Introduction to The Hot Standby Router Protocol (HSRP)

Arya MirNetworking and Communications

Oct 12, 2013 (3 years and 10 months ago)

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Often one of the least redundant parts of a network is the first hop between a host and the rest of the network; this is because they are typically configured with a default gateway IP address that links to a single device. Should this device fail, then all of the users who are on a specific segment using it as their default gateway will be unable to reach any other subnet including the Internet.

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Introduction to The Hot Standby Router
Protocol (HSRP)
Date: Sep 23, 2013 By
Sean Wilkins
.
Article is provided courtesy of
Cisco Press
.
In this article, networking consultant Sean Wilkins introduces HSRP and how it operates,
along with the main concepts that should be known before attempting to configure it.
Often one of the least redundant parts of a network is the first
hop between a host and the
rest of the network; this is because they are
typically configured with a default gateway IP
address that links to a single
device. Should this device fail, then all of the users who are on
a specific
segment using it as their default gateway will be unable to reach any other
subnet
including the Internet.
There are a number of different solutions to this problem, and most
of these are all grouped
together and referred to as
First Hop Redundancy Protocols
(FHRP). This article looks at the
Hot Standby Redundancy Protocol (HSRP), which Cisco developed several years ago
when
no other standardized solution existed. This article specifically looks at
HSRP and how it
operates along with the main concepts that should be known
before attempting to configure
it.
My next
HSRP Configuration article
includes the details of how to configure
HSRP.
How Does HSRP Work?
The general idea behind HSRP is rather simple: Configure multiple
devices that all exist on
the same subnet and are able to act as gateways to
the hosts on the subnet. Basic
redundancy can be configured by configuring some
of the hosts to use one gateway and the
other hosts to use another. However, in
this situation, if either one of the gateways was to
fail, then there are still
a large number of hosts that would lose outside network access.
HSRP provides a solution to this problem by allowing two of the
connected gateways to be
configured to provide redundancy. HSRP does this by
providing a
virtual
MAC and IP
address that is shared between these two devices; the
active
device between the two of
them will be responsible for the
handling of traffic to the virtual IP address, while the
standby
device will monitor the active device for signs of failure.
Should the active router fail, the
standby router will take over the duties of
handling traffic that is sent to the virtual IP address
by both accepting
traffic to the IP address and by taking over the virtual MAC address (using
the
Address Resolution Protocol (ARP)). Hosts that are configured on the subnet use
the
virtual IP address as their gateway IP address, and if there is a failure
between the HSRP
devices, the switchover requires no extra configuration on the
host device.
The determination as to which HSRP device on a subnet is elected
the active router (versus
the standby router) is completed initially by
referencing the configured HSRP priority (0 –
255). If configured, the device
with the highest priority will become the active HSRP router,
and the router
with the second highest priority will become the standby router. If there are
more than two devices, all other devices will monitor active and standby
devices and wait
until they are required when a failure occurs. If none of the
HSRP devices are configured
with a non-default priority (100), then HSRP will
use the device with the highest configured
IP address on the shared subnet.
HSRP also provides the ability to configure multiple HSRP groups on
a single interface.
There are a number of different reasons why multiple HSRP
groups would work in this way,
including the ability to offer further
redundancy configurations and load balancing
configuration options. The
utilization of a single HSRP group between devices has one
major disadvantage:
One of the gateways always sits idle until the active device fails. This
wastes
the forwarding potential of this second device; to solve this problem, utilize
multiple
HSRP groups.
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HSRP Versions
There are two versions of HSRP: version 1 and version 2. The
difference between them in
terms of general functionality is small, but there
are several improvements that Cisco made
when updating to version 2. Some of these
improvements include:
HSRP version 2 now advertises and learns
millisecond timer values, improving the
stability of HSRP.
The total number of HSRP group numbers increases
from 256 to 4096 (not
necessarily to utilize this many groups on a single interface,
but to match local VLAN
numbers—on sub-interfaces—for easy troubleshooting and
documentation).
HSRP version 2 includes the addition of an
identifier field (which contains the
physical MAC address of the source) within
the HSRP packet that allows the ability
to easily track which device sent an
HSRP packet (this is because the active HSRP
router will send traffic using the
virtual MAC address).
HSRP version 2 changes the multicast address
that is used for HSRP
communication to avoid conflict with Cisco Group
Management Protocol (CGMP).
Some other differences are not really improvements, per se. HSRP
version 1 utilizes the
MAC address range from 0000.0C07.AC00 through
0000.0C07.ACFF (00 = Group 0, FF =
Group 255), while HSRP version 2 utilizes
the MAC address range from 0000.0C9F.F000
through 0000.0C9F.FFFF (000 = Group
0, FFF = Group 4095). HSRP version 1 uses the
multicast address 224.0.0.2,
while HSRP version 2 uses the multicast address 224.0.0.102.
HSRP version 1 and
version 2 are not compatible and use different packet formats.
HSRP Preemption, Priority, and Object Tracking
By default, HSRP devices have preemption disabled. What this means
is that if a device with
a higher priority were to come up on an existing HSRP
network, it would not automatically
become the active HSRP device. It would
only gain this role should both the active AND
standby devices fail. If HSRP
preemption is enabled, then the device with higher priority will
assert itself
when it comes online to become the active HSRP device. As covered briefly
above, the default HSRP priority that is configured on interfaces is 100 with a
valid range
from 0 through 255 (higher being better).
Some other capabilities exist with HSRP that enable it to alter its
behavior (which device is
active and standby), should a separate tracked object
alter its status. For example, if two
HSRP devices were configured on a subnet
and both had separate connections that
connected them to the next level of
connectivity (closer to the center of the network). If one
of these connections
went down, it would not be an optional condition for that device to
become (or
stay) the active HSRP router. A simple example of this is shown in Figure 1.
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Figure 1
Object tracking example topology
In this case, if R1 was the active HSRP router and its primary
connection to the rest of the
network went down, it would not be the most
efficient choice to be the active HSRP router
anymore. Object tracking can
automate the process of decrementing the HSRP priority to
alter the selection
of HSRP active and standby devices.
Summary
The intention of this article is to cover the essential concepts
that you need to understand
before attempting to configure HSRP. Overall, it is
not a complicated concept, and should be
picked up easily by someone with
minimal network experience and exposure.
The next
article
will cover how HSRP can be configured on a device, and the configuration
commands
that are required to alter the HSRP properties and behavior based on
the specifics of a
situation.
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