10.3 IS-IS PDU Families and Neighbor Exchange
Key Takeaways
IIH discovers/maintains neighbors; LSP carries topology and prefix TLVs.
CSNP summarizes an LSP inventory; PSNP requests missing information and acknowledges LSPs on P2P links.
Junos default non-DIS hello/hold timers are 9/27 seconds, with LAN DIS timers scaled to 3/9 seconds.
IS-IS Adjacencies & PDU Mechanics
Link-state protocols require deterministic mechanisms to discover neighbors, negotiate operational parameters, synchronize topology databases, and maintain reachability state. In IS-IS, all communication is mediated through structured Protocol Data Units (PDUs) transmitted directly inside Layer 2 frames. Understanding the functional taxonomy of these PDUs, the three-way handshake sequence, and the role of the Designated Intermediate System (DIS) is essential for mastering enterprise campus and core routing.
1. The Four Core IS-IS Protocol Data Unit (PDU) Classes
IS-IS operates using four distinct PDU families, each divided into Level 1 and Level 2 variants where applicable:
+---------------------------------------------------------------------------------------------------+
| IS-IS PROTOCOL DATA UNIT (PDU) TAXONOMY |
+-------------------+-----------------------------------+-------------------------------------------+
| PDU FAMILY | SPECIFIC PDU TYPES | PRIMARY FUNCTION |
+-------------------+-----------------------------------+-------------------------------------------+
| 1. IIH | - Point-to-Point IIH | Discovers neighbors, establishes 3-way |
| (IS-IS Hello) | - LAN Level 1 IIH | adjacencies, tests full-size delivery with padding, and |
| | - LAN Level 2 IIH | maintains keepalive liveness. |
+-------------------+-----------------------------------+-------------------------------------------+
| 2. LSP | - Level 1 LSP | Originates and floods link-state metrics, |
| (Link-State | - Level 2 LSP | neighbor adjacencies, IP prefixes, |
| PDU) | | and protocol capabilities. |
+-------------------+-----------------------------------+-------------------------------------------+
| 3. CSNP | - Level 1 CSNP | Describes the entire inventory of LSPs |
| (Complete Seq. | - Level 2 CSNP | in the LSDB; sent periodically on LANs |
| Number PDU) | | and once at startup on P2P links. |
+-------------------+-----------------------------------+-------------------------------------------+
| 4. PSNP | - Level 1 PSNP | Acknowledges LSPs on point-to-point |
| (Partial Seq. | - Level 2 PSNP | links; requests missing or newer LSPs |
| Number PDU) | | on broadcast multi-access links. |
+-------------------+-----------------------------------+-------------------------------------------+
1. IS-IS Hello (IIH) PDUs
Hello packets are exchanged periodically to discover adjacent Intermediate Systems and verify bidirectional communication. IS-IS uses three specialized Hello formats:
- Point-to-Point IIH: Exchanged over dedicated serial, dark-fiber, or routed point-to-point Ethernet links. It utilizes a three-way handshake TLV (RFC 5303) traversing three states:
Down,Initializing, andUp. - LAN Level 1 IIH: Multicast to
01:80:C2:00:00:14on broadcast multi-access links to discover Level 1 neighbors and elect the Level 1 DIS. - LAN Level 2 IIH: Multicast to
01:80:C2:00:00:15on broadcast multi-access links to discover Level 2 neighbors and elect the Level 2 DIS.
Hello Timers and Padding
By default in Junos OS, non-DIS routers transmit Hellos every 9 seconds with a Holding Time of 27 seconds (a multiplier of 3). Junos uses Hello padding to test the supported IS-IS packet size during adjacency establishment. The documented default padded size is 1492 bytes, rather than automatically matching a 9000-byte physical jumbo MTU. The default loose mode pads until the adjacency becomes Up; strict pads continuously, adaptive uses neighbor-state information, and disable removes padding. A path unable to carry the padded PDU can fail adjacency establishment. Different physical MTU settings alone do not prove that Hellos exceed the smaller endpoint’s limit: inspect the actual ISO/PDU sizes and padding mode.
2. Link-State PDUs (LSPs)
LSPs are the IS-IS equivalent of OSPF Link-State Advertisements (LSAs) and Link-State Updates (LSUs). Each router generates its own LSP to describe its identity, connected interfaces, neighbor adjacencies, metric costs, and reachable IP prefixes. LSPs are identified by an 8-octet LSP ID:
+---------------------------------------------------------------------------------------------------+
| LSP IDENTIFIER STRUCTURE (8 OCTETS) |
| |
| [ System ID: 6 Octets ] . [ Pseudonode ID: 1 Octet ] - [ LSP Fragment Number: 1 Octet ] |
| |
| Example: 1921.6800.0001.00-00 (Non-pseudonode LSP originated by router 192.168.0.1) |
| Example: 1921.6800.0001.01-00 (Pseudonode LSP for LAN segment originated by DIS) |
+---------------------------------------------------------------------------------------------------+
- Lifetime: In Junos, LSPs are generated with a default Remaining Lifetime of 1200 seconds (20 minutes) and count downward to 0. This stands in direct contrast to OSPF LSA Age, which counts upward from 0 to 3600 seconds. If an LSP's lifetime expires, it is purged from the LSDB.
- Sequence Number: A 32-bit unsigned integer beginning at
0x00000001that increments each time the originating router floods an updated version of its LSP.
3. Complete Sequence Number PDUs (CSNPs)
A CSNP contains a complete summary of every single LSP present in a router's Link-State Database, specifying each LSP's identifier, sequence number, remaining lifetime, and checksum. CSNPs ensure that all routers on a segment maintain an identical LSDB:
- Broadcast Networks: The Designated Intermediate System (DIS) multicasts a CSNP onto the LAN every 10 seconds. All other routers on the LAN compare the CSNP against their local databases.
- Point-to-Point Networks: Routers exchange a CSNP once during initial link initialization when the adjacency first reaches the
Upstate.
4. Partial Sequence Number PDUs (PSNPs)
A PSNP contains an inventory of only a subset of LSPs. It serves two distinct operational functions depending on the media type:
- On Point-to-Point Links: Acts as an explicit acknowledgment (ACK). When Router A receives an LSP from Router B over a P2P link, Router A sends a PSNP listing that LSP ID and sequence number to confirm receipt.
- On Broadcast LAN Links: Acts as a Link-State Request (NACK). If a router inspects a periodic CSNP from the DIS and detects that it is missing an LSP or has an older sequence number, it sends a PSNP to the DIS to explicitly request the missing full LSP.
Official references checked 2026-10-18: Junos hello/hold timers.
Type-Length-Value Encoding in IS-IS
A TLV has a type code identifying what the information means, a length describing the value’s size and the value itself. The PDU header identifies the kind of exchange; its TLVs carry the variable neighbor, addressing, reachability or capability information. An LSP can contain multiple TLVs, and an extended reachability TLV can contain sub-TLVs describing a particular link. A TLV is therefore an encoding element inside a PDU, not a fifth PDU family.
The older IS reachability TLV 2 describes neighbors with narrow metrics. Extended IS reachability TLV 22 carries wider link metrics and can include link attributes in sub-TLVs. Extended IPv4 reachability TLV 135 carries IP prefixes and metrics; its prefix-control octet contains the Up/Down flag used for inter-level leaking. Hostname TLV 137 gives a human-readable name without changing the System ID. These examples show why a display can list both topology and naming information from one LSP.
When diagnosing an Up adjacency with missing routes, inspect the actual database encoding rather than assuming an interface metric command proves what the peer advertised. The neighbor/PDU exchange can succeed while a required prefix TLV is absent, unsupported or filtered during route installation. Unknown extensions are handled according to their defined encoding rules; flexible TLVs permit protocol evolution without changing every fixed PDU header. RFC 5305 defines the extended reachability and sub-TLV examples used here.
Official references checked 2026-10-18: IS-IS interface padding modes and packet size, Operational interface fields.
Which statement correctly describes the operational roles of CSNP and PSNP packets on IS-IS point-to-point links compared to broadcast LAN links?
On point-to-point links, PSNP serves as an explicit acknowledgment of received LSPs, whereas on broadcast LANs, PSNP is sent by non-DIS routers to request missing LSPs
On point-to-point links, CSNP is flooded continuously every 10 seconds, while on broadcast LANs CSNP is only sent during the initial 3-way handshake
On point-to-point links, PSNP carries the entire routing table inventory, whereas on broadcast LANs it carries interface metric overrides
On both network types, CSNP acts as an acknowledgment packet and PSNP acts as an unnumbered keepalive monitor
What does default Junos IS-IS Hello padding test during adjacency establishment?
Padding encrypts the Hello header with MD5 hashing; mismatched MTUs cause fallback to narrow metrics
Padding inserts dynamic hostname TLVs into the packet; mismatched MTUs result in duplicate System IDs
Whether the path carries the documented default 1492-byte IS-IS PDU; an undersized path can drop padded Hellos.
Padding compresses the Area ID down to a single byte; mismatched MTUs cause the DIS to permanently elect the router with the lower MAC address
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