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This chapter covers the following key topics:
Chapter 10, "Understanding Intermediate System-to-Intermediate System (IS-IS)," provides an overview of the IS-IS routing protocol, covering IS-IS protocol concepts and basic con-figuration on Cisco routers. In line with the overall theme of this book, this chapter covers troubleshooting of IS-IS routing problems. Cisco routers and IOS Software provide the framework for the ensuing discussions, which focus on only IP- related issues. Two main categories of IS-IS routing problems exist:
In the absence of any obvious misconfiguration or interoperability issues, any operational issues most likely would be the result of malfunctioning hardware or software bugs. In most such cases, this can be discerned after confirming the configuration is okay or the problem seems to be limited to a particular interface. Problems caused by hardware-and software-related bugs are beyond the scope of this chapter and are not discussed further. Any such problems should be referred to the Cisco Technical Assistance Center for further diagnosis. The discussions in this chapter largely focus on troubleshooting problems caused by misconfiguration, interoperability, or inadequate network resource issues. Network resource issues are triggered when some or all of the routers in the network are low on CPU or memory resources required for storing and computing large amounts of routing information. In general, however, IS-IS seems relatively easier to troubleshoot when compared to sim-ilarly complex routing protocols, such as OSPF. A major contributing factor to this is that IS-IS routers advertise routing information consolidated usually in single LSPs, which are easy to track throughout the network. LSPs can be fragmented , if necessary, but this is rare in today's large IS-IS domains that connect to the Internet. In contrast, OSPF, for example, uses multiple LSA types for carrying different kinds of link-state information. The multiple individual LSAs advertised by each router create a complex environment tracking routing information and troubleshooting problems. Another reason is that IS-IS has been deployed in some of the largest service-provider networks, even though in single-area topologies, for reasonably long enough to enable the Cisco implementation to be mature and stable. Additionally, inherent attributes of the IS-IS protocol allow for deployment in a large, flat network design with remarkable stability. In contrast, OSPF requires hierarchical deployment in large networks, for constraining areas into manageable sizes. In general, hierarchy is necessary for scaling any network, yet it undoubtedly introduces sophistication into the design, which, in turn , complicates trouble-shooting. In summary, it is significantly easier to troubleshoot a large, flat IS-IS network by tracking a single LSP for each router than it is to track multiple OSPF LSAs for each router in a hierarchical topology. This foregoing observation is not intended to pitch one protocol against the other. For the most part, IS-IS and OSPF are identical in functionality and demonstrate similar capabilities in a well-designed network. Probably the most challenging thing about IS-IS for the newly initiated is having to deal with two independent addressing schemes ‚ IP addressing and ISO CLNP addressing. In most cases, there is less familiarity with CLNP addresses, which are also known as NSAPs. The rather long NSAP addresses (up to 160 bits) can be daunting for many who are less exposed to them. On the other hand, IP addresses have a maximum size of 32 bits, with another 32 bits for the mask. CLNP addressing is covered as part of the introduction to IS-IS in Chapter 10. As pointed out in that chapter, even though IS-IS is used for IP routing, it operates within the framework of the ISO connectionless network protocol requiring the need for NSAPs to be configured on routers for node identification. As already indicated, Integrated IS-IS can be used for routing CLNP and IP simultaneously . In general, CLNP addressing is not as complicated as it might seem. Under-standing the structure of NSAPs should help alleviate any potential discomfort working with them. Additionally, familiarity with the NSAP format can provide significant advan-tages in troubleshooting some IS-IS problems ‚ in particular, adjacency-formation problems. Because of the complicity of CLNP in the IS-IS framework, you must be familiar with CLNP-specific show commands as well as IS-IS-specific commands, in addition to the generic IP com-mands for troubleshooting routing problems. Cisco IOS Software also features debugging commands for CLNP and IS-IS that you should become familiar with. As you might recall, CLNP is the Layer 3 protocol within the ISO connectionless network services (CLNS) frame-work. For historical reasons, the keyword clns is used in place of the more appropriate clnp in CLNP-related commands. This misnomer, however, has been retained in the Cisco IOS command line interface (CLI) for backward compatibility. The following key show commands commonly are used for troubleshooting IS-IS routing problems:
The following IS-IS debugging commands provide useful output and frequently are used in addition to show commands for troubleshooting complicated problems:
Unlike the show and debug commands, which are used reactively for troubleshooting problems, the log-adjacency-changes command, which is an IOS router-level configuration command, proactively enables logging of IS-IS adjacency changes. Any such logged information can be an indication of flaky links and potential connectivity problems. Sometimes, a simple reset of IS-IS ‚ related data structures with the command clear isis * can help save the day. In this case, the problem obviously might be caused by buggy code. Infre-quently, a network operator could get temporary relief from a problem by restarting the IS-IS process ‚ that is, removing the IS-IS router configuration and re-entering it. Problems resolved that way are the result of bugs and would always come back to haunt you. You might run into many problems that are caused by misconfiguration and malfunction of software and hardware. This chapter covers the following generic problem categories in detail and elaborates on the steps needed to troubleshoot them:
The rest of the chapter tackles these problems. |
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