EtherChannel (Link Aggregation)
Combining multiple physical links into a single logical "Port-Channel". Provides redundancy and increased bandwidth.
1. Protocols: LACP vs PAgP
| Feature | LACP (802.3ad) | PAgP (Cisco) | Static (On) |
|---|---|---|---|
| Standard | IEEE Open Standard | Cisco Proprietary | Manual |
| Modes | Active / Passive | Desirable / Auto | On / On |
| Recommendation | Preferred | Legacy | Avoid (No loop protection) |
Negotiation Logic
- Active + Passive: Link Comes UP.
- Active + Active: Link Comes UP.
- Passive + Passive: Link stays DOWN (Neither side starts).
2. Load Balancing (Hashing)
EtherChannel does NOT do round-robin packet balancing (e.g., Packet 1 -> Link 1, Packet 2 -> Link 2). This would cause out-of-order delivery, breaking TCP.
It uses a Hash Algorithm based on the packet header fields:
- Source MAC
- Destination MAC
- Source IP
- Destination IP
- TCP/UDP Ports (Layer 4)
Implication: A single file transfer (1 Flow) is limited to the speed of ONE physical link (e.g., 1Gbps), even if the Port-Channel is 4Gbps total. The aggregate bandwidth is only realized with multiple concurrent flows.
3. Configuration Requirements
For an EtherChannel to form, all member ports must match exactly:
- Speed & Duplex
- VLAN Mode (Access or Trunk)
- Native VLAN
- Allowed VLAN list
If settings mismatch, the ports will go into Err-Disable or "Suspended" state.
References
- IEEE 802.3ad (LACP) - The standard for Link Aggregation Control Protocol.
- Understanding EtherChannel Load Balancing - Explanation of hash algorithms (src-dst-ip, etc).