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Course 5: LAN SwitchingLesson 1.2 (2 of 6 in this course)17 of 91 in the CCNA series

EtherChannel (Port-Channel)

Bundling several links into one logical link: LACP, PAgP, load balancing and verification.

Intermediate · 10 min read

EtherChannel is a Cisco technology that bundles several parallel physical Ethernet links between two devices into one logical link, called a port channel. Traffic is shared across the member links, and Spanning Tree treats the bundle as a single port, so none of the links is blocked. A bundle can be negotiated with LACP or PAgP, or configured statically.

In simple terms: Instead of Spanning Tree blocking all but one of several cables between two switches, EtherChannel makes them work as one bigger cable. If one cable fails, the others keep carrying the traffic.

The problem with parallel links

Situation: one 1 Gbps link between two switches is too slow, so you add a second. Spanning Tree sees a loop and blocks one of them: you've paid for 2 Gbps and get 1.

Gi1/0/23SW1Gi1/0/23-24SW2Gi1/0/23-24
  1. 1. Half the capacity idle. STP blocks the second link to prevent a loop between the two switches.

EtherChannel: many links, one logical link

EtherChannel bundles 2 to 8 (active) physical links into one logical interface, a port-channel. Spanning Tree sees a single link, so nothing is blocked. All members carry traffic, and if one fails the others carry on without any STP recalculation.

Port-channel 1 (2 × 1 Gbps)SW1Gi1/0/23-24SW2Gi1/0/23-24
  1. 1. Flows are spread across members. The switch hashes each flow's addresses to choose a member link; flow A uses one link…
  2. 2. …and flow B the other. Each flow stays on one link so its frames arrive in order.
  3. 3. A member fails: the bundle stays up. Flows on the failed member move to the remaining link within moments; Spanning Tree doesn't notice anything.

💡 Load balancing is per flow, not per frame. A single large transfer between two hosts uses one member link, so one flow never gets more than one link's speed.

Negotiation: LACP, PAgP or static

ProtocolModesForms a channel when
LACP (IEEE 802.3ad / 802.1AX)active, passiveactive + active, or active + passive (passive + passive never starts)
PAgP (Cisco)desirable, autodesirable + desirable, or desirable + auto
Staticonon + on only; no negotiation and no checks

Use LACP: it's the standard, works with other vendors and servers, and checks the far end is really part of the same bundle. LACP can hold up to 16 ports, with 8 active and the rest in hot standby.

Members must match

Every member link must have the same:

  • speed and duplex,
  • switchport mode (access or trunk),
  • access VLAN, or allowed VLANs and native VLAN on a trunk.

A port that doesn't match is suspended from the bundle.

Configure

interface range GigabitEthernet1/0/23 - 24 channel-group 1 mode active interface Port-channel1 switchport mode trunk switchport trunk allowed vlan 10,20

On both switches. After the bundle forms, configure it on the Port-channel interface; changes are copied to the members.

port-channel load-balance src-dst-ip

Global: hash on source and destination IP, which spreads traffic better than MAC-only hashing when most traffic goes to one router.

Verify

Example output · based on Cisco documentation; exact format varies by platform and software version
SW1#show etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator
...
Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi1/0/23(P)   Gi1/0/24(P)
Po1(SU): a Layer 2 port-channel in use. (P) on each member: bundled. Watch for (s) suspended (a mismatch) or (I) stand-alone (the other side isn't negotiating).
show etherchannel load-balance

Which fields the hash uses.

Check yourself

Predict · scenario 1

Both ends of a link bundle are set to LACP passive. What happens?

Predict · scenario 2

One member shows (s) in show etherchannel summary. What's the likely cause?