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 Protocol (STP). 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
Because WAN links are often critical pieces of the internetwork, WAN environments often deploy redundant media. As shown in Figure 2-17, you can provision backup links so that they become active when a primary link goes down or becomes congested.
Figure 2-17 Backup Links Can Provide Redundancy problems.
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 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 circuit routing. Different carriers sometimes use the same facilities, meaning that your backup path might be 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.
You can combine backup links with load balancing and channel aggregation. Channel aggregation means that a router can bring up multiple channels (for example, ISDN B channels) as bandwidth requirements increase.
Cisco supports 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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