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Describe the purpose of HSRP (Hot Standby Router Protocol).

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Hot Standby Router Protocol (HSRP) is a Cisco proprietary redundancy protocol designed to provide high availability for routers, particularly in scenarios where network reliability and minimal downtime are critical. HSRP ensures seamless failover by allowing multiple routers to work together in a group, with one router acting as the active or primary router and the others as standby or backup routers. The purpose of HSRP is to maintain continuous network operation by quickly and transparently transitioning to a backup router in the event of a failure in the active router.

Here's a more detailed technical explanation of HSRP:

  1. Router Redundancy:
    • HSRP is primarily used to provide router redundancy in a network. In a typical HSRP setup, two or more routers are configured to work together to provide a virtual IP address and MAC address. Clients in the network use this virtual IP address as their default gateway.
  2. Active and Standby Routers:
    • Within an HSRP group, one router is elected as the active router, and the others become standby routers. The active router is responsible for forwarding traffic, while the standby routers remain in a hot standby state, ready to take over if the active router fails.
  3. Virtual IP Address:
    • HSRP assigns a virtual IP address to the group, and this address is shared between the active and standby routers. The virtual IP address is what clients in the network use as their default gateway. The active router responds to ARP requests for this virtual IP, ensuring seamless traffic flow.
  4. Virtual MAC Address:
    • HSRP also assigns a virtual MAC address to the group. The virtual MAC address is associated with the virtual IP address and is used in ARP responses. This allows for a quick switch between the active and standby routers without requiring clients to update their ARP caches.
  5. Hello Messages:
    • HSRP routers exchange "Hello" messages at regular intervals to monitor the status of each other. If a router misses a certain number of consecutive Hello messages, it is considered to be in a failed state, and the standby router with the highest priority takes over as the new active router.
  6. Priority and Preemption:
    • Each router in the HSRP group is assigned a priority value. The router with the highest priority becomes the active router. In case of a tie, the router with the highest IP address is elected as the active router. Preemption is a feature that allows a higher priority router to reclaim the role of the active router when it becomes available again after a failure.
  7. Tracking and Interface Status:
    • HSRP allows for tracking the status of certain interfaces or parameters. If a tracked interface or parameter goes down, the router dynamically adjusts its HSRP priority, potentially causing a failover to a standby router.