3ad)
   • mode passive → waits for the other side to initiate
   • mode active → actively initiates formation
   • Works across different vendors.

Important Note: Both sides must match in protocol and mode; otherwise, EtherChannel fails.

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3. Key Configuration Requirements for EtherChannel

EtherChannels are strict. Misconfiguration is the most common cause of failure.

EtherChannel Rules:

1. Same speed and duplex: All interfaces must match (e.g., 1 Gbps full-duplex).
2. Same VLAN configuration (Layer 2): All member interfaces must belong to the same VLAN/trunk configuration.
3. Same trunking mode: All links in trunk mode must use the same encapsulation (dot1q or ISL).
4. Same EtherChannel protocol: PAgP vs LACP must match.
5. No port errors: Ports must be up and not administratively down.

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4. Common Problems & Troubleshooting Commands

When EtherChannel fails, check for the following:

Common Problems:

1. Physical Layer issues: Cable or interface down.
2. Speed/duplex mismatch: Even 1 mismatch breaks the channel.
3. VLAN mismatch: Trunks with different allowed VLANs fail.
4. Protocol mismatch: One side uses PAgP, the other LACP.
5. Misconfigured modes: One side passive, other side passive → won’t form EtherChannel.
6. EtherChannel flapping: Rapid up/down due to errors or misconfiguration.

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Key Troubleshooting Commands (Cisco)

1. Check EtherChannel summary:show etherchannel summary
   • Displays the port-channel, its protocol (PAgP/LACP), member interfaces, and status.
   • Example Output: Group Port-channel Protocol Ports 1 Po1 LACP Gi0/1(P) Gi0/2(P)
     • (P) → bundled in port-channel
     • (I) → standalone, not bundled
2. Check interfaces in channel:show running-config
   • Verify interface configuration matches requirements.
3. Check LACP/PAgP details:show lacp neighbor show pagp neighbor
   • Verifies the negotiation and remote partner info.
4. Check port status:show interfaces status show interfaces trunk
   • Confirm interfaces are up, trunking, and allowed VLANs match.
5. Check errors:show interfaces counters errors
   • Look for mismatched duplex or physical errors.

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5. Practical Troubleshooting Steps

Step-by-step approach:

1. Check physical connectivity: Make sure all cables and interfaces are up.
2. Verify EtherChannel protocol: PAgP or LACP must match.
3. Verify mode configuration:
   • For LACP: active ↔ active or active ↔ passive
   • For PAgP: desirable ↔ desirable or desirable ↔ auto
4. Check speed/duplex: All links must match exactly.
5. Verify VLAN/trunk configuration:
   • Layer 2 EtherChannel: Trunks must match encapsulation (dot1q/ISL) and allowed VLANs.
6. Check summary/status: Use show etherchannel summary to see if ports are bundled.
7. Check for error counters or flapping: Ensure no physical layer problems.

Tip for exams:

• If asked why an EtherChannel isn’t forming, common answers:
  • Mode mismatch (active vs passive, desirable vs auto)
  • Protocol mismatch (LACP vs PAgP)
  • Speed/duplex mismatch
  • VLAN mismatch
  • One interface is down

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✅ Key Exam Points to Remember

• Static vs Dynamic:
  • Static: mode on → no negotiation
  • Dynamic: LACP/PAgP → requires compatible modes
• All ports in an EtherChannel must match in:
  • Speed & duplex
  • VLAN/trunk config
  • EtherChannel protocol
• Use show etherchannel summary to confirm channel status.
• Mode mismatches are the most common reason dynamic EtherChannels fail.
• LACP is standard; PAgP is Cisco proprietary.

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