Step-by-Step With Cisco’s Hot Standby Router Protocol


The Hot Swappable Router Protocol (HSRP) is a way to build redundancy into
your network by allowing two or more routers to continuously test each
other for connectivity, and take over if a router fails. For
purposes of discussing a basic HSRP configuration, let’s assume we want to
make the 10.10.10./24 network dynamically redundant and have two building
routers at our disposal.


Using HSRP terminology, both building routers together are referred to as a
standby group and appear to the subnet as a single default gateway. Through
an election process, one router is designated as active and the other
router is designated as standby. Both the active and standby router listen
to routing updates from the core router, but only the active router
processes IP packets as the default gateway for the subnet. The active and
standby routers are constantly sending “hello” packets back and forth. If the
active router fails, as soon as a predetermined number of “hello” packets
from the standby router to the active router go unanswered, the standby
router becomes the active router and starts processing IP packets for the
subnet.


Both routers are configured with standard IP addresses on their interface
into the subnet. Each router also has a special virtual IP address which is
the same on both routers, configured via standby commands under the
specific interfaces.

Configuring the Routers

Given a standard 24-bit subnet, we can build the following:

  • Network: 10.10.10.0
  • Subnet Mask: 255.255.255.0
  • Default Gateway: 10.10.10.251


Physical IP Address

  • Building Router A:      10.10.10.252
  • Building Router B:      10.10.10.253


The configuration commands are as follows:

building router a#
interface fa0/2
ip address 10.10.10.252 255.255.255.0


This sets the IP address for the interface

standby 100 timers 3 10

100 denotes the HSRP process number as “hot standby group 100”. You
can have multiple HSRP standby groups on the same interface. The
timers command sets the interval to 3 seconds between HELLO messages, and waits 10 seconds before the other router is declared down.

standby 100 priority 200 preempt

Defines the priority of this router. The highest priority number will win
the election. “Preempt” allows the router to take over control even if
there is not an election in process if it comes on line with the highest
priority.

standby 100 authentication fnord


Optional – Authentication [word] creates an unencrypted authentication process for each HSRP packet.

standby 100 ip 10.10.10.251


Establishes 10.10.10.251 as the virtual interface. This IP address should be the same on both HSRP routers.

standby 100 track fa0/0


The Hot Swappable Router Protocol (HSRP) is a way to build redundancy into
your network by allowing two or more routers to continuously test each
other for connectivity, and take over if a router fails. For
purposes of discussing a basic HSRP configuration, let’s assume we want to
make the 10.10.10./24 network dynamically redundant and have two building
routers at our disposal.


Using HSRP terminology, both building routers together are referred to as a
standby group and appear to the subnet as a single default gateway. Through
an election process, one router is designated as active and the other
router is designated as standby. Both the active and standby router listen
to routing updates from the core router, but only the active router
processes IP packets as the default gateway for the subnet. The active and
standby routers are constantly sending “hello” packets back and forth. If the
active router fails, as soon as a predetermined number of “hello” packets
from the standby router to the active router go unanswered, the standby
router becomes the active router and starts processing IP packets for the
subnet.


Both routers are configured with standard IP addresses on their interface
into the subnet. Each router also has a special virtual IP address which is
the same on both routers, configured via standby commands under the
specific interfaces.

Configuring the Routers

Given a standard 24-bit subnet, we can build the following:

  • Network: 10.10.10.0
  • Subnet Mask: 255.255.255.0
  • Default Gateway: 10.10.10.251


Physical IP Address

  • Building Router A:      10.10.10.252
  • Building Router B:      10.10.10.253


The configuration commands are as follows:

building router a#
interface fa0/2
ip address 10.10.10.252 255.255.255.0


This sets the IP address for the interface

standby 100 timers 3 10

100 denotes the HSRP process number as “hot standby group 100”. You
can have multiple HSRP standby groups on the same interface. The
timers command sets the interval to 3 seconds between HELLO messages, and waits 10 seconds before the other router is declared down.

standby 100 priority 200 preempt

Defines the priority of this router. The highest priority number will win
the election. “Preempt” allows the router to take over control even if
there is not an election in process if it comes on line with the highest
priority.

standby 100 authentication fnord


Optional – Authentication [word] creates an unencrypted authentication process for each HSRP packet.

standby 100 ip 10.10.10.251


Establishes 10.10.10.251 as the virtual interface. This IP address should be the same on both HSRP routers.

standby 100 track fa0/0

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