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.Finally, the eighth line of the output shows that Frame Relay SVC is disabled(LAPF, the protocol that is used with SVCs for call setup, is down).In other words, thisconnection is PVC based.In summary, the show interface serial n command allows you to check the status of theinterface hardware, the link status, the encapsulation used, whether keepalives are beingsent, the LMI type, the standard statistics on the sent and received data, queue accounting,and so on.The LMI type, as said earlier, may be set as one of Cisco, ANSI, or Q933a.The LMI typeconfigured on the router must match the LMI type configured on the service provider switchto which the router connects.The LMI type at the other end of a connection between thedestination router and its service provider s switch may be different.Table 10-1 illustrateshow the seventh line of the show interface serial n would be different based on the LMItype configured on that interface.Notice that, depending on the LMI type used, a specialDLCI number is reserved for the local LMI usage.If Cisco LMI is used, dlci 1023 shall beused for LMI purposes.If ANSI or Q933a LMI is used, dlci 0 will be dedicated to LMI.Table 10-1 Indication of the Configured LMI Type on a Serial InterfaceWhat the output of the show interfaceThe Configured LMI type serial n indicates:Cisco LMI dlci 1023 LMI type is CiscoANSI LMI dlci 0 LMI type is ANSI Annex DQ933a LMI dlci 0 LMI type is CCITTThe number of resets and the number of carrier transitions are two important indicators tocheck while troubleshooting.In Example 10-1, line number 23 reports four interface resetsand line number 25 indicates six carrier transitions.Because line number 11 of the outputstates that the interface counters have never been cleared, the shown numbers haveaccumulated since the last router reload.You may discover the router s uptime using theshow version command.The number of interface resets reported on the output of any show interface command tellsyou how many times the interface buffers have been flushed (since the last time the interfacecounters were cleared or the router has reloaded).An interface reset may be forced by an316 Chapter 10: Diagnosing and Correcting Frame Relay Problemsadministrator through execution of the clear interface command, for instance or it mayhappen for any of the following reasons:" An interface has packets queued for transmission, but those packets are not sentwithin a reasonable amount of time (a few seconds)." There is a hardware problem (on the interface, cable, or the CSU)." There are clocking problems." The interface is looped." The router is attempting to restart an interface that has line protocol problems (therouter tries to do that periodically).The carrier transitions counter tells you how many times the DCD (Data Carrier Detect) haschanged state.A large number of carrier transitions, or at least a number much differentfrom the baseline value, should make you curious about the carrier s facility.You mustknow that every carrier transition causes the line to drop and the interface to reset.Now take a deeper look at the first line of the output from the show interface serial ncommand.This line may be reported as one of the following:" Serial1 is up, line protocol is up" Serial1 is up, line protocol is down" Serial1 is up, line protocol is down (looped)" Serial1 is administratively down, line protocol is down" Serial1 is down, line protocol is downThe first possible output tells you that the physical interface is up and that the link (lineprotocol) is up.It means that the serial interface is properly connected to the CSU (and isreceiving the clock signal).You also can conclude that the interface is communicating withthe service provider s switch (using LMI) via the local loop.The second possible output indicates that the router considers the serial interface (physicalhardware) as good, that the interface is not administratively shut down or looped, and thatthis interface s connection is properly clocked.The line protocol being reported as down,however, is usually due to lack of synchronization between the Frame Relay DTE (usuallythe router) and DCE device (usually the service provider s switch).This is usually the resultof LMI mismatch between the router and the switch.On the other hand, if your showcommand specified a subinterface (for example, show interfaces serial 1.3), then the lineprotocol being down is directly related to whether that particular subinterface s DLCI isactive or not.Other possible reasons for the line protocol to be down are:" The interface is not sending keepalives (keepalives can be stopped using the nokeepalive interface configuration command).The Frame Relay Diagnostic Tools: show 317" The service provider s switch has failed or has been misconfigured." The leased line is experiencing trouble such as too much noise." The CSU is not working properly.NOTE Note that in all of the preceding cases the bottom line result is that the router and the serviceprovider s switch lose their communication path, which is why the router reports the lineprotocol as down.One way of finding out the exact point of failure is to perform loopbacktesting.This is discussed in the next section.The third possible output also indicates that the serial interface is up and the line protocolis down.However, the output indicates that the interface is looped.Obviously, if aninterface is looped it will not be able to communicate with the service provider s switch.The fourth possible output indicates that the interface has been administratively shut down.Again, while in this state, of course, the router cannot communicate with the switch.Finally, the fifth possible condition of a serial interface that the show interface serial nwould indicate is: Serial1 is down, line protocol is down.This condition is usually the resultof a bad connection (or a lack of connection) between the router and the CSU [ Pobierz całość w formacie PDF ]