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JUNOS High Availability by Orin Blomberg, Senad Palislamovic, Kieran Milne, James Sonderegger

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Bidirectional Forwarding Detection

Lowering the timers of OSPF and IS-IS can be a reasonable solution to contain network failures, but it comes at a cost. It is possible to lower the timers to the point that they cause problems for the router. Imagine a scenario in which you have hundreds of interfaces all announcing subsecond OSPF Hellos—that’s a lot of work for a router! Now, of course, the amount of hardship the router incurs depends on the size and capacity of the router, how many interfaces are in play, and how exactly you configure the timers. But suffice it to say, there comes a point when IGP timers are not the solution to fast link failure detection. And beyond these two IGPs, what about the other protocols that don’t have fast failure detection mechanisms? Or what if there is no protocol at all?

BFD is a simple protocol designed to perform one function: link failure detection. BFD provides a mechanism to directly monitor a link and to notify the device if it stops receiving messages from a neighbor on the other end of that link. BFD relieves the IGP of having to provide fast failure detection; in fact, it can alleviate the load that Hello messages can place on a router. With BFD, it is no longer necessary to lower timers, or even to have them run at the default settings. In fact, you can configure higher timer values, virtually eliminating load issues on the router that result from generating Hello messages.

Another benefit of BFD is that it provides a centralized, common, ...

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