Designing Link Redundancy
It is often necessary to provision redundant media in locations where mission-critical application traffic travels. In Layer 2-switched networks, redundant links are permitted as long as STP is running. STP guarantees one, and only one, active path within a broadcast domain, avoiding problems such as broadcast storms (when a broadcast continuously loops). The redundant path automatically activates when the active path goes down.
Because WAN links are often critical pieces of the internetwork, redundant media are often deployed in WAN environments. As is the case in Figure 3-20, where a Frame Relay circuit is used in parallel with a backup IPsec connection over the Internet, backup links can use different technologies. It is important that the backup provide sufficient capacity to meet the critical requirements if the primary route fails.
Figure 3-20 Example of Enterprise Edge Link Redundancy
Remote Site
Primary WAN
Access
Router
Backup WAN
Access Router
Primary WAN
Access
Router
Remote Site
Redundancy in Access
Backup WAN
Building Distribution Multilayer Switching
Redundancy in Access
Access Router
Backup links can be always-on or become active when a primary link goes down or becomes congested.
Backup Links
Backup links often use a different technology. For example, a leased line can be parallel with a backup IPsec connection over the Internet.
Using a floating static route, specify that the backup route has a higher administrative distance (used by Cisco routers to select which routing information to use) than the primary route learned from the routing protocol in use. Doing so ensures that the backup link is not used unless the primary route goes down.
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. Do some investigative work to ensure that the backup really is a backup.
Backup links can be used for load balancing and channel aggregation. Channel aggregation means that a router can bring up multiple channels (such as ISDN B channels) as bandwidth requirements increase.
Cisco supports the Multilink Point-to-Point Protocol (MLP), also referred to as MPPP, which is an Internet Engineering Task Force (IETF) standard for ISDN B channel (or asynchronous serial interface) aggregation. MLP 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. 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.
Continue reading here: IP Telephony Components
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