How to verify the Secondary Root?

As you know  we can configure the designated switch as the "Primary root role" or the "Secondary root role" using the following commands:

spanning-tree vlan 10 root primay
spanning-tree vlan 20 root secondary
  

Although these commands are just "macros" that we cannot see them in the running configuration, it is still easy to verify the "Primary root role" is correctly configured: We just use "show spanning-tree" command to check the designated switch is really the current "Root Bridge".

Observe and configure designated switch as Spanning Tree Protocol's root role (CNA-03-005)

CCNA Exploration 4.0, Semester 3, "Observe and configure designated switch as Spanning Tree Protocol's root role" Packet Tracer 5.0 practice file (CNA-03-005).

• CNA-03-005.pkt, (Backup Link)
• CNA-03-005-sol.pkt, (Backup Link)

It will not be difficult for you to maintain VLAN connectivity for both VLAN 10 and VLAN 20. We have four previous examples. Practice 1, Practice 2, Practice 3, and Practice 4.

This practice is focused on the root role switch of a Spanning Tree Protocol domain.

How important is the root role switch? Because all broadcast frames in that VLAN would go through the root role switch. That is, if your root role switch's performance is modest, you will absolutely have poor network performance as a whole.

Practically, we usually choose the best performance switch (maybe also the most expensive switch!) as the root role in a Spanning Tree Protocol domain.

Root role is automatic selected according to the Spanning Tree Protocol. If all switches are using default Spanning Tree configuration, the root role is almost "randomly" chosen, because switch with the smallest MAC addresses (treat that MAC address as a long hexadecimal number) will be elected as root role.

See below the "show spanning-tree" result on "S2". It is the elected root role switch for VLAN 1.

S2#show spanning-tree 
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0002.4AEE.B73B
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0002.4AEE.B73B
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Desg FWD 19        128.1    P2p
Fa0/2            Desg FWD 19        128.2    P2p
Fa0/6            Desg FWD 19        128.6    P2p
Fa0/11           Desg FWD 19        128.11   P2p
Fa0/18           Desg FWD 19        128.18   P2p

S2#

In this practice, if we want to designate S1 as the root for VLAN 10, and S3 as the root for VLAN 20, we can issue the following commands on respective switches:

(These commands are the few exceptions of commands that won't be listed in the running-config. You have to use "show spanning-tree" command instead to observe!)

S1(config)# spanning-tree vlan 10 root primary

S3(config)# spanning-tree vlan 20 root primary

I believe you would guess there should be a "secondary" command of the same form. Exactly! This secondary command is for us to designate "backup switch" for the root role. When the root role switch dies, it will take over to be the new root role switch.

In this example we assume we also want S1 to be the backup for VLAN 20, and S3 to be the backup for VLAN 10, as root role switch. The we should issue the following commands:

S1(config)# spanning-tree vlan 20 root secondary

S3(config)# spanning-tree vlan 10 root secondary

We can use these commands to choose which switch to be the primary root role, and which switch to be the backup (secondary) root role, as the way we want!

What is a "Spanning Tree"? Learn it by her song!

The inventor of Spanning Tree algorithm, Dr. Radia Perlman, wrote this "Algorhyme", to explain the essence of "Spanning Tree algorithm" itself. It was published in her book: "Interconnections: Bridges, Routers, Switches, and Internetworking Protocols, 2nd Ed", on page 58.

In the video it also mentioned the "Piano" melody was also by her. Well, I am not sure about it!

Do you have clearer picture of what "Spanning Tree Protocols" is, now?

Algorhyme

I think that I shall never see
A graph more lovely than a tree.

A tree whose crucial property
Is loop-free connectivity.

A tree that must be sure to span
So packets can reach every LAN.

First the root must be selected.
By ID, it is elected.

Least-cost paths from root are traced.
In the tree, these paths are placed.

A mesh is made by folks like me,
Then bridges find a spanning tree.


- Radia Perlman

Go to the following URL of Google Books, then search with keyword "Algorhyme". Then you will get the source page! (On Page 58.)

http://books.google.com/books?id=AIRitf5C-QQC&pg=PA58&vq=spanning+tree&dq=book+interconnections+spanning+tree&source=gbs_search_s&cad=0