Examkin

Soru

Zorluk: ZorSTP Port States and PortFast Benefits

An enterprise desktop administrator reports that newly provisioned Linux endpoints attached to switch port GigabitEthernet0/1 consistently fail to receive an IPv4 address via DHCP during system startup. However, if the DHCP client service is manually restarted 40 seconds after the operating system boots, an address is acquired successfully. The switch port is configured as a standard 802.1D access port. Which interface configuration command combination immediately resolves the boot-time DHCP timeout while ensuring the switch port is protected against unauthorized switches creating Layer 2 loops?

  1. Configure 'spanning-tree portfast' together with 'spanning-tree bpduguard enable' on GigabitEthernet0/1.Cevap
  2. B
    Configure 'switchport mode trunk' and 'spanning-tree portfast trunk' on GigabitEthernet0/1 to suppress outgoing BPDUs.
  3. C
    Configure 'spanning-tree vlan 1 priority 4000' globally to reduce convergence timers on access interfaces.
  4. D
    Configure 'ip helper-address' on interface GigabitEthernet0/1 to bypass Spanning Tree Protocol port state transitions.

Cevap

The correct option specifies configuring 'spanning-tree portfast' along with 'spanning-tree bpduguard enable' directly on the edge access interface.
Enabling PortFast on access port GigabitEthernet0/1 causes the switch port to immediately transition from the Disabled/Blocking state directly into the Forwarding state when the physical link comes UP. This eliminates the 30-second 802.1D delay (15s Listening + 15s Learning), allowing the host's initial boot-time DHCP Discover request to pass through successfully. Furthermore, enabling BPDU Guard on the interface adds loop prevention: if a user plugs an unauthorized switch into GigabitEthernet0/1, the port receives BPDUs and is automatically placed into an err-disabled state.

Adım Adım Çözüm

1
Analyze the cause of the host DHCP timeout during bootup.
Standard 802.1D STP forces a newly connected link to pass through Listening (15 seconds) and Learning (15 seconds) states before reaching Forwarding (total 30 seconds delay). During this 30-second window, data frames including DHCP Discover messages are dropped.
Host operating systems typically issue DHCP requests immediately upon link UP and time out before STP transitions the port to Forwarding.
2
Determine the STP feature required to bypass listening and learning delays.
Cisco STP PortFast allows edge ports connected to single end-user devices to immediately enter the Forwarding state upon link UP.
PortFast bypasses the 30-second convergence delay, allowing instant frame transmission for DHCP initialization.
3
Identify the protection mechanism to prevent Layer 2 loops if a switch is plugged into a PortFast port.
STP BPDU Guard disables the PortFast-enabled port (placing it in err-disable state) upon receiving any BPDU frame.
Because PortFast ports transition directly to Forwarding without standard convergence analysis, receiving a BPDU indicates a potential switch or bridge attachment that could form a Layer 2 loop.

Anahtar Kavram

STP PortFast Immediate Forwarding and BPDU Guard Security Mechanics
Bu soruyu puanla

Konular

STP Port States and PortFast Benefits

Bu konudan daha fazla soru