More bandwidth between switches without STP blocking your investment.
You need more bandwidth between your access and distribution switches. You add a second cable — STP blocks it
to prevent loops. EtherChannel bundles multiple physical links into one logical channel that STP sees as a single link.
Spanning Tree prevents loops by putting redundant links into blocking state — a necessary but frustrating tradeoff.
Access Switch
SW-ACCESS
Active (forwarding)
Blocked by STP
Distribution Switch
SW-DIST
Without EtherChannel
Bandwidth wasted, no failover speed
You run two 1Gbps links. STP elects one as active, blocks the other. You get 1Gbps effective bandwidth.
If the active link fails, STP convergence takes 30-50 seconds (classic STP) to unblock the redundant link.
With EtherChannel
Combined bandwidth, instant failover
EtherChannel groups both links into one logical interface. STP sees a single link — no blocking.
You get up to 2Gbps aggregate bandwidth with load balancing. If one member link fails, traffic
instantly shifts to the remaining links — sub-second recovery.
What STP sees: EtherChannel presents the bundle as one logical interface to STP.
STP treats it as a single path, so it does not block any member links. The aggregation is transparent to Layer 2 protocols.
Slide 3 of 8
LACP vs PAgP
Two negotiation protocols — one open standard, one Cisco-only.
LACP — 802.3ad
Industry Standard
Link Aggregation Control Protocol. IEEE 802.3ad standard (now part of 802.1AX). Works between
different vendor switches. The exam-preferred and production-preferred choice.
Modes: active (initiates) and passive (responds, waits).
PAgP — Cisco Proprietary
Cisco-Only
Port Aggregation Protocol. Only works between Cisco devices.
Modes: desirable (initiates) and auto (responds, waits).
Same behavior as LACP active/passive — just different names and a different wire protocol.
Static / On Mode
No negotiation
Sets the channel unconditionally with no protocol. Both sides must be set to on.
Fastest to configure, most dangerous — no detection of misconfiguration. If one side fails silently,
the other side continues sending into a broken channel.
Key difference — negotiation: LACP and PAgP actively exchange protocol data units (PDUs) to
confirm both sides agree to form the channel. Static mode has no such verification — mismatches are silent failures.
Always prefer LACP in multi-vendor environments.
Your access switch is Cisco, your distribution switch is also Cisco. You could use either LACP or PAgP.
Use LACP — when you eventually replace a switch with a different vendor, you will not need to reconfigure.
Slide 4 of 8
Mode Compatibility Matrix
Whether a channel forms depends on the mode combination on both sides. Know this table.
LACP (802.3ad)
Side A \ Side B
Active
Passive
On
Active
Forms
Forms
No
Passive
Forms
No
No
On
No
No
Forms
PAgP (Cisco)
Side A \ Side B
Desirable
Auto
On
Desirable
Forms
Forms
No
Auto
Forms
No
No
On
No
No
Forms
Critical rule: passive/passive and auto/auto do NOT form a channel — both sides are waiting for
the other to initiate. At least one side must actively negotiate. Never mix LACP and PAgP on the same channel.
On + On caveat: Works, but neither side is monitoring the channel health. A silent failure on
one side causes the other to keep sending into a black hole. Only use "on" mode in a controlled lab environment.
Slide 5 of 8
EtherChannel Requirements
All member links must be identical. One mismatch prevents the channel from forming.
Speed & Duplex
Must match
All interfaces in the bundle must run at the same speed (e.g., all 1Gbps) and duplex (full-duplex). You cannot mix 100Mbps and 1Gbps members.
VLAN Configuration
Must match
If access ports: same access VLAN. If trunk ports: same allowed VLAN list, same native VLAN, same trunk encapsulation. Any mismatch suspends the channel.
Port Mode
Must match
All members must be the same: all access or all trunk. You cannot mix access and trunk ports in the same EtherChannel bundle.
Spanning Tree
Same STP configuration
STP parameters (port cost, priority) should match across members. STP operates on the Port-Channel interface, not individual member ports.
Max Members
Up to 8 active links
LACP supports up to 8 active + 8 standby links (16 total configured). PAgP supports up to 8 active. Cisco's EtherChannel typically supports 8 active member ports per bundle.
Same Switch
Or MLAG/VSS for multi-chassis
Standard EtherChannel requires all member ports on the same switch. Multi-chassis variants (VSS, vPC, MLAG) span two physical chassis for redundancy at the switch level.
Slide 6 of 8
Configuration Basics
Bundle Gi0/1 and Gi0/2 into Port-Channel 1 using LACP.
Create LACP EtherChannel (access side)
SW-ACCESS(config)# interface range gi0/1 - 2
SW-ACCESS(config-if-range)# channel-group 1 mode active
Configuration order matters: Configure the physical member ports first, then configure the
Port-Channel interface. Member ports inherit the Port-Channel configuration. Any mismatch between member port
configs and the Port-Channel config causes the member to be suspended (err-disabled or individually blocked).
Slide 7 of 8
Load Balancing & Troubleshooting
EtherChannel does not split a single flow across links — it hashes flows to links.
How Load Balancing Works
Per-flow, not per-packet
A hash algorithm assigns each traffic flow to one member link. The same conversation always
takes the same link (maintaining packet order). Common hash inputs: src/dst MAC, src/dst IP,
src/dst port. All flows get distributed across available links — the more flows, the better the spread.
Changing the Hash Method
Tune for your traffic pattern
If most traffic comes from one IP (e.g., a single server farm), src-ip hashing puts it all on
one link. Switch to src-dst-ip to spread it. Configure globally: port-channel load-balance src-dst-ip.
Verify: show etherchannel load-balance.
R-Layer3 S-Layer2 U-in use f-failed-alloc M-not in use
Number of channel-groups in use: 1
Group Port-channel Protocol Ports
1 Po1(SU) LACP Gi0/1(P) Gi0/2(P)
SU = Layer2 in use. P = bundled in port-channel. This is healthy.
Suspended state (s flag): A member port is suspended due to a configuration mismatch.
Check that speed, duplex, VLAN, and port mode all match across every member link. One bad port suspends itself
but leaves the rest of the channel intact.
Slide 8 of 8 | N10-009 Obj 2.3
EtherChannel: More Bandwidth, One Logical Link
You ran a second cable between your access and distribution switches. STP blocked it. After configuring
EtherChannel with LACP active/passive, STP now sees one logical Port-Channel interface.
Both physical links are active and load-balancing traffic. If one fails, the other carries the full load
within milliseconds. You doubled your bandwidth without STP ever knowing.
5 Facts to Carry Out of This Presentation
1EtherChannel bundles physical links into one logical Port-Channel interface — STP sees it as a single link, so nothing is blocked.
2LACP (802.3ad) is the open standard. PAgP is Cisco-only. Prefer LACP. Modes: active/passive (LACP) and desirable/auto (PAgP).
3passive/passive and auto/auto do NOT form a channel. At least one side must actively initiate.
4All member links must match: same speed, duplex, VLAN config, and port mode. One mismatch suspends that member.
5Troubleshoot with show etherchannel summary. Healthy members show P (bundled). Suspended members show s.