information on behalf of the LAN. The DIS represents a pseudonode. The DIS sends out the separate Level 1 or Level 2 LSPs for the pseudonode. The Level 1 DIS and the Level 2 DIS on a LAN may or may not be the same router because an interface can have different Level 1 and Level 2 priorities. LSPs on point-to-point links are sent as unicast, whereas on broadcast media (LANs) LSPs are sent as multicast. Level 1 and Level 2 IIH
IIHs establish and maintain neighbor adjacency between ISs. The default hello interval is every 10 seconds; however, the hello interval timer is adjustable. On a LAN, separate Level 1 and Level 2 IIHs are sent periodically as multicasts to a multicast MAC address. Level 1 announcements are sent to the AllL1IS multicast MAC address 0180.C200.0014, and Level 2 announcements are sent to the AllL2IS multicast MAC address 0180.C200.0015. The default hello interval for the DIS is three times faster (that is, three times smaller) than the interval for the other routers so that DIS failures can be quickly detected. Unlike the DR and BDR in OSPF, there is no backup DIS in IS-IS. A neighbor is declared dead if hellos are not received within the hold time. The hold time is calculated as the product of the hello multiplier and hello time. The default hello time is 10 seconds, and the default multiplier is three; therefore, the default hold time is 30 seconds. Unlike LAN interfaces with separate Level 1 and Level 2 IIHs, point-to-point links have a common point-to-point IIH format that specifies whether the hello relates to Level 1 or Level 2 or both. Point-to-point hellos are sent to the unicast address of the connected router.

Content 4.3 IS-IS Operation 4.3.7 Comparing Broadcast and Point-to-Point Topologies Figure summarizes the differences between broadcast and point-to-point links.

Content 4.3 IS-IS Operation 4.3.8 Link-State Database Synchronization An IS-IS update process is responsible for flooding the LSPs throughout the IS-IS domain. An LSP is typically flooded to all adjacent neighbors, except the neighbor from which it was received. Level 1 LSPs are flooded within their local areas. Level 2 LSPs are flooded throughout the backbone. Each IS originates its own LSP (one for Level 1 and one for Level 2). These LSPs are identified by the system ID of the originator and an LSP fragment number starting at 0. If an LSP exceeds the maximum transmission unit (MTU), it is fragmented into several LSPs, numbered 1, 2, 3, and so on. IS-IS maintains the Level 1 and Level 2 LSPs in separate LSDBs. When an IS receives an LSP, it examines the checksum and discards any invalid LSPs, flooding them with an expired lifetime age. If the LSP is valid and newer than what is currently in the LSDB, it is retained, acknowledged, and given a lifetime of 1200 seconds. The age is decremented every second until it reaches 0, at which point the LSP is considered to have expired. When the LSP has expired, it is kept for an additional 60 seconds before it is flooded as an expired LSP.

Content 4.3 IS-IS Operation 4.3.9 LSDB Synchronization Sequence number PDUs (SNPs) are used to acknowledge the receipt of LSPs and to maintain LSDB synchronization. There are two types of SNPs:

CSNP
PSNP

The use of SNPs differs between point-to-point and broadcast media. CSNPs and PSNPs share the same format. Each carries summarized LSP information. The main difference is that CSNPs contain summaries of all LSPs in the LSDB, while PSNPs contain only a subset of LSP entries. Separate CSNPs and PSNPs are used for Level 1 and Level 2 adjacencies. Adjacent IS-IS routers exchange CSNPs to compare their LSDB. In broadcast subnetworks, only the DIS transmits CSNPs. All adjacent neighbors compare the LSP summaries received in the CSNP with the contents of their local LSDBs to determine if their LSDBs are synchronized (in other words, if they have the same copies of LSPs as other routers for the appropriate levels and area of routing). CSNPs are periodically multicast (every 10 seconds) by the DIS on a LAN to ensure LSDB accuracy. If there are too many LSPs to include in one CSNP, the LSPs are sent in ranges. The CSNP header indicates the starting and ending LSP ID in the range. If all LSPs fit in the CSNP, the range is set to default values. Adjacent IS-IS routers use PSNPs to acknowledge the receipt of LSPs and to request transmission of missing or newer LSPs. On point-to-point networks, CSNPs are sent when the link comes up to synchronize the LSDBs.

Content 4.3 IS-IS Operation 4.3.10 LAN LSDB Synchronization Example On a LAN, the DIS sends CSNPs that list the LSPs that it has in its LSDB every 10 seconds. This update is sent as a multicast to all Level 1 or Level 2 IS-IS routers on the LAN. In Figure , router R1 compares this list of LSPs with its topology table and realizes that it is missing one LSP. Therefore, it sends a PSNP to the DIS (router R2) to request the missing LSP. The DIS reissues only that missing LSP (LSP 77), and R1 acknowledges it with a PSNP.

Content 4.3 IS-IS Operation 4.3.11 Point-to-Point LSDB Synchronization Example In contrast to broadcast links such as LAN links, CSNPs are not sent periodically on point-to-point links. A CSNP is sent only once when the point-to-point link first becomes active. After the CSNP is sent, LSPs are sent to describe topology changes, and they are acknowledged with a PSNP. Figure shows what happens on a point-to-point link when a link failure is detected. The sequence is as follows:

A link fails.
Router R2 notices this failure and issues a new LSP noting the change.
Router R1 receives the LSP, stores it in its topology table, and sends a PSNP back to R2 to acknowledge receipt of the LSP.

Content 4.3 IS-IS Operation 4.3.12 LAN Adjacencies IIH PDUs announce the area address. On LANs, separate IIH packets announce the Level 1 and Level 2 neighbors. For example, the following applies where a LAN has routers from two areas attached:

The routers from one area accept Level 1 IIH PDUs only from their own area, so they establish adjacencies only with their own area routers.
The routers from a second area similarly accept Level 1 IIH PDUs only from their own area.
The Level 2 routers (or the Level 2 process within any Level 1–2 router) accept only Level 2 IIH PDUs and establish only Level 2 adjacencies.

Content 4.3 IS-IS Operation 4.3.13 WAN Adjacencies On point-to-point WAN links, the IIH PDUs are common to both levels but announce the level type and the area address in the hellos as follows:

Level 1 routers in the same area (which includes links between Level 1 and Level 1–2 routers) exchange IIH PDUs that specify Level 1 and establish a Level 1 adjacency.
Level 2 routers (in the same area or between areas, and including links between Level 2 only and Level 1–2 routers) exchange IIH PDUs that specify Level 2 and establish a Level 2 adjacency.
Two Level 1–2 routers in the same area establish both Level 1 and Level 2 adjacencies and maintain these with a common IIH PDU that specifies the Level 1 and Level 2 information.

Two Level 1 routers that are physically connected, but are not in the same area, can exchange IIHs. However, they do not establish an adjacency, since the area addresses do not match. Figure illustrates a variety of IS-IS connections. In the figure, roll over the router pair to display the adjacency type. Interactive Media Activity Matching: IS-IS Level 1 Adjacencies Upon completion of this activity, the student will be able to identify the Level 1 adjacencies between routers in an IS-IS network. Interactive Media Activity Matching: IS-IS Level 2 Adjacencies Upon completion of this activity, the student will be able to identify the Level 2 adjacencies