Information Technology Reference
In-Depth Information
Link Media Redundancy
In mission-critical applications, it is often necessary to provide redundant media.
In switched networks, switches can have redundant links to each other. This redundancy is
good because it minimizes downtime, but it can result in broadcasts continuously circling
the network, which is called a
broadcast storm
. Because Cisco switches implement the
IEEE 802.1d spanning-tree algorithm, you can avoid this looping in Spanning Tree Proto-
col (STP). The spanning-tree algorithm guarantees that only one path is active between
two network stations. The algorithm permits redundant paths that are automatically acti-
vated when the active path experiences problems.
STP has a design limitation of only allowing one of the redundant paths to be active. VSS
can be used with Catalyst 6500 switches to overcome this limitation.
Because WAN links are often critical pieces of the internetwork, WAN environments of-
ten deploy redundant media. As shown in Figure 2-18, you can provision backup links so
that they become active when a primary link goes down or becomes congested.
Primary
Backup
Access Sites
Figure 2-18
Backup Links Can Provide Redundancy
Often, backup links use a different technology. For example, a leased line can be in paral-
lel with a backup dialup line or ISDN circuit. However, it is more common to use DSL
lines as backup in today's networks. By using floating static routes, you can specify that
the backup route have a higher administrative distance (used by Cisco routers to select
routing information) so that it is not normally used unless the primary route goes down.
This design is less available than the partial mesh presented previously. Typically, on-
demand backup links reduce WAN charges.
Note:
When provisioning backup links, learn as much as possible about the physical cir-
cuit routing. Different carriers sometimes use the same facilities, meaning that your backup
path might be susceptible to the same failures as your primary path. Do some investigative
work to ensure that your backup really is acting as a backup.
