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Link Aggregation — EtherChannel

STP solves switching loops, but pays a price for it: redundant links between two switches get blocked, so you lose bandwidth. Link aggregation is the workaround — multiple physical connections are bundled into one logical link, so STP sees them as a single unit and blocks nothing.

Benefits

  • Fault tolerance — a physical link can go down without the connection dropping
  • Load balancing — traffic is automatically spread across the bundle
  • Increased capacity — the logical link has more bandwidth than a single cable
  • Cost-saving — cheaper than investing in faster hardware

EtherChannel

EtherChannel is Cisco's implementation of link aggregation.

  • Multiple FastEthernet or GigabitEthernet ports together form 1 logical port channel interface
  • Configuration is done on the port channel interface, not on the individual physical ports
  • STP sees the entire bundle as 1 connection — STP only recalculates if all physical links in the bundle fail
  • Losing 1 physical link changes nothing about the network topology from STP's perspective

Limitations

  • Interface types may not be mixed — no bundling FastEthernet with GigabitEthernet
  • Max. 8 physical ports per EtherChannel (Catalyst 2960: max 6)
  • 8x FastEthernet = 800 Mbps — 8x Gigabit = 8 Gbps
  • All ports in an EtherChannel must have an identical configuration:
    • Same duplex and speed
    • As a trunk: same native VLAN and the same allowed VLAN range
    • As access: same VLAN

Always configure on the port channel

Once an interface is part of an EtherChannel, don't make individual changes on that interface anymore. Everything goes through the port channel interface.

Creation Methods

Method Protocol Type
Static (ON) None Unconditional — always on, no negotiation
PAgP Cisco proprietary Auto-negotiation
LACP IEEE 802.3ad Auto-negotiation, multi-vendor

PAgP — Port Aggregation Protocol

Cisco proprietary. Identifies ports that can form an EtherChannel together and announces the bundle to STP as one link. Sends packets every 30 seconds.

Modes

Mode Behavior
ON Static — no PAgP packets, always active
Desirable Takes the initiative, actively sends out PAgP packets
Auto Waits — responds to desirable, sends nothing itself

Mode combinations

S1 S2 Channel?
On On Yes
On Desirable/Auto No
Desirable Desirable Yes
Desirable Auto Yes
Auto Desirable Yes
Auto Auto No

IEEE 802.3ad — vendor-neutral, so it also works in mixed environments. Cisco devices support it too. Works similarly to PAgP.

Modes

Mode Behavior
ON Static — no LACP packets
Active Takes the initiative, sends out LACP packets
Passive Waits — responds to active

Mode combinations

S1 S2 Channel?
On On Yes
On Active/Passive No
Active Active Yes
Active Passive Yes
Passive Active Yes
Passive Passive No

LAN Scalability

Scalability = a network's ability to grow or shrink without disrupting availability and reliability. EtherChannel contributes to this by increasing bandwidth without topology changes and without forcing STP to block redundant paths.