Chapter 9

"Do I Know This Already?"

1.A, B, D
2.A, B, D
9.A and B
10.A, B, C, D
14.A and B
16.B and C
18.A, B, C
20.B and C


1.The DIS generates the pseudonode, using its own system ID and setting the following octet to be a nonzero number.
2.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.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.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.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.IS-IS is capable of carrying both IP and CLNS.
7.The DIS sends out hellos every 3.3 seconds, three times the speed of other routers on the multiaccess link.
8.The name of the link-state algorithm is the Dijkstra algorithm.
9.The default dead 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 OSPF stub areas.
11.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.The address is a NET address because the last octet is set to 0x00. Thus, there is no network service defined. This is the address of a router, not an end system.
13.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 between each of the ISs, there is one link per IS to the pseudonode. The DIS generates link-state PDUs on behalf of the pseudonode. These LSPs are sent to all the connected ISs.
14.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.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.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.A Level 1-2 router has two SPF (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.IS-IS packets run directly on top of the data link layer.
19.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.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.

21.It is necessary to configure routers that straddle more than one area as Level 1-2 routers so that they can receive updates from both Level 1 and Level 2 routers and thus forward datagrams from Level 1 routers out of their area. Some designs allow for every router to be configured as a Level 1-2 router; this is the default configuration on Cisco routers. This eliminates errors but is a drain on network resources.
22.The four stages of the routing process are update, decision, forwarding, and receiving.
23.An LSP contains the list of neighbors connected to the originating router.
24.LSPs are generated whenever there is a change in the network, often because of a configuration change. However, any of the following instances trigger a new LSP to be flooded throughout the network:
  • An adjacency comes either up or down (for example, a new router comes online).

  • An interface on the router changes state or is assigned a new metric.

  • An IP route changes (for example, because of redistribution).

25.The following list describes the flooding process on a point-to-point link:
  1. When an adjacency is established, both sides send a CSNP packet with a compressed version of their link-state databases.

  2. If there are any LSPs in the receiving router's database that were not in the CSNP it received, it sends a copy of the missing LSPs to the other router.

  3. Likewise, if the database is missing any LSPs received in the CSNP, the receiving router requests the detailed LSP to be sent.

  4. The individual LSPs are requested, sent, and acknowledged via PSNPs.

  5. When an LSP is sent, the router sets a timer. If no explicit acknowledgement has been received before the timer expires, the LSP is resent. This timer is the minimumLSP-TransmissionInterval and can be configured; the default on a Cisco router is 5 seconds.

26.The LSP contains three fields that help determine whether the LSP that has been received is more recent than that held in the database and whether it is intact or has been corrupted. These three fields are as follows:
  • Remaining Lifetime— This is used to age-out old LSPs. If an LSP has been in the database for 20 minutes, it is assumed that the originating router has died. The refresh timer is set to 15 minutes.

    If the lifetime expires, the LSP has the content removed, leaving only the header. The lifetime is set to show that it is a new LSP, and then it is flooded through the network. All receiving routers accept the mutilated LSP, recognize that this means the route is bad, and purge the existing LSP from their databases.

  • Sequence Number— This is an unsigned 32-bit linear number. The first LSP is allocated the sequence number 1, and the following LSPs are incremented by 1.

  • Checksum— If a router receives an LSP and the checksum does not compute correctly, the LSP is flushed and the lifetime is set to 0. The router floods the LSP, all routers purge the LSP, and the originating router retransmits a new LSP.

27.Each router builds a shortest path tree (SPT) with itself as the root. This is achieved by taking all the LSPs from the link-state database and using the Dijkstra algorithm to create the SPT. The SPT is used in turn to create the forwarding table, which is also known as the routing table.
28.If there is more than one path to a remote destination, the criteria by which the lowest cost paths are selected and placed in the forwarding database are as follows:
  • If there is more than one path with the lowest value metric, Cisco IOS places up to six equal-cost paths into the table. The default number of equal-cost paths is four.

  • Optional metrics are chosen before the default metric, but because Cisco supports only the default metric, this is a moot point.

  • Internal paths are chosen before external paths, because going outside the autonomous system is likely to be a suboptimal route and might be the result of a routing loop.

  • Level 1 paths within the area are more attractive. If the path is within the area, not only is it more efficient to route directly to it, but also going outside the area and returning can be the cause of a routing loop, demanding greater resources and time.

  • The address with the longest match or most specific address in IP is the address with the longest IP subnet mask. This ensures that the closest router is chosen, because prefix routing is configured by summarization that can occur only on area boundaries.

29.A narrow metric is the default metric, which has a 6-bit field. Cisco increased the size of the field to 24 bits; this 24-bit field is referred to as a wide metric.
30.The IS-IS metric is applied to the outgoing interface.
31.If an LSP fragment is incomplete, the routing process ignores it, safe in the knowledge that it will be retransmitted if the sending router does not receive an acknowledgment within a specified time frame.
32.Suboptimal routing decisions occur when Level 1 areas have knowledge of networks only within their own areas. To reach another area, packets are sent to the nearest Level 2 router.

Without additional configuration, the Level 1 router determines the nearest Level 2 router to be the one with the lowest hop count. The metrics used are the default metric of 10 on each outbound interface; therefore, the best route translates to that with the lowest hop count. As you know, the router two hops away might include a 16-Mbps Token Ring and a 56-kbps link as opposed to the three hops of Fast Ethernet and ATM.

33.Route summarization can be configured on a Level 1-2 router at the area boundary.
34.A DIS is elected on a WAN when the NBMA cloud is configured as multipoint.
35.Frame Relay and ATM are examples of NBMA networks, which are not accommodated in Integrated IS-IS. OSPF has a point-to-multipoint configuration option, but Integrated IS-IS does not. The solution in Integrated IS-IS is to configure the link as multipoint, allowing the election of a DIS. The alternative is to configure the interfaces with subinterfaces that are point-to-point.