| 1: | Which system generates the pseudonode? |
| A1: | The DIS generates the pseudonode, using its own system ID and setting the following octet to be a nonzero number. |
| 2: | What is a CSNP? When is it used? |
| A2: | The complete sequence number packet describes every link in the link-state database. It is sent on point-to-point links when the link comes up to synchronize the link-state databases. The DIS on a multicast network will send out CSNPs every 10 seconds. |
| 3: | What is a PSNP? When is it used? |
| A3: | Partial Sequence Number Packets are sent on point-to-point links to explicitly acknowledge each LSP they receive. A router on a broadcast subnetwork will send a PSNP requesting the LSPs it needs to synchronize its link-state database. |
| 4: | A new router comes online on a multiaccess link, and the priority is the same as that of the DIS for the segment. What action is taken? |
| A4: | The routers exchange Hellos and immediately see that both routers have the same priority. If the new router had a higher priority, it would take over as the new DIS. However, if both routers have the same priority, the router with the highest MAC address will reign as the DIS. |
| 5: | What happens in the event of the DIS dying? |
| A5: | There is no backup designated router in IS-IS. Therefore, if the DIS meets an untimely death, a new DIS would be elected, based on priority or highest MAC address. If another router comes online with a higher priority, it will dislodge the existing DIS and rule in its place. This behavior is different from that of OSPF. Once a new DIS is elected, the link-state databases are purged and new LSPs are flooded. |
| 6: | Integrated IS-IS can be used to send information about which routed protocols? |
| A6: | IS-IS is capable of carrying both IP and CLNS. |
| 7: | How often does the DIS send out a Hello packet? |
| A7: | The DIS sends out hellos every 3.3 seconds, three times the speed of other routers on the multiaccess link. |
| 8: | What is the name of the link-state algorithm used to create entries for the routing table? |
| A8: | The name of the link-state algorithm is the Dijkstra algorithm. |
| 9: | What is the relationship between the Hello timer and when the path is considered to have died? |
| A9: | The default timer is three times that of the Hello timer; thus the path will wait for 30 seconds before declaring the path dead and flushing the LSPs from the link state database. |
| 10: | Integrated IS-IS areas are similar to which type of areas in OSPF? |
| A10: | Integrated IS-IS areas are similar to OSPF stub areas. |
| 11: | Describe one design restriction in configuring Level 2 routing. |
| A11: | There is only one hard and fast rule for the design of a Level 2 network: Level 2 routers must be contiguous; that is, the area cannot be fractured. |
| 12: | Given the following address: 49.0001.2222.2222.2222.00 Is this a NET or NSAP address? Give reasons for your choice. |
| A12: | The address is a NET address because the last octet is set to 0x00. This there is no network service defined. This is the address of a router, not an end system. |
| 13: | What is a pseudonode and what is its purpose? |
| A13: | The pseudonode is the LAN identifier for a broadcast subnetwork. The pseudonode is the System ID of the DIS plus the Circuit ID. The pseudonode has links to each of the ISs, and each IS has a single link to the pseudonode. The use of the pseudonode reduces the number of links required. Instead of n-1 links to each of the other ISs, there is one link per IS. The DIS generates link-state PDUs on behalf of the pseudonode. These LSPs are sent to all the connected ISs. |
| 14: | State two reasons why a router might not be able to find a neighbor. |
| A14: | For an adjacency to be formed and maintained , both interfaces must agree on the following: -
- The same MTU. -
- Both are Level 1. If both are Level 1, they must be in the same area. -
- Both are Level 2. -
- At least one is Level 1-2. -
- The authentication must be the same. -
- The Hello timers (including the holddown timer) must match. If one router has a Hello timer of 40 seconds, the defaults on the other router would time out the holddown timer and purge the LSP, resulting in a flapping link and endless SPF calculations. |
| 15: | Explain briefly why two routers cannot have the same system ID within the area. |
| A15: | The system ID is the unique identifier for the area. The first part of the address is a very long area address, of which only the last six octets are available for addressing the router or host. |
| 16: | What does TLV stand for? Briefly explain its purpose. |
| A16: | TLV is the same as CLV, but some literature refers to the variable length fields as Type/Length/Value in accordance to the IP terminology. Although the IS-IS PDUs are fixed, the TLV fields are variable length and can expand as needed. This design allows great flexibility and movement to develop in step with technological advances. The development of TLV code 128 extended IS to carry integrated IS-IS. |
| 17: | How many link-state databases does a Level 1-2 router have? |
| A17: | A Level 1-2 router has two link-state databases, one for the Level 1 routes and the other for the Level 2 routes. A separate SPF algorithm is run for each database. |
| 18: | Integrated IS-IS packets run directly on top of which layer? |
| A18: | IS-IS packets run directly on top of the data-link layer. |
| 19: | What is the NET address associated with in the IS-IS addressing scheme? |
| A19: | The NET address is associated with the end system, but not with a process on the end system. The address is that of an entire system, as opposed to an interface on the system, as is the case with IP. Because the NET (unlike the NSAP) does not identify a process, the address is that of a transitional or intermediate system. Therefore, the NET address is associated with the router or IS and is the destination address of the next hop in the life of a routed or routing packet. |
| 20: | Describe briefly the main characteristics of a Level 1 IS. |
| A20: | Characteristics of a Level 1 IS include the following: -
- An intra-area router. -
- Similarity to an OSPF stub router. -
- Knowledge of the network limited to the area. -
- A link-state database with all the routing information for the area. -
- The address of the closest Level 2 router to which to send traffic destined for another area. -
- Neighbors must be in the same area. -
- A DIS is elected on LANs. |
