Advanced Spanning Tree Protocol: Etherchannel Load Balancing + Flex Links

Load Balancing

port-channel load-balance ?

dst-ip, dst-mac, src-ip, src-mac, src+dst ip

This configuration is entered under global config leading with the port-channel command.

Flex Links

If you don’t want to run STP. Why would you not?!

Using Flex Links, only the active port will be carrying data and the other link is used as a standby.

The configuration is entered on the interface: ‘switchport backup interface CHOICE

‘show interface switchport backup’

  • 1. Enabling a Flex Link disables STP
  • 2. Once backup interface per active interface
  • 3. Ports inside an Etherchannel can’t be assigned only the complete Etherchannel

Advanced Spanning Tree Protocol Features: Configuring and Troubleshooting Etherchannels

channel-protocol lacp/pagp

channel-group NUMBER (Must match at each end) mode on – No protocol used?

This will enable Etherchannel only with no specified protocol

Logical interfaces always appear at the top of a configuration

Use the interface range command to avoid having to do each port manually, be aware that if set 1 side with a delay, this can err disable ports locally as the other side has no configuration

Etherchannel Verification

show etherchannel ? (several options)

summary and detail are helpful commands

detail shows you how long a physical port has been in the bundle

show pagp/lacp neighbor – details neighbours in Etherchannel bundles

show etherchannel brief

Look out for vlan masking on physical interfaces, will impact the logical interface.

Ports in an Etherchannel can be source SPAN ports but NOT destination SPAN ports.

Advanced Spanning Tree Protocol: RSTP Values and Intro to Etherchannels


Port states – disabled, blocking and listening are combined into DISCARDING


Edge and Point to Point ports are connected to a single host.

If a BPDU is received on an edge port, the port is demoted to a regular RSTP port and generates a TCN BPDU.

P2p ports is any port running in Full Duplex mode

RSTP considers a topology change to be a port that moves into Forwarding mode

Switch that discovers the topology change will send a BPDU with the TC bit set

RSTP switches generate a BPDU every 2 seconds regardless of receiving a BPDU from the root switch

RSTP misses 3 BPDUs and will immediately age out the superior BPDUs info

UplinkFast, Portfast, BackboneFast are all built into RSTP and no config required

MST = 802.1s – multiple VLANs to be mapped into 1 instance of STP

For MST to agree:

  • 1. The MST config name
  • 2. The MST instance to VLAN mapping table
  • 3. The MST config revision number

Up to 16 instances of MST can exist in a region

  • Turn on MST
  • Create a REGION
  • Create a REVISION number
  • Create a INSTANCE and map VLANs


Etherchannels use an algorithm ‘Exclusive OR’ (XOR) to determine which channel in the EC to use to transmit data to the remote switch.

Values to match: Remote port group number and Device ID of all remote ports must be the same.

IF you change the physical speed on 1 interface in a Pagp bundle, this will reflect on the logical po interface

802.3d – LACP

Allows 16 ports , but only 8 ports will go into the bundle with the lowest port priority

PAGP – Dynamic and Auto, Dynamic will negotiate and Auto will wait for the remote switch to instruct

LAGP uses Active and Passive modes

ON option is with NO negotiation