re 20-7 shows another red alarm scenario. In this example, Telecom Switch C sending garbled frames to Router D, which sees consecutive OOF problems and declares a red alarm. When Router D declares the red alarm, a yellow alarm signal is sent back out the link. Locally undetected 00F signal Red alarm problem Yellow alarm Telecom Router A Router D Telecom Telecom Switch B Switch C Figure 20-7. Yellow alarm Alarms The way these alarms behave can be a bit confusing. A red alarm generally indicates that something serious is happening on the local equipment, but in the example in Figure 20-7, Router D is receiving SO many OOF errors that the signal is useless. Because Router D cannot figure out how to read the frames from the far side, it triggers a local red alarm and sends out a yellow alarm. Yellow Alarm (RAI) A yellow alarm is also called a remote alarm indication (RAI). A yellow alarm indi- cates a remote problem. A yellow alarm is defined in RFC 1232 as follows: A Yellow Alarm is declared because of an incoming Yellow Signal from the far-end. In effect, the circuit is declared to be a one way link. One vendor specifies that a yellow alarm to be declared for SF links when bit six of all the channels has been zero for at least 335 ms, and that it be cleared when bit six of at least one channel is not zero for less than one to five seconds. For an ESF link, a yellow alarm is declared if the signal pattern occurs in at least 7 of 10 contiguous 16-bit pattern intervals, and is cleared if the pattern does not occur in 10 contiguous 16-bit pattern intervals. Wow, what a mouthful! The simple truth is that unless you're designing T1 CSU/ DSUs, you don't need to know all of that. Here's what you need to know: a yellow alarm does not necessarily indicate a problem with your device; rather, it's a prob- lem being reported by the device to which you are connected. Figure 20-8 shows a simple network. Router A has a T1 link to Router D. The T1 is actually terminated locally at the T1 provider's central office, where it is usual