Tutorial

MPLS Network Reliance and Recovery
(continued)

The other failure detection system is called the error-message detection method (Figure 4).  When a device on the network detects an error, it sends a message to its neighbors to redirect traffic to a path or router that is working.  Most routing protocols use adaptations of this method.  The advantage of the error message is that network overhead is low.  The disadvantage is that it takes time to send the error-and-redirect message to the network components. Another disadvantage is that the error messages may never arrive at the downstream routers.

Figure 4: Error Message

If switchover time is not critical (as it has historically been in data networks), the error-message method works fine; however, in a time-critical switchover, the heartbeat method is often the better choice.

Reviewing Routing

Remember that, in a routed network (Figure 5), data is connectionless, with no real quality of service (QoS). Packets are routed from network to network via routers and routing tables. If a link or router fails, an alternative path is eventually found and traffic is delivered. If packets are dropped in the process, a layer-4 protocol such as TCP will retransmit the missing data.

Figure 5: Standard Routing

This works well when transmitting non-real time data, but when it comes to sending real-time packets, such as voice and video, delays and dropped packets are not tolerable. To address routing-convergence problems, the OSPF and IGP working groups have developed IGP rapid convergence, which reduces the convergence time of a routed network down to approximately one second.  

The benefits of using IGP rapid convergence include both increased overhead functions and traffic on the network; however, it only addresses half of the problem posed by MPLS.  The challenge of maintaining QoS parameter tunnels is not addressed by this solution.

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