IS-IS Control Messages

Intermediate System to Intermediate System (IS-IS) routers implement routing by exchanging control messages. This section describes IS-IS control messages.

IS-IS PDU Formats

Nine types of IS-IS protocol data units (PDUs) are available for processing control information. Each PDU is identified by a 5-digit type code. IS-IS has three major types of PDUs: Hello PDUs, link state PDUs (LSPs), and Sequence Number PDUs (SNPs). Table 1 shows the mapping between PDUs and type values.

**Table 1** Mapping between PDUs and type values
PDU Type	Acronym	Type Value
Level-1 LAN IS-IS Hello PDU	L1 LAN IIH	15
Level-2 LAN IS-IS Hello PDU	L2 LAN IIH	16
Point-to-Point IS-IS Hello PDU	P2P IIH	17
Level-1 Link State PDU	L1 LSP	18
Level-2 Link State PDU	L2 LSP	20
Level-1 Complete Sequence Numbers PDU	L1 CSNP	24
Level-2 Complete Sequence Numbers PDU	L2 CSNP	25
Level-1 Partial Sequence Numbers PDU	L1 PSNP	26
Level-2 Partial Sequence Numbers PDU	L2 PSNP	27

The first eight bytes in all IS-IS PDUs are public. Figure 1 shows the IS-IS PDU structure.

Figure 1 IS-IS PDU format

The main fields are as follows:

Intradomain Routing Protocol Discriminator: network layer protocol identifier assigned to IS-IS, which is always 0x83.
Length Indicator: length of the fixed header in bytes.
ID Length: length of the system ID of network service access point (NSAP) addresses or NETs in this routing domain.
PDU Type: type of a PDU. For details, see Table 1.
Maximum Area Address: maximum number of area addresses permitted for this IS-IS area. The value 0 indicates that a maximum of three area addresses are supported in this IS-IS area.

Type/Length/Value (TLV): encoding type that features high efficiency and expansibility. Each type of PDU contains a different TLV. Table 2 shows the mapping between TLV codes and PDU types.

**Table 2** Mapping between TLV codes and PDU types
TLV Code	TLV Code Name	PDU Type
1	Area Addresses	IIH, LSP
2	IS Neighbors (LSP)	LSP
4	Partition Designated Level2 IS	L2 LSP
6	IS Neighbors (MAC Address)	LAN IIH
7	IS Neighbors (SNPA Address)	LAN IIH
8	Padding	IIH
9	LSP Entries	SNP
10	Authentication Information	IIH, LSP, or SNP
128	IP Internal Reachability Information	LSP
129	Protocols Supported	IIH or LSP
130	IP External Reachability Information	L2 LSP
131	Inter-Domain Routing Protocol Information	L2 LSP
132	IP Interface Address	IIH or LSP

Hello Packet Format

Hello packets, also called the IS-to-IS Hello PDUs (IIHs), are used to set up and maintain neighbor relationships. Level-1 LAN IIHs are applied to the Level-1 routers on broadcast LANs. Level-2 LAN IIHs are applied to the Level-2 routers on broadcast LANs. P2P IIHs are applied to non-broadcast networks. IIHs in different networks have different formats.

LAN IIHs: Figure 2 shows the format of IIHs on a broadcast network.
Figure 2 Level-1/Level-2 LAN IIH format
P2P IIHs: Figure 3 shows the format of IIHs on a P2P network.
Figure 3 P2P IIH format

As shown in Figure 3, most fields in a P2P IIH are the same as those in a LAN IIH. The P2P IIH does not have the priority and LAN ID fields, but has a local circuit ID field. The local circuit ID indicates the local link ID.

LSP Format

LSPs are used to exchange link-state information. There are two types of LSPs: Level-1 and Level-2. Level-1 IS-IS transmits Level-1 LSPs. Level-2 IS-IS transmits Level-2 LSPs. Level-1-2 IS-IS can transmit both Level-1 and Level-2 LSPs.

Level-1 and Level-2 LSPs have the same format, as shown in Figure 4.

Figure 4 Level-1 or Level-2 LSP

The main fields are as follows:

ATT: Attached bit

It is generated by a Level-1-2 router to identify whether the originating router is connected to other areas. When a Level-1 router receives a Level-1 LSP with ATT as 1 from a Level-1-2 router, the Level-1 router generates a default route destined for the Level-1-2 router so that data can be transmitted to other areas.

Although ATT is defined in both the Level-1 LSP and Level-2 LSP, it is set only in the Level-1 LSP only by the Level-1-2 router.
OL: LSDB overload

LSPs with the overload bit are still flooded on the network, but the LSPs are not used when routes that pass through a router configured with the overload bit are calculated. After a router is configured with the overload bit, other routers ignore the router when performing shortest path first (SPF) calculation. Only the direct routes of the router are considered.
IS Type: type of the IS-IS generating the LSP

This field is used to specify whether the IS-IS type is Level-1 IS-IS or Level-2 IS-IS. The value 01 indicates Level-1; the value 11 indicates Level-2.

SNP Format

SNPs describe the LSPs in all or some of the databases and are used to synchronize and maintain all link-state databases (LSDBs). SNPs consist of complete SNPs (CSNPs) and partial SNPs (PSNPs).

CSNPs carry summaries of all LSPs in LSDBs, which ensures LSDB synchronization between neighboring routers. On a broadcast network, the designated intermediate system (DIS) sends CSNPs at regular intervals. The default interval is 10 seconds. On a P2P link, neighboring devices send CSNPs only when a neighbor relationship is established for the first time.

Figure 5 shows the CSNP format.

Figure 5 Level-1/Level-2 CSNP format

The main fields are as follows:
- Source ID: system ID of the router that sends SNPs
- Start LSP ID: ID of the first LSP in a CSNP
- End LSP ID: ID of the last LSP in a CSNP
PSNPs list only the sequence numbers of recently received LSPs. A PSNP can acknowledge multiple LSPs at a time. If an LSDB is not updated, PSNPs are also used to request a neighbor to send a new LSP.

Figure 6 shows the PSNP format.

Figure 6 Level-1/Level-2 PSNP format