CLV Fields

The variable-length fields following the PDU-specific fields are Code/Length/Value(CLV)m triplets, as shown in Figure 10.17. The Code is a number specifying the information content of the value field, the Length specifies the length of the Value field, and the Value field is the information itself. As the one-octet size of the Length field implies, the maximum size of the Value field is 255 octets.

[17] The acronym CLV is not used in ISO 10589, but is used here for convenience. You are already familiary with the concept of CLVs from the coverage of EIGRP TLVs in Chapter8, "Enhanced Interior Gateway Routing Protocol (EIGRP)." In fact, RFC 1195 referes to Integrated IS-IS CLVs as TLVs.

Figure 10.17. IS-IS Code/Length/Value triplets perform the same function for IS-IS as Type/Length/Value triplets perform for EIGRP.

Figure 10.17. IS-IS Code/Length/Value triplets perform the same function for IS-IS as Type/Length/Value triplets perform for EIGRP.

Table 10.2 lists the IS-IS CLV codes. The table also indicates whether the CLV is specified in ISO 10589 or in RFC 1195. The ISO-specified CLVs are designed for use with CLNP, although most of them are also used with IP. The RFC-specified CLVs are designed only for IP. If a router does not recognize a particular CLV code, it will ignore the CLV. This arrangement allows CLVs for CLNP, IP, or both to be carried in the same PDU.

Table 10.2. CLV codes used with IS-IS.

Code

CLV Type

ISO 10589

RFC 1195

1

Area Addresses

X

2

IS Neighbors (LSPs)

X

3

ES Neighbors

X

4

Partition Designated level 2 IS*

X

5

Prefix Neighbors

X

6

IS Neighbors (Hellos)

X

8

Padding

X

9

LSP Entries

X

10

Authentication Information*

X

128

IP Internal Reachability Information

X

129

Protocols Supported

X

130

IP External Reachability Information

X

131

Inter-Domain Routing Protocol Information

X

132

IP Interface Address

X

133

Authentication Information

X

The ES-Neighbors and Prefix Neighbors CLVs are not relevant to IP routing ^This CLV is used for partition repair, ^RFC 1195 specifies this code for IP

and are not covered in this book. which Cisco does not support. authentication, but Cisco uses the ISO code of 10.

Although many of the CLVs are used by more than one IS-IS PDU type, only one (Authentication) is used by all PDUs. As the formats of the IS-IS PDUs are described in the following sections, the CLVs used by each PDU are listed. The format of each CLV is described only once, the first time it is listed. Table 10.3 summarizes which CLVs are used with which PDUs.

Table 10.3. The CLVs used by each IS-IS PDU.

PDU Type

CLV Type

15

16

17

18

20

24

25

26

27

Area addresses

X

X

X

X

X

IS Neighbors (LSPs)

X

X

ES Neighbors

X

Partition Designated Level 2 IS

X

Prefix Neighbors

X

IS Neighbors (Hellos)

X

X

Padding

X

X

X

LSP Entries

X

X

X

X

Authentication Information

X

X

X

X

X

X

X

X

X

IP Internal Reachability Information

X

X

Protocols Supported

X

X

X

X

X

IP External Reachability Information

X

Inter-Domain Routing Protocol Information

X

IP Interface Address

X

X

X

X

X

The IS-IS Hello PDU Format

The IS-IS Hello PDU Format

The purpose of the IS-IS Hello PDU is to allow IS-IS routers to discover IS-IS neighbors on a link. Once the neighbors have been discovered and have become adjacent, the job of the Hello PDU is to act as a keepalive to maintain the adjacency and to inform the neighbors of any changes in the adjacency parameters.

The upper limit on the size of an IS-IS PDU is defined by either the buffer size of the originating router or the MTU of the data link on which the PDU is transmitted. ISO 10589 specifies that IS-IS Hellos must be padded to within one octet less than this maximum, partly to allow a router to implicitly communicate its MTU to its neighbors. More important, sending Hellos at or near the link MTU is intended to help detect link failure modes in which small PDUs pass but larger PDUs are dropped. The benefit of this design decision, weighed against the expense of sending such large Hellos over low-speed serial links, is debatable.

There are two kinds of IS-IS Hellos: LAN Hellos and point-to-point Hellos. The LAN Hellos can be further divided into L1 and L2 LAN Hellos. The format of the two types of LAN Hellos is identical, as shown in Figure 10.18. Figure 10.19 shows an analyzer capture of a level 2 LAN Hello.

Figure 10.18. The IS-IS LAN Hello PDU format.

Figure 10.18. The IS-IS LAN Hello PDU format.

Figure 10.19. This analyzer capture of a LAN Hello shows the fields unique to a Hello PDU.

Circuit Typeis a two-bit field (the preceding six bits are reserved and are always zero) specifying whether the router is an L1 (01), L2 (10), or L1/L2 (11). If both bits are zero (00), the entire PDU is ignored.

Source ID is the System ID of the router that originated the Hello.

Holding Time is the period a neighbor should wait to hear the next Hello before declaring the originating router dead.

PDU Length is the length of the entire PDU in octets.

Priority is a seven-bit field used for the election of a DR. The field carries a value between 0 and 127 with the higher number having the higher priority. L1 DRs are elected by the priority in L1 LAN Hellos, and L2 DRs are elected by the priority in L2 LAN Hellos.

LAN ID is the System ID of the DR plus one more octet (the Pseudonode ID) to differentiate this LAN ID from another LAN ID that might have the same DR.

The following CLVs can be used by an IS-IS LAN Hello:[18]

[18] As a reminder, RFC 1195 also specifies an Authentication Information CLV with a type number of 133. Cisco uses the ISO-specified type number of 10 to identify its Authentication Information CLVs.

• Intermediate System Neighbors (type 6)

• Authentication Information (type 10)

• Protocols Supported (type 129)

• IP Interface Address (type 132)

Figure 10.20 shows the format of the IS-IS point-to-point Hello PDU. It is almost identical to the LAN Hello except that there is no Priority field and there is a Local Circuit ID field instead of a LAN ID field. Unlike LAN Hellos, L1 and L2 information is carried in the same point-to-point Hello PDU.

Figure 10.20. The IS-IS point-to-point Hello PDU.

Figure 10.20. The IS-IS point-to-point Hello PDU.

Local Circuit ID is a one-octet ID assigned to this circuit by the router originating the Hello and is unique among the router's interfaces. The Local Circuit ID in the Hellos at the other end of the point-to-point link may or may not contain the same value.

The IS-IS point-to-point Hello does not use the IS Neighbors CLV. With that exception, the same CLVs are used as the LAN Hello.

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