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 may result in broadcasts continuously circling the network, which is called a broadcast storm. Because Cisco switches implement the IEEE 802.1d Spanning-Tree Algorithm, this looping can be avoided in the Spanning-Tree Protocol. The Spanning-Tree Algorithm guarantees that only one path is active between two network stations. The algorithm permits redundant paths that are automatically activated when the active path experiences problems.

Because WAN links are often critical pieces of the internetwork, redundant media is often deployed in WAN environments. As shown in Figure 4-8, backup links can be provisioned so they become active when a primary link goes down or becomes congested.

Figure 4-8 Backup Links Can Be Used to Provide Redundancy

Figure 4-8 Backup Links Can Be Used to Provide Redundancy

Often, backup links use a different technology. For example, a leased line can be in parallel with a backup dialup line or ISDN circuit. By using floating static routes, you can specify that the backup route has a higher administrative distance (used by Cisco routers to select which routing information to use) so that it is not normally used unless the primary route goes down.

NOTE When provisioning backup links, learn as much as possible about the actual physical circuit routing. Different carriers sometimes use the same facilities, meaning that your backup path is susceptible to the same failures as your primary path. You should do some investigative work to ensure that your backup really is acting as a backup.

Backup links can be combined with load balancing and channel aggregation. Channel aggregation means that a router can bring up multiple channels (for example, Integrated Services Digital Network [ISDN] B channels) as bandwidth requirements increase.

Cisco supports the Multilink Point-to-Point Protocol (MPPP), which is an Internet Engineering Task Force (IETF) standard for ISDN B channel (or asynchronous serial interface) aggregation. MPPP does not specify how a router should accomplish the decision-making process to bring up extra channels. Instead, it seeks to ensure that packets arrive in sequence at the receiving router. Then, the data is encapsulated within PPP and the datagram is given a sequence number.

At the receiving router, PPP uses this sequence number to re-create the original data stream. Multiple channels appear as one logical link to upper-layer protocols.

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