HI Naveen,

I hope it will helps

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Spanning Tree Protection

BPDU Guard

• Prevent      loops if another switch is attached to Portfast port enabled port.

• When enabled      on interface,  port goes in error-disabled state if BPDU is received
  • Switch(config-if)#spanning-tree       bpduguard enable

               (note portfast may not be configured)

• Can be      enabled on global configuration mode
  • Switch(config)#spanning-tree       portfast bpduguard default

BPDU Filter:

• If       enabled on interface mode it prevents the port from sending or       receiving BPDU
  • Switch(cofig-if)#spanning-tree       bpdufilter enable

• If       enabled globally if portfast interface receive the BPDU it take out       portfast status
  • Switch(config)#spanning-treee       portfast bpdufilter  default

Root Guard

• Prevent      other switch becoming root bridge
• Enabled      on  ports other than the root port and on the switches other than      root switch
• Root guard      port  receives a BPDU that might cause it to become a root port, then      the  port is put into “root inconsistent” state and does not pass      traffic through it.
• But if port      stops  receiving these BPDU, it automatically re-enables itself
  • Switch(config)#spanning-tree       guard root.

UDLD (Unidirectional Link Detection)

• Detects      physical broken physical link in absence of layer 1 electrical       keepalives (Ethernet calls this a link beat).However, sometimes a      cable  is intact enough to maintain keepalives, but no to pass data      in both  directions. This is unidirectional link.

• UDLD detects      a unidirectional link by sending periodic hellos out to  the      interface. It also uses porbes, which must be acknowledged by the       devices on the other end of the link. UDLD operates at Layer 2. The      port  is shutdown if a unidirectional link is found.

• UDLD will      not consider a link eligible for disabling until it has seen  a      neighbor on the interface already.This prevents it from disabling an       interface when only one end of the link has been configured to      support  UDLD.

• The default      UDLD message timer is 7 or 15 seconds (depending on the  platform),      allowing it to detect a unidirectional link before STP has  time to      transition the interface to forwarding mode

• UDLD has two      modes of operation:
  • Normal mode       – UDLD will notice and log a unidirectional link condition, but the       interface is allowed to continue operating.
  • Aggressive mode       – UDLD will transmit 8 additional messages (1 per  second); if none       of these are echoed back the interface is placed in the        error-disabled state.
  • UDLD can be       enabled globally for all fiber interfaces, or per-interface

• Global      command but it applies only to fiber ports
  • Switch(config)#       udld { enable | aggressive | message time }

• The UDLD      message time can be from 7 to 90 seconds

• To enable      udld for non fiber port enable same command on interface mode
  • Switch(config-if)#       udld { enable | aggressive | disable }

• To disable      UDLD on a specific fiber port, use the following command:
  • Switch(config-if)#       udld disable

• To disable      UDLD on a specific non-fiber port, use the following command:
  • Switch(config-if)#no       udld enable

• To re-enable      all interfaces shut by UDLD, use the following:    
  • Switch#udld       reset

• To verify      UDLD status, use the following:
  • Switch#show       udld interface

Loop guard

• Loop guard      prevents form loop which occurs as  result of blocking port      transiting to forwarding state
  • If no        Bpdu are received on a blocked port for a specific length of  time.       Loop guard puts that port into “loop inconsistent”  blocking state
  • Loop Guard       automatically re-enables the port if it starts receiving BPDUs again.
  • It is most       effective when enabled in the entire switched network in conjunction with       UDLD.

• To enable      Loop Guard for all point-to-point links on the switch, use the following      command:
  • Switch(config)#       spanning-tree loopguard default

• To enable      Loop Guard on a specific interface, use the following:
  • Switch(config-if)#       spanning-tree guard loop

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Regards