| 1: | What type of information is stored in an Link-State database, and how is this information collected? |
| A: | There are two types of Link-State databases in IS-IS, Level 1 and Level 2. These databases store link-state packets, which are generated by routers in an area or the backbone, respectively, and spread out by a process called flooding. |
| 2: | Describe the use of the Link-State database in IS-IS. |
| A: | As a link-state protocol, IS-IS requires each router in an area to have the same view of the area's topology. The Link-State database contains LSPs from other routers in the area and describes the topology of the area. Each router bases its route calculation on the information in the Link-State database. The route calculation is done using the shortest path first (Dikjstra) algorithm. |
| 3: | What is the difference between Level 1 and Level 2 Link-State databases? |
| A: | The Level 1 Link-State database describes a single IS-IS area. It contains only LSPs from the routers in that area. A separate Level 1 Link-State database exists for each area in an IS-IS domain (if more than one exists). An IS-IS domain with multiple areas has a backbone, which interconnects the areas. The Level 2 database describes the backbone and consists of Level 2 LSPs from the routers connected to the backbone. |
| 4: | What is the meaning of Link-State database synchronization? |
| A: | Link-State database synchronization is the process of using reliable or periodic flooding mechanisms to ensure that routers in the same area or the backbone have identical Level 1 or Level 2 databases, respectively. |
| 5: | Describe the general format of a link-state packet. |
| A: | A link-state packet is composed of a 27-byte header and variable-length TLV fields. |
| 6: | List the four fields in the LSP header that adequately describe the LSP. |
| A: | LSP ID, Sequence Number, Remaining Lifetime, and Checksum fields. |
| 7: | What are TLVs? |
| A: | TLV stands for Type, Length, and Value. TLVs are special information fields that are appended as necessary to the header of an IS-IS packet. Every type of IS-IS packet supports a specific set of TLVs. The same TLV type can be used in different IS-IS packet types. |
| 8: | List five TLVs that carry metric information and where they are originally specified. |
| A: | IS-neighbor TLV (ISO 10589) Internal IP Reachability TLV (RFC 1195) External IP Reachability TLV (RFC 1195) Extended IS Reachability TLV (RFC Draft) Extended IP Reachability TLV (RFC Draft) |
| 9: | What is the format of the LSP Identifier (LSPID)? Give an example. |
| A: | The LSPID is composed of the system ID of the originating router, a pseudonode number, and an LSP fragment number. Example: 000.0000.0001.00-00 or RTA.00-00, where the first 6 bytes represent the system ID, the seventh byte is the pseudonode number, and the last byte is the LSP fragment number. If ISIS dynamic name resolution is enabled on the router or CLNS host names are configured, the system ID is replaced by the host name of the source router. |
| 10: | Name the types of metric information specified by ISO 10589. |
| A: | ISO 10589 specifies four types of metrics: default, reliability, monetary cost, and delay. |
| 11: | What are the limitations of the default metric specified by IS0 10589? |
| A: | Only 7 bits are dedicated to the value of the default metric, allowing a maximum value of only 63 units. In addition, the maximum metric for an entire path is 1023 units. Even though the default metric has a connotation of bandwidth, it is not assigned automatically based on the interface bandwidth of a router. |
| 12: | How are the metric limitations of ISO 10589 addressed by TLVs Type 22 and Type 135 recently proposed in the IETF IS-IS Working Group ? |
| A: | Because all widely deployed IS-IS implementations use only the default metric, TLV Types 22 and 135 propose the extension of the default metric field with the fields of the unused metric types. The extended default metric field allows larger metric values and provide more flexibility to network administrators in designing and expanding their networks. |
| 13: | What are sequence number packets? List all types. |
| A: | Sequence number packets contain LSP summary TLVs and aid the flooding process, as well as database synchronization. Two types of sequence number packets exist: complete sequence number packets (CSNP) and partial sequence number packets (PSNPs). CSNPs contain summaries of all known LSPs. They are exchanged on point-to-point links only once after the adjacency is brought up. However, they are advertised periodically on broadcast links by the DIS. PSNPs that normally contain summaries of a subset of known LSPs in the area are used to request complete copies of LSPs on both point-to-point links and broadcast links. PSNPs are also used as acknowledgments on point-to-point links to support reliable flooding. |
| 14: | What are IS-IS mesh groups? |
| A: | IS-IS mesh groups are used to limit redundant flooding in NBMA environments with point-to-point subinterfaces and highly meshed underlying PVCs. |
| 15: | What is maxage and maximum LSP regeneration interval? List the Cisco IOS commands that can be used to change their values. |
| A: | Maxage specifies the maximum life span of an LSP, from the time it is generated at the source until it expires . The IOS router-level command max-lsp-interval is used to modify the value of maxage. The default value in Cisco IOS is 20 minutes (120 seconds). Maximum LSP regeneration interval, also known as the LSP refresh interval, is the periodic interval between the times at which a router must refresh its LSP throughout the whole network. LSP refresh occurs before maxage is reached. The IOS router-level command lsp-refresh-interval is used to modify this timer. |
| 16: | Can two routers in the same area have the same system ID? Briefly describe what happens when two routers in the same area are configured with the same system ID. |
| A: | No. All routers in the same area must have different system IDs to make them unique so that packets sourced from them ( Hellos, LSPs, SNPs) can be uniquely identified. When two or more routers in the same area share the same system ID, their LSPs get mixed up and each router sees incorrect information in what seems to be its LSP. The routers, therefore, continuously try to update each others LSP creating unnecessary resource consuming churn in the network. An error is logged if LSP regeneration exceeds a frequency threshold. |
