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.
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