MultiChassis Redundancy – Review notes for Exam day of VSS vs Stackwise, the differences and benefits both reviewed and illustrated for clarity!


A review of VSS (Virtual Switching System) and StackWise “Chassis Redundancy” systems for the CCNP SWITCH exam, I already have a full blow Stackwise post here, however VSS doesn’t need to be beat to death to get the must knows for exam day down!

Lets get started first with VSS otherwise known as Virtual Switching System!

  • Pair up to 2 x 6500 series switches (500 ports each)
  • Up to 8 x 10GB Ethernet wires to connect them (can be 40km apart theoretically)
  • Provides MultiChassis EtherChannel (Diagram Below)
  • Best suited for Campus Area Networks because it can be physically far apart
  • Active = Handles all L2 / L3 Control traffic (CDP, NTP, SSH, Routing Updates, Etc)
  • Standby = Handles local packet forwarding, sends Control traffic to Active switch
  • VSL = Virtual Switch Link = The physical link(s) between VSS Active / Passive pair
  • VSLP = Virtual Switch Link Protocol = Mgmt Traffic for VSS Active / Passive pair
  • VSLP uses two sub-protocols to elect and manage the link
  • LMP = Link Mgmt Protocol = Exchanges info between Chassis to form VSS Pair, runs on all VSL links, detects and brings down Unidirectional links between Pair
  • RRP = Role Resolution Protocol = Used to negotiate Active / Standby Role of Pair

VSS is for 6500 series switches which are monster switches, with a monster throughput of 80gbps via VSL Trunks, which can be on different sides of a campus or city!

It also provides MultiChassis EtherChannel similar to how it was explained in my larger Stackwise lab, visually demonstrated here:



The VSL will inherently be bundled into an EtherChannel, however you can have one logical Port-Channel running to both the Active and Passive Chassis as pictured.

(PAgP EtherChannel may not work with VSS as demonstrated below)

The VSS Chassis will also send regular traffic from common VLANs over the VSL link if the hosts are on opposite Chassis as very poorly illustrated here:


Yes, I’ll go to that length to illustrate my point, I have the humility to do it!

(Best drawn arrows I’ve ever seen!!!)

Now lets take another quick look at Stackwise and what it brings to the table!


Note the full ring Topology here, as the switches sort of side-wind the Stackwise cables down the switches, and the bottom switch connects back up to the top completing the Full Ring Topology for full throughput.

Some details bullet point style:

  • Stackwise is specifically for 3750 switch models and requires special Stackwise cables as pictured above
  • Stackwise can interconnect 9 x 3750 switches
  • Stackwise provides 32gbps throughput with a Full Ring Topology
  • If not Full Ring or a cable breaks, throughput goes down to 16gbps
  • Switches elect Master based on the following criteria:
  • Best IOS Featureset – #1
  • Default Configuration – #2
  • Highest Uptime – #3
  • Lowest MAC Address – #4
  • Each switch keeps a copy of its local MAC Table along with a copy of the entire stack MAC Table provided by the Master
  • One management IP for all switches, if you plug into any Console port you will be on the elected Master Switch
  • Switches are hot swappable in the stack, if a Master detects inferior IOS version on a new switch joining stack, will update its IOS before allowing it to join
  • Ideal for smaller deployments in a wiring closet / rack mount

I’ve gone through this exhaustively again in this post however those basics of knowing the differences between VSS vs Stackwise should get you through CCNP SWITCH.

There is one final issue to draw attention to with MultiChassis EtherChannel

For some reason Cisco’s own Proprietary EtherChannel protocol PAgP does not do MultiChassis EtherChannel when I tested it with Stackwise, I’ve pulled some of a config from my Stackwise post to demonstrate what I mean:

SW1(config)#int ra fa1/0/1 – 2
SW1(config-if-range)#channel-group 13 mode desirable <<<— PAgP
Creating a port-channel interface Port-channel 13

(Lots of output from physical interfaces bouncing Down / Up)
SW1(config-if-range)#int ra fa2/0/1 – 2
SW1(config-if-range)#channel-group 13 mode desirable
%With PAgP enabled, all ports in the Channel should belong to the same switch
Command rejected (Port-channel13, Fa2/0/1): Invalid etherchnl mode

% Range command terminated because it failed on FastEthernet2/0/1

Not sure if many would catch it, but with Stackwise your switches become one logical switch with Chassis that start with Fa1/0/1, Fa2/0/1, Fa3/0/1 as pictured above.

LACP and EtherChannel ON mode both work with MultiChassis, I’ve never tested it on a 6500 Series switch, but it does not work on Stackwise so I would assume it would not work on VSS as well.

Moral of that story – PAgP is not MultiChassis friendly!

That is it for this post, quick bullet points for easy points on exam day, loopy out!


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