What is the primary function of VRRP (Virtual Router Redundancy Protocol)?
Correct Answer: B
Detailed Explanation: Understanding Gateway Redundancy and VRRP: In many networks, a default gateway (typically a router) is a single point of failure. If the default gateway router fails, devices relying on it to reach external networks will lose connectivity. Gateway redundancy protocols like VRRP, HSRP (Hot Standby Router Protocol - Cisco proprietary), and GLBP (Gateway Load Balancing Protocol - Cisco proprietary) address this single point of failure issue. VRRP (Virtual Router Redundancy Protocol): VRRP is an industry-standard protocol (RFC 5798) that allows multiple routers to work together to provide a virtual router service for a LAN. It achieves gateway redundancy by: Virtual Router Concept: VRRP creates a virtual router, which is an abstract entity represented by: One Virtual IP Address: A single IP address that acts as the default gateway for hosts on the LAN. This IP address is configured on the VLAN interface of the virtual router. One Virtual MAC Address: A MAC address associated with the virtual router IP address. Master and Backup Routers: In a VRRP group, one router is elected as the master router, and one or more other routers are designated as backup routers. Master Router: The master router actively forwards packets destined for the virtual router IP address. It "owns" the virtual IP and MAC addresses and responds to ARP requests for the virtual IP. Backup Routers: Backup routers listen passively for VRRP advertisements from the master. They are in standby mode and do not actively forward traffic unless the master router fails. VRRP Advertisements (Hello Messages): The master router periodically sends VRRP advertisement messages (VRRP hellos) to announce its presence and priority. Backup routers listen for these advertisements. Master Router Election and Failover: Priority Based Election: Routers in a VRRP group are configured with a priority value. The router with the highest priority becomes the master router by default. Preemption (Optional): If preemption is enabled, a router with a higher priority that comes online later will preempt the current master router and become the new master. Failover Mechanism: If the master router fails (stops sending VRRP advertisements or becomes unreachable), a backup router with the next highest priority is automatically elected as the new master. This failover is typically very fast (within seconds). Virtual IP and MAC Address Migration: When a failover occurs, the virtual IP address and virtual MAC address "migrate" to the new master router. Hosts on the LAN continue to send traffic to the virtual IP address, and the new master router now handles the forwarding, providing seamless gateway redundancy. Why Option B is Correct: High Availability and Redundancy for Gateways: Option B perfectly describes VRRP's primary function – to provide gateway redundancy and high availability by creating a virtual router from a group of physical routers. This ensures that if one router fails, another automatically takes over the gateway role, minimizing network downtime. Why Other Options are Incorrect: A. To increase network bandwidth by aggregating multiple physical router interfaces into a single logical interface. This describes the function of link aggregation or EtherChannel (at Layer 2) or potentially technologies like port channeling in some router contexts. VRRP does not aggregate bandwidth; it's about gateway redundancy. VRRP uses multiple routers, not multiple interfaces on a single router, to provide redundancy. C. To dynamically route traffic based on application type and Quality of Service (QoS) requirements. This describes the function of Policy-Based Routing (PBR) or QoS routing mechanisms. While routers involved in VRRP can also implement PBR or QoS, VRRP itself is not for dynamic application-based routing or QoS routing. VRRP is specifically for gateway failover. D. To secure routing updates by encrypting routing protocol messages and authenticating routing neighbors. This describes security features of routing protocols themselves, such as routing protocol authentication (e.g., MD5 authentication in OSPF or EIGRP) or potentially more advanced security mechanisms like IPsec-protected routing protocol sessions. VRRP is not about securing routing protocol updates; it's about gateway redundancy and failover. In Conclusion: VRRP is a fundamental high availability protocol for network gateways. It enables multiple routers to cooperate as a single virtual router, providing automatic failover and ensuring continuous network connectivity even if one router fails. Understanding VRRP (and similar protocols like HSRP and GLBP) is crucial for designing resilient network infrastructures and is a key topic in CCNA studies. This question directly tests your understanding of VRRP's core purpose in providing gateway redundancy.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.