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Difficulty: HardFirst Hop Redundancy Protocols (HSRP and VRRP)

A network engineer is configuring first-hop redundancy for VLAN 20 on two Cisco IOS routers, R1 and R2. Interface GigabitEthernet0/0 on R1 is assigned IPv4 address 10.20.0.2/24, and GigabitEthernet0/0 on R2 is assigned 10.20.0.3/24. The engineer attempts to configure HSRP group 20 on R1 using the command `standby 20 ip 10.20.0.2`. The Cisco IOS CLI rejects the command with an error message. However, when configuring VRRP using `vrrp 20 ip 10.20.0.2`, the command is accepted successfully. What fundamental operational difference between HSRP and VRRP causes this behavior?

  1. VRRP permits the virtual IP address to match the physical IP address of a router interface, appointing that router as the IP address owner with a default priority of 255, whereas HSRP strictly prohibits assigning any router's physical IP address as the virtual IP.Answer
  2. B
    HSRP requires the virtual IP address to be identical to the active router's physical interface IP address, whereas VRRP requires a separate IP address outside the interface subnet range.
  3. C
    HSRP requires preemption to be explicitly enabled before a virtual IP address can be specified under the interface configuration, whereas VRRP automatically enables preemption by default.
  4. D
    VRRP uses a dedicated multicast group address to resolve physical-to-virtual IP mapping conflicts, whereas HSRP relies on broadcast traffic that prevents physical IP reuse.

Answer

VRRP permits the virtual IP address to match the physical IP address of a router interface, appointing that router as the IP address owner with a default priority of 255, whereas HSRP strictly prohibits assigning any router's physical IP address as the virtual IP.
In Cisco IOS, HSRP mandates that the virtual IP address must be a unique IP address on the subnet and cannot be identical to any physical IP address configured on a router's interface. In contrast, VRRP allows the virtual IP address to match the interface physical IP address; the router whose physical IP matches the VRRP group virtual IP assumes the role of Master with an unalterable priority of 255 (IP address owner).

Step-by-Step Solution

1
Analyze the HSRP virtual IP configuration restriction.
HSRP requires that the virtual IP address defined in `standby <group> ip <address>` must be a unique, unused IP address on the subnet, separate from all physical interface IP addresses of the participating routers.
Assigning an existing physical IP address as an HSRP virtual IP address causes an IP overlap/conflict on the interface and is rejected by the Cisco IOS CLI.
2
Analyze the VRRP virtual IP configuration capability.
VRRP allows the virtual IP address to be identical to the primary physical IP address of one of the VRRP routers. The router owning that physical IP becomes the IP Address Owner and automatically receives a VRRP priority of 255.
VRRP standard RFC 5798 explicitly defines the IP address owner role to allow interface IP address sharing.
3
Select the option that accurately contrasts this behavior between HSRP and VRRP.
The correct response highlights that VRRP allows IP address ownership with a priority of 255 while HSRP prohibits physical IP reuse as the virtual IP.
This directly explains why HSRP rejected the configuration while VRRP accepted it.

Key Concept

HSRP vs VRRP Virtual IP Address Assignment and IP Address Ownership
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Topics

First Hop Redundancy Protocols (HSRP and VRRP)

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