HSRP Written Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.

HSRP

*** Theory ****

  • Hot Standby Routing Protocol (HRSP): Allows two or more routers to share an IP address, e,g. Default gateway so that if one goes down the other router can still serve. Create a standby address that the standy group will share. Cisco calls this a phantom router.
    • MAC Address: Has its own (virtual) MAC address to avoid long arp cache timeouts and the HSRP primary router will respond to it.
    • How it works: The group sends multicast hellos to eachother 1 in every 3 seconds
    • Primary router: elected via priority.
    • Failover: Next router becomes primary and then announces it is the primary and all other routers will re-arrange their roles / priority to know who will be next
    • Resign: failover router resigns if primary router comes back up and sends hello which still has the higher priority if pre-empt is enabled
    • Enabling it: conf-if# standby 10 ip 1.1.1.1
    • Pre=empt:allows it to reclaim as primary
    • Interface tracking – Monitors additional interface
    • Defaults: priority is 100 and no pre-empt

 

 

  • Hot Standby Routing Protocol (HSRP) – Cisco. 1 router is primary. Host uses virtual MAC/IP. (pseudorouter).  Hello timers etc can be changed. Highest priority determines primary router (pre-empt is disabled by default , so changing priority will not take immediate effect)
    • 00-00-0c-070ac0xx = HSRP well known MAC. Xx is group number, e.g. 05 or 17 = 11 (16 + 1)
    • States
      • Disabled
      • Initial (init) – interface is up but HSRP not running
      • Learn – Learn about the active router etc
      • Listen – knows the VMAC and listening for Hellos
      • Speak – Sending hellos
      • Standby  – Sends hello and is candidate for active
      • Active – Router is forwarding to VIP
    • HSRP Interface tracking – Monitor additional interface e.g. (WAN). If WAN goes down, the router priority goes down allowing the other router to be high priority and takeover (pre-empt enabled) – config-if#standby 1 track serial 0 decrement
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