Summary
Now that you have completed this chapter, you should a have a firm understanding of the following concepts:
  • HSRP provides automatic router backup when configured on Cisco routers that run IP over Ethernet.
  • Because many end-user devices usually are configured statically with the address of a single router, these devices do not automatically assume a new default gateway when routing protocols converge.
  • With HSRP, a set of routers works together to present the illusion of a single virtual router to the hosts on the LAN.
  • Although multiple routers can exist in an HSRP group, only the active router forwards the packets sent to the virtual router.
  • The router with the highest standby priority in the group becomes the active router.
  • The standby router automatically assumes the function of the active router if the active router fails.
  • Routers participating in an HSRP group communicate to each other via a multicast UDP-based hello packet.
  • Routers receive hello messages from the active router and consider that hello message to be valid for one holdtime.
  • HSRP defines six states in which a router may exist. When a router exists in one of these states, the router performs the necessary actions required in that state.
  • Multiple hot standby groups may coexist on a LAN segment. There can be up to 255 standby groups on any LAN.
  • While running HSRP, the end-user station must not discover the primary MAC addresses of the routers in the standby group. Any protocol that informs a host of the router primary address must be disabled.