Most IP hosts have an IP address of a single device configured as the default gateway. When HSRP is used, the HSRP virtual
IP address is configured as the host’s default gateway instead of the IP address of the device.
HSRP is useful for hosts that do not support a discovery protocol (such as ICMP Router Discovery Protocol [IRDP]) and cannot
switch to a new device when their selected device reloads or loses power. Because existing TCP sessions can survive the failover,
this protocol also provides a more transparent recovery for hosts that dynamically choose a next hop for routing IP traffic.
When HSRP is configured on a network segment, it provides a virtual MAC address and an IP address that is shared among a group
of devices running HSRP. The address of this HSRP group is referred to as the virtual IP address. One of these devices is
selected by the protocol to be the active device. The active device receives and routes packets destined for the MAC address
of the group. For n devices running HSRP, n+ 1 IP and MAC addresses are assigned.
HSRP detects when the designated active device fails, at which point a selected standby device assumes control of the MAC
and IP addresses of the Hot Standby group. A new standby device is also selected at that time.
HSRP uses a priority mechanism to determine which HSRP configured device is to be the default active device. To configure
a device as the active device, you assign it a priority that is higher than the priority of all the other HSRP-configured
devices. The default priority is 100, so if you configure just one device to have a higher priority, that device will be the
default active device.
Devices that are running HSRP send and receive multicast UDP-based hello messages to detect device failure and to designate
active and standby devices. When the active device fails to send a hello message within a configurable period of time, the
standby device with the highest priority becomes the active device. The transition of packet forwarding functions between
devices is completely transparent to all hosts on the network.
You can configure multiple Hot Standby groups on an interface, thereby making fuller use of redundant devices and load sharing.
The figure below shows a network configured for HSRP. By sharing a virtual MAC address and IP address, two or more devices
can act as a single virtual router. The virtual device does not physically exist but represents the common default gateway
for devices that are configured to provide backup to each other. You do not need to configure the hosts on the LAN with the
IP address of the active device. Instead, you configure them with the IP address (virtual IP address) of the virtual device
as their default gateway. If the active device fails to send a hello message within the configurable period of time, the standby
device takes over and responds to the virtual addresses and becomes the active device, assuming the active device duties.
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