HSRP- Another FHRP protocol is HSRP which is a Cisco proprietary protocol. This protocol functions in the same way as VRRP does for the most part with a few differences, so let’s dive into it. The main router that is responsible for forwarding traffic for the HSRP group is called the “active” router. The active router is chosen from the HSRP by having the highest priority, and if there is a tie then the highest IP address wins the active router spot. The router with the next highest priority (next highest IP if there is a tie-breaker) becomes the standby router, any other additional routers beyond this become listening routers. This router is not forwarding any traffic but paying attention to the health of the active router and if it were to fail for any reason it will take over the active role immediately. Just like VRRP every router in the group is assigned a virtual IP and MAC address. HSRP has two versions, one called V1 and one called V2. The virtual MAC address of version 1 is 0000.0C07.AC** and the MAC address of version 2 is 0000.0C9F.F***. So if you are looking at configurations and see a consistency of those addresses assigned to hosts you will be able to tell which version of HSRP is being used.
Hello Packets- HSRP monitors the health of the active router by sending hello packets. Unlike VRRP where only the master router sends advertisements, hello packets are sent from both the active and standby routers. By sending hello packets routers in the HSRP can elect the active/standby roles, and monitor the health of the active router. The hello timer works in conjunction with the hold timer. The typical HSRP hello timer is 3 seconds while the hold timer is usually 10 seconds. If a standby router does not receive a hello packet from the active router within 10 seconds (hold timer), the active router will be declared dead and the next standby router will take over the active default router position. Hello packets are sent via multicast 224.0.0.2 for version 1 and 224.0.0.102 for version 2. Just like VRRP, a gratuitous ARP will be sent to refresh hosts ARP cache and to update the new port to MAC mapping on the CAM tables in the topology. Gratuitous ARPs are only sent when the standby router takes over the role of the active router.
Preemption- Preemption is disabled by default for HSRP, so when the active router goes down having another standby router takes its role when the active router comes back online it will remain as a standby router unless something triggers a re-election.
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