Chapter 12: Hot Standby Routing Protocol
In Depth
Dynamic environments are constantly growing. I happen to work in one and I see the demand for 99.99
percent reliability increasing every day. However, even in a worldwide Enterprise network, high availability
solutions are not prepared for various network failures. Here’s an example: I manage a few hundred servers at
a Fortune 100 company. I’m trying to get to a local intranet site and my browser just hangs. Then I try to ping
the server that is across subnets and the request times out. I can ping everything and anything within my
subnet, but then I try the default gateway. Does this sound all too familiar? Think of the customer impact, the
data warehouses that are unreachable, or online training sessions where students cannot connect. I find it hard
to believe sometimes that redundancy is not in place.
The point I’m trying to make is that department budgets or deep pockets do not always solve the problem.
Network and systems administrators need help, especially when it is usually a team of about four to six
engineers that manage a dynamic networked enterprise environment. If this was a Cisco switched campus
model, which would overcome these IP−related issues and provide network redundancy, every device would
have a backup device standing by in case of a failure. Route processor devices (such as internal cards in
multilayer switches and routers) would have been among the hardest devices to configure for fast convergence
redundancy.
The Cisco switched campus model builds redundancy into the Layer 2 switch block level devices. However,
Hot Standby Routing Protocol (HSRP) is designed to build redundancy into the Layer 3 routing devices found
in the Distribution layer of a network. It also provides convergence in seconds without manual intervention
from a network administrator while remaining transparent between other interfaces on the network. HSRP can
be applied to almost any LAN environment.
One primary feature of HSRP, which allows it to be so transparent to users, is its use of priority schemes.
These priorities are used to determine which router is set as the default active router. When a router is
manually assigned a priority, the standby interface that has a higher priority is selected as the active router.
HSRP is one of the best solutions when host interfaces on a local LAN segment require continuous access to
network resources.

Routing Problems

Within a standard client/server network, the ability to exchange routing information between segments is
allowed by Layer 3 address translation. However, although the clients may route the packet to its default
gateway, they cannot route beyond their local LAN segment.
Note The default gateway is the protocol address for the route processor to which data packets containing a
destination address outside the local segment are sent.
In order for the client to route information out of its local LAN segment, it must use a manually configured IP
stack; or, the client may be configured for Dynamic Host Configuration Protocol (DHCP) to determine a
default gateway. In any event, if the Layer 3 routing device assigned as the default gateway fails or is offline,
all devices located on that particular subnet or network will only be allowed to communicate with each other.
The local collision or broadcast domain becomes the entire network in the eyes of each device. You can place
another default gateway on the network, but there is no clear way to provide a secondary configuration to
another default route without manually resetting the default gateway on either the client or server.
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