5.4. Loops in a Remotely Bridged (WAN) Catenet
In the preceding discussions, all of the links were presumed to be high-speed LANs. However, bridges are often used to interconnect geographically separated locations using intermediate Wide Area Network (WAN) links, as discussed in Chapter 3, "Bridging Between Technologies." While the STP will properly resolve loops in such a configuration (and is sometimes even used for this purpose), some important issues arise from the differences between LAN and WAN technologies. In Chapter 3, we discussed the issues relating to data flow on wide area bridged catenets; in this section we examine issues relating to the use of the STP on such links.
5.4.1. There's More Than a One-Letter Difference
There are two significant differences that affect the behavior and operation of bridged catenets using the STP:
Link speeds are lower. In general, the data rates of WAN links are much lower than those of local networks. While LANs usually operate in the range of 10 to 1000 Mb/s or more, WAN bridge interconnections today typically operate in the range of 56 kb/s to 155 Mb/s. The most popular site interconnection links operate at rates of 56/64 kb/s to 2 Mb/s. STP link costs will be higher on WAN links than on LANs. The range of values for the link cost (1 to 65, 535) can accommodate links as slow as 15 kb/s using the default recommendation for cost calculation.
Links cost money every month. Unlike LANs, WAN links usually incur a recurring monthly cost. ...
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