IS-IS PDUs
PDU
IS-IS PDUs are encapsulated into link layer frames. An IS-IS PDU has two parts, the headers and the variable length fields. The headers comprise the PDU common header and the PDU specific header. All PDUs have the same PDU common header. The specific headers vary by PDU type.
Figure 39: PDU format
Table 11: PDU types
Type | PDU Type | Acronym |
---|---|---|
15 | Level-1 LAN IS-IS hello PDU | L1 LAN IIH |
16 | Level-2 LAN IS-IS hello PDU | L2 LAN IIH |
17 | Point-to-Point IS-IS hello PDU | P2P IIH |
18 | Level-1 Link State PDU | L1 LSP |
20 | Level-2 Link State PDU | L2 LSP |
24 | Level-1 Complete Sequence Numbers PDU | L1 CSNP |
25 | Level-2 Complete Sequence Numbers PDU | L2 CSNP |
26 | Level-1 Partial Sequence Numbers PDU | L1 PSNP |
27 | Level-2 Partial Sequence Numbers PDU | L2 PSNP |
Hello PDU
IS-to-IS hello (IIH) PDUs are used by routers to establish and maintain neighbor relationships. On broadcast networks, Level-1 routers use Level-1 LAN IIHs, and Level-2 routers use Level-2 LAN IIHs. The P2P IIHs are used on point-to-point networks.
LSP
The LSPs carry link state information. LSPs include Level-1 LSPs and Level-2 LSPs. The Level-2 LSPs are sent by the Level-2 routers, and the Level-1 LSPs are sent by the Level-1 routers. The Level-1-2 router can send both types of LSPs.
SNP
A sequence number PDU (SNP) describes the complete or partial LSPs for LSDB synchronization.
SNPs include CSNP and PSNP, which are further divided into Level-1 CSNP, Level-2 CSNP, Level-1 PSNP, and Level-2 PSNP.
A CSNP describes the summary of all LSPs for LSDB synchronization between neighboring routers. On broadcast networks, CSNPs are sent by the DIS periodically (every 10 seconds by default).
A PSNP only contains the sequence numbers of one or multiple latest received LSPs. It can acknowledge multiple LSPs at one time. When LSDBs are not synchronized, a PSNP is used to request missing LSPs from a neighbor.
CLV
The variable fields of PDU comprise multiple Code-Length-Value (CLV) triplets.
Figure 40: CLV format
Table 12 shows that different PDUs contain different CLVs. Codes 1 through 10 are defined in ISO 10589 (code 3 and 5 are not shown in the table). Codes 128 through 132 are defined in RFC 1195. Codes 222 through 237 are defined in RFC 5120.
Table 12: CLV codes and PDU types
CLV Code | Name | PDU Type |
---|---|---|
1 | Area Addresses | IIH, LSP |
2 | IS Neighbors (LSP) | LSP |
4 | Partition Designated Level 2 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, SNP |
128 | IP Internal Reachability Information | LSP |
129 | Protocols Supported | IIH, LSP |
130 | IP External Reachability Information | L2 LSP |
131 | Inter-Domain Routing Protocol Information | L2 LSP |
132 | IP Interface Address | IIH, LSP |
222 | MT-ISN | LSP |
229 | M-Topologies | IIH, LSP |
235 | MT IP. Reach | LSP |
237 | MT IPv6 IP. Reach | LSP |