Root Guard

For ports which must participate in STP, but never accept a superior BPDU.

Downlinks

Unlike BPDU Guard, the port still participates in spanning tree.

Example

Normally SW1 is the root bridge

┌──────┐                                                      ┌──────┐
│SW1 DP├─ 0/1/52:54:00:e8:3a:ff ─── 4096/1/52:54:00:4b:99:08 ─┤RP SW2│
└──────┘                                                      └──────┘

Someone configures SW2 to be the root by making the switch priority 0.

┌──────┐                                                      ┌──────┐
│SW1 RP├─ 0/1/52:54:00:e8:3a:ff ────── 0/1/52:54:00:4b:99:08 ─┤DP SW2│
└──────┘                                                      └──────┘

Config

This can be prevented with this config. Root guard goes onto DPs.

!
! SW1
!
interface 1
  spanning tree guard root

Verification

sw1# show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    4097
             Address     5254.004f.110e
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    4097   (priority 4096 sys-id-ext 1)
             Address     5254.004f.110e
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Desg BKN*4         128.1    P2p *ROOT_Inc 
Gi0/2               Desg BKN*4         128.3    P2p *ROOT_Inc

Logs

*May  3 20:14:45.169: %SPANTREE-2-ROOTGUARD_BLOCK: Root guard blocking port GigabitEthernet0/0 on VLAN0001.

Picture Example

loopguard-rootguard-locations

Image courtesy of BRKENS-2031.

References

Cisco Live - Enterprise Campus Design - Marcin Hamróz, and Jarosław Gawron - BRKENS-2031

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