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Difficulty: MediumFirst Hop Redundancy Protocols (HSRP and VRRP)

A network technician is configuring Hot Standby Router Protocol (HSRP) on Router R1 to provide default gateway redundancy for hosts in VLAN 20. Interface GigabitEthernet0/0/1 on R1 has been assigned the IPv4 address 172.16.20.1/24172.16.20.1/24. The technician enters the following commands in interface configuration mode:

text
R1(config-if)# ip address 172.16.20.1 255.255.255.0
R1(config-if)# standby 20 ip 172.16.20.1

Which statement accurately describes the outcome of executing these configuration commands?

  1. Cisco IOS rejects the virtual IP configuration because HSRP does not allow the virtual IP address to match a physical interface IP address on any participating router.Answer
  2. B
    The router accepts the command and automatically enables HSRP preemption to ensure Router R1 becomes the permanent Active router for group 20.
  3. C
    Cisco IOS accepts the configuration and assigns Router R1 a default HSRP priority of 255 because it acts as the IP address owner for the standby group.
  4. D
    The router accepts the virtual IP address, but HSRP stays permanently in the Listen state until a secondary standby IP address is defined.

Answer

Cisco IOS rejects the virtual IP configuration because HSRP does not allow the virtual IP address to match a physical interface IP address on any participating router.
In Cisco IOS Hot Standby Router Protocol (HSRP), the virtual IP address must be a unique, unassigned IPv4 address within the same IP subnet as the participating router interfaces. Cisco IOS will generate an error and reject any attempt to configure the virtual IP address to be identical to a physical interface IP address. Therefore, the statement noting that Cisco IOS rejects the configuration because HSRP does not allow matching physical IP addresses is correct.

Step-by-Step Solution

1
Analyze the proposed HSRP configuration command on Router R1.
The command attempts to assign 172.16.20.1172.16.20.1 as the HSRP virtual IP address for group 20, which is already configured as the physical IP address of GigabitEthernet0/0/1.
Identifying potential IP address conflicts between physical interface addressing and FHRP virtual addressing.
2
Evaluate HSRP protocol rules regarding virtual IP address assignment.
HSRP requires the virtual IP address to be an unused IP address within the same logical subnet as the physical interface IP addresses.
HSRP routers share a virtual IP and virtual MAC address. If the virtual IP matches a physical IP, ARP and routing table resolution conflicts occur.
3
Contrast HSRP behavior with VRRP operational rules.
While VRRP permits a router to act as the IP address owner (where the virtual IP matches the physical interface IP and automatically receives priority 255), HSRP strictly prohibits this practice and rejects the command in Cisco IOS.
Distinguishing vendor-standard HSRP mechanics from IEEE standard VRRP mechanics.

Key Concept

HSRP Virtual IP Address Assignment Restrictions
Estimated Time:1m 15s
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Topics

First Hop Redundancy Protocols (HSRP and VRRP)

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