ISO Addressing for Integrated IS-IS

13 Mar

ISO Addressing for Integrated IS-IS
Because IS-IS is the product of a committee, it has the feel of an academic solution that is intended to resolve every eventuality. Its addressing scheme thinks not just locally, but globally. The large address space is one characteristic that is responsible for IS-IS’s new popularity.

When Integrated IS-IS is routing IP traffic, the routing information is carried in the IS-IS updates; therefore, the participating routers need an ISO address. The ISO address comes in two forms: the NSAP and the NET address (depending on the type of device being addressed). The addressing scheme of IS-IS is long and complex, but clear rules define the address space, allowing the IS-IS protocol to find the routers to forward the data traffic to the end destination.

The IS-IS address is a variable-length address from 8 to 20 octets in length, much larger than the 4 octets in the TCP/IP address. ISO 10589 defines three parts to the address—Area, ID, and SEL—as shown in Figure 10-1.

ISO Addressing for Integrated IS-IS 351
Figure 10-1 The Three Sections of an ISO Address

AREA       ID         SEL

IS-IS addressing can become complicated because these three fields are subdivided to allow greater granularity in routing. If you are confused, however, just remember that it all condenses to the three elements of the address, which perform the following functions:

■ Area—The Area field is used to route between areas using Level 2 routing.
■ ID—The ID field is used to route to a host or router within the area using Level 1 routing.
■ SEL—The SEL field is used to route to an entity within the host or ES.

Although the three parts to the address describe how to get to the area, how to find the host, and how to find the application within the host, finding the destination host uses only the first two parts; the last part of the address is used after the end host has received the packet. Therefore, Integrated IS-IS has two levels of hierarchy: basically, how to get to the area and then to the host.

The first part of the address, routing to the area, might require many decisions to be made. These decisions might involve determining the country and then the organization and many suborganizations. So although the address has only two levels of hierarchy, several levels of hierarchy can be hidden within the first part of the address, which states how to get to the autonomous system and forms the external portion of the area.

Figure 10-2 shows the division between external and internal routing—the Initial Domain Part (IDP) and the Domain Specific Part (DSP)—and how this division is broken down to reveal greater granularity for routing. In the figure, the three original elements have been placed underneath as reference.

Figure 10-2 An ISO Address

ISO Address

The following list explains the parts of an ISO address and their relationship:

■ IDP—External routing. The IDP is used to route to the domain, or autonomous system. The IDP is given by the ISO and identifies the organization responsible for assigning the format for the rest of the address by defining the DSP structure. The IDP comprises the following two parts:

— Authority and format identifier (AFI) —AFI is the first octet of the address. It is defined as one octet. It states the authority in charge of issuing addresses and the format of the rest of the address, within the constraints of IDI and DSP, to describe Area, ID, and SEL.
— Initial domain identifier (IDI) —IDI is a suborganization to the AFI; as an analogy, the U.S. government is the AFI, and the General Services Agency (GSA) is the IDI,
otherwise known as GOSIP.

■ DSP—DSP is used to route within the autonomous system. The authority specified by the IDP may further subdivide the DSP and may assign subauthorities responsible for parts of the DSP. The DSP can be broken down into the following fields:

— High Order DSP —High Order DSP is typically the area within the autonomous system.
— System ID —System ID can have a value between one to eight octets. It must have the same length throughout all systems in the autonomous system. Cisco uses six octets, which is a common solution because it allows the use of the MAC address to autoconfigure the system.
— NSEL—NSEL is one byte and identifies the particular service at the network layer to which to hand the packet.

The fact that the address can take so many forms can cause confusion; remember, however, that there are only two layers of hierarchy. By providing such flexibility in the address space, the ISO has ensured a decentralized address allocation and management, in addition to the ability to scale the network.

As with TCP/IP, the addressing scheme within an autonomous system can be the result of the creative genius of the network administrator or can be obtained from the AFI, an authorized ISO body such as ANSI or GOSIP.

The next sections discuss ISO addresses for Integrated IS-IS, including examples of NETs and NSAP, in addition to the rules for IS-IS addressing.

NETs and NSAP
NETs and NSAPs are ISO addresses. The differences between the NET and NSAP addresses are subtle. The NET address is the address of the host, where the value in the NSEL field is set to 0×00. Therefore, there is no upper-layer protocol identified within the host. With no application identified within the end host, the packet can be routed to the destination, but it cannot be handed off to a process after it has been delivered. However, routers do not have upper-layer protocols to identify because they are transitory ISs. Therefore, the NSAP of the router is referred to as a NET because the NSEL field is set to 00.

The NSAP is the full ISO address. It not only defines the area and destination host within the area, but also specifies where to send the incoming packet after it has reached the host. The NSEL field at the end of the ISO address specifies the upper-layer protocol and is similar to the Protocol field of the IP header.

Rules of ISO Addressing
The following list indicates a few rules that clarify ISO addressing:

■ The ISO address is assigned to the system, not to the interface.
■ Typically, the router has one NET address. The limit of three NETs is in a conventional IS-IS implementation; the limit is three NETs per area in a multiarea Integrated IS-IS implementation. Multiple addresses are used during transitions.
■ If multiple NETs are configured on the same router, they must all have the same system ID.
■ The area address must be the same for all routers in the same area.
■ All Level 2 routers must have a system ID that is unique for the entire domain.
■ All Level 1 routers must have a system ID that is unique for the entire area.
■ The system ID must be the same length for all ISs and ESs within a routing domain.

Example of a NET Address
The following are examples of NET addresses. The first two addresses are used for routing within the autonomous system and, therefore, the IDI portion of the address has not been defined.

■ A simple OSI NET address, using the host MAC address as the system ID:
47.0005.aa00.0301.16cd.00
← To the Domain →← Within the Domain →

HO-DSP

354 Chapter 10: Fundamentals of the Integrated IS-IS Protocol
■ A GOSIP version 2 address, showing the external routing information:
47. 0005.80ff.f800.0000. 0001.0000.0c00.1234.00

System ID

Integrated IS-IS Hierarchical Structure
As the addressing structure shows, there are potentially many levels of hierarchy within the addressing scheme, even though the protocol has only two layers of hierarchy. To accommodate the two levels of hierarchy, two types of router are defined: a Level 1 router that deals with the first level of routing, finding the end destination within the area, and a Level 2 router that finds the area within which the end destination resides. Both of these routers are combined in the Level 1-2 router, which runs both the Level 1 and the Level 2 processes and could be viewed as a third type of router.

The Level 1 Router
The Level 1 router locates the destination host within the area and, as such, is known as the intraarea router. It is similar to a stub router in OSPF. Like a stub router in OSPF, its knowledge of the network is limited to the area, using a default route to the nearest Level 2 router as the means of routing traffic external to the area.

Every Level 1 router has a link-state database containing all the routing information for the area. The nature of the link-state protocol means that each database is identical. Because the database is limited to routes within the area, there is a requirement that neighbors must be in the same area so that the routers can communicate.

The Level 2 Router
To route traffic between areas, a Level 2 router is needed. Routing between areas is referred to as interarea routing. This router is similar to a backbone internal router in OSPF, and as in OSPF, the backbone must be contiguous. Level 2 routers communicate via Hellos that are understood only by other Level 2 routers. To fracture the area means the loss of routing information. As in Level 1 routing, the link-state database is identical on all Level 2 routers, although the database contains prefixes of addresses in other areas as opposed to internal area addresses.

The Level 1-2 Router
The router that has everything is, of course, the Level 1-2 router. This is both an intra-area and interarea router. Its characteristics are similar to those of an ABR in OSPF. This router might have neighbors in different areas because it sends both Level 1 and Level 2 Hellos and can thus communicate with everyone. It holds both a Level 1 database for the Level 1 area to which it is connected and a Level 2 database with all the information for interarea routing.

The Level 1-2 configuration is convenient because the router informs Level 1 routers that it is a Level 2 router and can forward traffic to other areas. It can also inform other Level 2 routers of the areas to which it is connected. Although convenient, it consumes more resources on the router in terms of memory and CPU and uses more bandwidth in maintaining the link-state databases. This configuration is the default configuration on Cisco routers.

Basic Principles of Area Routing
Having reviewed the complexities of ISO addressing and the corresponding hierarchical nature of Integrated IS-IS routing, you now need to pull it all together. This section explains the basics of Integrated IS-IS routing. The following list shows what happens when a router receives a packet to forward and how decisions are made in area routing, based on the ISO destination address of the incoming packet:

1. When a router receives traffic to route to a remote destination, it performs the customary routing table lookup.
2. The router strips off the system ID and the SEL to reveal the Area portion of the address. If the Area address is the same as that of the router, it routes the packet toward the host system ID using the Level 1 database.
3. If the Area is different, the router does one of the following:

(a) Sends the packets to the nearest Level 2 router (if the router is a Level 1 router).
(b) Looks up the route in the forwarding database to find a match, if the router is a Level 2 router.
(c) Resolves the address to the longest match, ensuring the greatest level of accuracy. Summarization, as described in Chapter 2, “IP Addressing,” is used by Integrated IS-IS, and therefore prefix routing can reduce the routing tables and speed up the forwarding process.

Areas in Integrated IS-IS are defined on the link, meaning that the entire router is in the area, requiring the router to define the area and not the interface like in OSPF. Figure 10-3 shows how backbone routers are used to connect different areas.

For the Level 2 routing updates to be exchanged, all the routers capable of sending Level 2 updates must be contiguous; Figure 10-4 shows this.

356 Chapter 10: Fundamentals of the Integrated IS-IS Protocol
Figure 10-3 Areas and Backbone Routers

Areas and Backbone Routers

Adjacencies for the Topology

Because Level 1 routers communicate only with other Level 1 routers and, likewise, Level 2 routers communicate only between themselves, it can get confusing. Figure 10-5 shows the logical communications or adjacencies set up between the routers shown in Figure 10-4. In Figure 10-5, the area 0001 becomes the backbone area, connecting another area (area 0100). It also shows the communication among L1, L1-2, and L2 routers.

Integrated IS-IS Networks and Interfaces
Routers sharing a common data-link layer become Integrated IS-IS neighbors if the Hello packets that they exchange meet the criteria for forming an adjacency. Although the process of finding a neighbor differs slightly depending on the media, the information sent in the Hellos is essentially the same. Each Hello states the originator of the Hello and the capabilities of its interface. If the Hellos are exchanged and the criteria are met, an adjacency is formed and the Integrated IS-IS neighbors exchange routing information in the form of LSPs. In this way, every router gathers the connected networks of every router to create identical detailed topology maps of the network.

For an adjacency to be formed and maintained, both interfaces must agree on the following:

■ The maximum packet size (MTU) that can be transmitted by the interface must be the same.
■ Each router needs to be configured as the same level of routing—that is, either Level 1 or Level 2—so that they can decode the Hellos sent by the other router.

■ If the routers are both Level 1, they must be in the same area.
■ Level 1 routers form adjacencies with each other, and Level 2 routers form adjacencies with other Level 2 routers. For a Level 1 router to communicate with a Level 2 router, one of the routers needs to configured as a Level 1-2 router. Therefore, to connect to another area at least one of the routers must be configured as a Level 1-2, allowing the Level 2 router to receive the packets destined for another area from the Level 1 router.

■ The system ID must be unique to each router.
■ If authentication is configured, it must be configured identically on both routers.

The Hello timers (including the holddown timer) must match; otherwise, it could result in a flapping link and endless SPF calculations.

Integrated IS-IS defines two network types—broadcast subnetworks and point-to-point networks— compared with the five types used in OSPF. A broadcast network, as in OSPF, is a multiaccess data link that supports broadcasts and multicasts. The point-to-point links are deemed to be nonbroadcast and can be permanent virtual circuits (PVCs) or dynamically set up switched virtual circuits (SVCs).

The following sections describe adjacencies on point-to-point links, broadcast links, and
nonbroadcast multiaccess (NBMA) links.

Establishing Adjacencies on a Point-to-Point Link
A point-to-point link connects two routers. After a Hello packet has been received, each side declares the other side reachable. The routers are neighbors. At this point, each side sends a CSNP. The CSNP is a list of all the links held in the link-state database, which triggers a synchronization of the link-state database on each machine.

Periodic Hellos maintain the adjacency. If a router does not hear a Hello within the Hello holdtime, the router is declared dead and the database is purged of any entries associated with the router. Cisco sets the default Hello multiplier to three. The holdtime is defined as the Hello time multiplied by the Hello multiplier, making the holdtimer expire every 30 seconds.

Establishing Adjacencies on a Broadcast Link
On broadcast links, all the routers running Integrated IS-IS receive packets sent by one router—the DIS—to control the amount of traffic that needs to be generated to maintain the adjacencies and, thus, the databases. The DIS has the responsibility of flooding the LSPs to all connected systems running Integrated IS-IS. More accurately, the DIS floods the LSPs for the pseudonode.

The pseudonode represents the LAN, with each router simulating an interface on the imaginary router. This imaginary router is called the pseudonode. As if it were a real router, the pseudonode floods a new pseudonode LSP when there is a change in the status of its connections (for example, when a new neighbor comes online).

The adjacencies with the other routers are maintained by the DIS, which sends out Hellos every 3.3 seconds, three times the speed of other routers. This is to ensure the integrity of the adjacencies by identifying a problem very quickly. If there is a problem with the DIS, or a router with a higher priority appears, it is quickly identified and a new router is elected in the place of the old DIS, which is forced into retirement. The election is based on priority and stated on the interface in the event
that all the interfaces are set at the default of 64. Then the highest numeric SNPA (data-link address) determines the DIS.

Establishing Adjacencies on an NBMA Link
The creation and maintenance of adjacencies on both point-to-point and multiaccess links is straightforward. It becomes more complicated when the format of the technology cloud allows either link technology to be used.

An NBMA link is neither a broadcast medium nor a point-to-point link; it is a bit of both. Frame Relay, ATM, and X.25 are examples of NBMA. Using PVCs, NBMAs provide multiple connections, which could be viewed as a LAN. The confusion occurs when Integrated IS-IS sees the link is multiaccess. Having no knowledge of multiaccess WAN clouds, Integrated IS-IS believes that the medium is some form of LAN and therefore has broadcast capabilities. However, the medium is a WAN, not a LAN, and although the LAN can be simulated, the WAN cloud has no inherent broadcast capabilities.

To avoid complications and possible errors, Cisco recommends that the links be configured as a series of point-to-point links. Figure 10-6 shows a comparison of the broadcast and point-to-point topologies supported by IS-IS.

Figure 10-6 Network Topologies Supported by IS-IS

Topologies Supported by IS-IS

360 Chapter 10: Fundamentals of the Integrated IS-IS Protocol

Network Layer Protocols Used in Integrated IS-IS
The PDU is created by the network layer and encapsulated directly into the data-link frame. All the Integrated IS-IS packets share the same eight-octet header. After the fixed header, there are a number of optional variable-length fields that contain specific routing-related information. These variablelength fields are called TLV or CLV.

The fields found in the fixed header of every IS-IS PDU are each one octet in length. Table 10-5 explains the fixed header.

Table 10-5 The Fixed Header Common to All IS-IS PDUs

Field Length of Field in Octets

After this common header are the PDU-specific fields and the variable-length fields, creating the different packet types seen in Integrated IS-IS. The PDU-specific fields are discussed in the sections of this chapter relevant to the different packet types: Hellos, LSPs, and SNPs. The variable-length fields are discussed in the section on TLVs.

There are three Integrated IS-IS packets, as the following list describes:

■ Hello—These packets create and maintain neighbor relationships and adjacencies. There are three types of Integrated IS-IS Hello packet. The type of packet is defined in the fixed header under the Type field and allows the packet to be handed off to the appropriate process. The different types are as follows:

— LAN Level 1 —Generated by Level 1 and Level 1-2 routers
— LAN Level 2 —Generated by Level 2 and Level 1-2 routers
— Point-to-point—Generated by Level 1, Level 2, and Level 1-2 routers

■ LSP—LSPs hold information on the neighbors connected to the router. There are two types of LSP, as follows:

— Level 1 —Generated by Level 1 and Level 1-2 routers
— Level 2 —Generated by Level 2 and Level 1-2 routers

■ Sequence number packet (SNP) —SNPs describe the LSPs in the transmitting router’s linkstate database. The information is condensed and is never flooded but only sent between neighbors. SNPs ensure link-state databases synchronization by:

— Distributing groups of LSPs on a LAN without explicit individual acknowledgements
— Acknowledging individual LSPs
— Requesting LSPs at startup

There are two types of SNP for each level of routing, as follows:

— Complete SNP (CSNP) —Includes every LSP in the database:
Level 1
Level 2
— Partial SNP (PSNP) —Includes a subset of LSPs, used to request individual LSPs and to acknowledge receipt of these LSPs:

Level 1
Level 2
The following sections describe Hello packets, LSPs, and SNPs in more detail.

The Format of the Hello Packet There are three different Hellos, as follows:
■ Point-to-point Hello
■ LAN Level 1 Hello
■ LAN Level 2 Hello

Because the point-to-point and broadcast media work differently, the adjacencies are formed in a different manner. There need to be separate Hellos for the point-to-point networks and the broadcast networks. A point-to-point network has only one other router with which to communicate and, therefore, after the level of routing has been established, the appropriate updates can be sent. However, a broadcast network is a multiaccess network and can have a mixture of both Level 1 and Level 2 routers. For this reason, the broadcast or LAN network has two Hello formats, the Level 1 format and the Level 2 format. The Hellos for the broadcast media are referred to as LAN Hellos.

Point-to-point Hello packets are used over point-to-point links. Table 10-6 shows the point-to-point Hello.

Table 10-6 The Point-to-Point Hello

Common to all Integrated IS-IS PDUs

LAN Hello packets are used over broadcast links. The PDU is the same for all router types, although the values within the fields differ.

Table 10-7 shows the LAN Hello.

Table 10-7 LAN Hello Packet Format

States whether the transmitting

The Format of the LSP
The LSP from a Level 1 router is flooded to all routers in the area. The LSP contains a list of all the adjacencies.

Likewise, a Level 2 router floods the LSP to every other Level 2 router in the domain. However, this LSP contains the list of adjacencies to other Level 2 routers and the areas that the transmitting router can reach. The TLVs hold the Level 1 and Level 2 information, allowing the LSP format to be the same for both Level 1 and Level 2 routers.

Table 10-8 shows the format of the LSP.
364 Chapter 10: Fundamentals of the Integrated IS-IS Protocol
Table 10-8 LSP Packet Format

Fixed Integrated IS-IS header

The Format of the SNP
As with the LSP, the SNP format is the same for both Level 1 and Level 2 routers. The CSNP has two additional fields to the PSNP. The extra fields define the address range, which states all the LSPs that are included in the CSNP. This is to ensure that if the database is so large that several PDUs need to be sent, the information can be reassembled at the receiving router.

Table 10-9 shows the format of the SNP.
Table 10-9 SNP Packet Format

Packet Length

TLVs
TLVs, sometimes called CLVs, are one of the main strengths of Integrated IS-IS. TLVs provide flexibility and extend the functionality of the protocol. The TLV fields are variable in length and are appended to the various packet formats. This means that the protocol can adapt to the changing needs and advances in technology by defining a new TLV.

The structure of the TLV is as follows:

■ Type or Code —State the Type of field. This code identifies the TLV and all the characteristics that pertain to it. (For example, TLV 128 defines the capability to carry IP routes in IS-IS packets.) In essence the TLV 128 is Integrated IS-IS.
■ Length —The length of the following field. This is important because the next field can be any length. By identifying the length of the field, error detection is possible.
■ Value—The information, whether it is IP routes, IS-IS neighbors, or authentication. One of the most radical TLVs was the introduction of TLV 128, as defined in RFC 1195 in 1992. This extended Integrated IS-IS to support IS-IS and IP routing. In the future, it is expected that a TLV will be created to support IPv6.

To read the definition of these TLVs and, in particular, the various codes, you must read the standards. ISO/IEC 10589 defines the TLV codes 1 to 10; RFC 1195 defines the TLV codes 128 to 133.

It is important to note which TLVs are supported by your equipment because this determines the design and configuration of the network. The receiving router ignores TLVs that are not supported.

Table 10-10 describes the main TLVs, states the document that defines them, and names the PDUs to which they are appended.
Table 10-10 TLVs Implemented by Cisco

TLV Name

Table 10-10 TLVs Implemented by Cisco (Continued)

TLV Source

Foundation Summary
The “Foundation Summary” section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a wellprepared candidate should, at a minimum, know all the details in each “Foundation Summary” before going to take the exam.

Figure 10-7 shows the format of a CLNS address.
Figure 10-7 An ISO Address

ISO Address

For routing purposes, ISO 10589 specifies three parts to the address, namely how to get to the area, how to find the host, and how to find the application within the host. However, there are only two elements in the address used to find a destination host: the address that locates the area and the host within that area. Therefore, Integrated IS-IS has two levels of hierarchy, as follows:

■ IDP—The IDP is used to route to the domain, or autonomous system. The IDP is given by the ISO and identifies the body responsible for assigning the format for the rest of the address by defining the DSP structure. The following two parts comprise the IDP:
— AFI (authority and format identifier) —AFI is the first octet of the address. It is defined as one octet. It states the authority in charge of issuing addresses and the format of the rest of the address, within the constraints of IDI and DSP, to describe Area, ID, and SEL.
— IDI (initial domain identifier) —IDI is the suborganization to the AFI. (As an analogy, the U.S. government is the AFI and the General Services Agency is the IDI, otherwise known as GOSIP.)
■ DSP—DSP is used to route within the autonomous system. The authority specified by the IDP may further subdivide the DSP and may assign subauthorities responsible for parts of the DSP. The DSP can be broken down into the following fields:
— High Order DSP —High Order DSP is typically the area within the autonomous system.
— System ID —System ID can have a value between one and eight octets. It must have the same value throughout all systems in the autonomous system. Cisco uses six octets, which is a common solution because it allows the use of the MAC address to autoconfigure the system.
— NSEL—NSEL identifies the particular service at the network layer to which to hand the packet.

Characteristics of a Level 1 router 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 that is elected on LANs Characteristics of a Level 2 router include the following:
■ An interarea router
■ Similarity to a backbone internal router in OSPF
■ Level 2 routers must be contiguous (that is, the area cannot be fractured)
■ Neighbors can be in different areas
■ A Level 2 database with all the information for interarea routing Characteristics of a Level 1-2 router include the following:
■ An intra-area router and an interarea router
■ Similar to an area boundary router (ABR) in OSPF
■ Neighbors can be in different areas
■ A Level 2 database with all the information for interarea routing
■ A Level 1 database for each area to which it is connected
■ Informs Level 1 routers that it is a Level 2 router and can forward traffic to other areas
■ Informs other Level 2 routers of the areas to which it is connected For an adjacency to be formed and maintained, both interfaces must agree on the following:
■ The maximum packet size (MTU) that can be transmitted by the interface must be the same.
■ Each router needs to be configured as the same level of routing—that is, either Level 1 or Level 2—so that they can decode the Hellos sent by the other router.

■ If the routers are both Level 1, they must be in the same area.
■ Level 1 routers form adjacencies with each other, and Level 2 routers form adjacencies with other Level 2 routers. For a Level 1 router to communicate with a Level 2 router, one of the routers needs to be configured as a Level 1-2 router. Therefore, to connect to another area, at least one of the routers must be configured as a Level 1-2, allowing the Level 2 router to receive the packets destined for another area from the Level 1 router.
■ The system ID must be unique to each router.
■ If authentication is configured, it must be configured identically on both routers. There are three Integrated IS-IS packets, as the following list describes:
■ Hello—These packets create and maintain neighbor relationships and adjacencies. There are three types of Integrated IS-IS Hello packet. The type of packet is defined in the fixed header under the Type field and allows the packet to be handed off to the appropriate process. The different types are as follows:
— LAN Level 1 —Generated by Level 1 and Level 1-2 routers
— LAN Level 2 —Generated by Level 2 and Level 1-2 routers
— Point-to-point—Generated by Level 1, Level 2, and Level 1-2 routers
■ LSP—LSPs hold information on the neighbors connected to the router. There are two types of LSP, as follows:
— Level 1 —Generated by Level 1 and Level 1-2 routers
— Level 2 —Generated by Level 2 and Level 1-2 routers
■ Sequence number packet (SNP) —SNPs describe the LSPs in the transmitting router’s linkstate database. The information is condensed and is never flooded but only sent between neighbors. SNPs ensure link-state databases synchronization by:
— Distributing groups of LSPs on a LAN without explicit individual acknowledgements
— Acknowledging individual LSPs
— Requesting LSPs at startup
There are two types of SNP for each level of routing, as follows:
— Complete SNP (CSNP) —Includes every LSP in the database:
Level 1
Level 2
— Partial SNP (PSNP) —Includes a subset of LSPs, used to request individual LSPs and to acknowledge receipt of these LSPs:
Level 1
Level 2
There are three different Hellos, as follows:
■ Point-to-point Hello
■ LAN Level 1 Hello
■ LAN Level 2 Hello

Q&A
The following questions test your understanding of the topics covered in this chapter. After you have answered the questions, check your answers in Appendix A. If you get an answer wrong, review the answer and ensure that you understand the reason for your mistake. If you are confused by the answer, refer to the appropriate text in the chapter to review the concepts.

1. Which system generates the pseudonode?
2. What is a CSNP? When is it used?
3. What is a PSNP? When is it used?
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?
5. What happens in the event of the DIS dying?
6. Integrated IS-IS can be used to send information about which routed protocols?
7. How often does the DIS send out a Hello packet?
8. What is the name of the link-state algorithm used to create entries for the routing table?
9. What is the relationship between the Hello timer and when the path is considered to have died?
10. Integrated IS-IS areas are similar to which type of areas in OSPF?
11. Describe one design restriction in configuring Level 2 routing.
12. Given the following address: 49.0001.2222.2222.2222.00 Is this a NET or NSAP address? Give reasons for your choice.
13. What is a pseudonode and what is its purpose?
14. State two reasons why a router might not be able to find a neighbor.
15. Explain briefly why two routers cannot have the same system ID within the area.
16. What does TLV stand for? Briefly explain its purpose.
17. How many link-state databases does a Level 1-2 router have?
18. Integrated IS-IS packets run directly on top of which layer?
19. What is the NET address associated with in the IS-IS addressing scheme?
20. Describe briefly the main characteristics of a Level 1 IS.


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