Nokia IS-IS Routing Protocol cheat sheet

IS-IS is a link-state interior gateway protocol that runs directly on the data link layer (OSI Layer 2), not over IP; PDUs are identified by NLPID 0x83 in an 802.3 frame or by a 0xFE padding byte in a PPP frame.

Level 1 routing stays inside one area and uses the nearest L1/L2 router as its default exit; Level 2 routing is the backbone that connects areas. An L1/L2 router sets the ATT bit in its L1 LSP to advertise itself as an attached exit point.

Level 1 and Level 2 have separate link-state databases, separate SPF runs, and separate LSP lifetimes. A router can hold both databases simultaneously; the L1 database never leaks into L2 and vice versa.

The NET (Network Entity Title) is 8 to 20 bytes: the System ID is exactly 6 bytes, the area address is 1 to 13 bytes, and the last byte is the NSEL, which must be 0x00 for a router. Example: 49.0001.1921.6800.1001.00.

Every IS-IS router in a domain must have a unique 6-byte System ID. The System ID is normally derived from a loopback address or a MAC address; duplicate System IDs break the database and cause LSP purging conflicts.

The DIS (Designated Intermediate System) is elected per broadcast link by highest interface priority, then highest SNPA/MAC address. Unlike OSPF's DR, the DIS preempts a lower-priority DIS immediately and there is no backup DIS.

On broadcast links the DIS advertises a pseudonode LSP that lists all attached routers, so each router reports only adjacency to the pseudonode rather than a full mesh of adjacencies. Point-to-point links have no pseudonode.

The LSP ID is the 6-byte System ID plus a 1-byte pseudonode ID plus a 1-byte LSP fragment number. The remaining lifetime counts down from the configured max age (default 1200 seconds, 20 minutes).

IS-IS uses three timers on broadcast links: hello interval (default 10 s), hello multiplier (default 3, so hold time 30 s), and the DIS default hello of 3 s with a multiplier of 10. P2P hellos default to 10 s with multiplier 3.

The PDU types are: LAN Hello (IIH) type 15, P2P Hello type 17, LSP type 18, CSNP type 24, and PSNP type 26. All are carried in the same 802.3 frame format with a 0x83 NLPID.

The CSNP (Complete Sequence Number PDU) is sent periodically by the DIS (default every 10 s) and lists every LSP in the database with its sequence number, checksum, and remaining lifetime. The PSNP is used for partial acknowledgement and requests missing LSPs.

LSP flooding is reliable: a received LSP is acknowledged by a PSNP, and the sender retransmits after the retransmit interval (default 5 s) if no acknowledgement arrives. The SRM/SSN flags track send-routing-message and send-sequence-number-message state per circuit.

The LSP sequence number is a 32-bit value starting at 0x00000001 and increments with each new version. A router that receives an LSP with a higher sequence number installs it; an equal sequence number with a different checksum triggers a purge/flood.

The overload bit (OL) in an LSP tells other routers not to use this router as a transit hop, but it remains reachable as a directly attached destination. It is commonly set during startup or maintenance to avoid black-holing traffic.

The attach bit (ATT) is set in L1 LSPs by an L1/L2 router to signal that it can reach other areas. An L1 router installs a default route toward the nearest attached L1/L2 router.

The TLV structure is the extension mechanism: TLV 1 area addresses, TLV 2 IS neighbors, TLV 6 IS neighbors (with MAC), TLV 22 extended IS reachability, TLV 128 IP internal reachability, TLV 130 IP external reachability, TLV 132 IP interface address, TLV 135 extended IP reachability, TLV 137 hostname, TLV 236 IPv6 reachability.

Wide metrics (TLV 22 and TLV 135) use a 24-bit metric field and support traffic engineering; narrow metrics are 6 bits per link and cap at 63. Enabling wide metrics requires consistent configuration across the domain to avoid metric mismatch.

The default administrative distance for IS-IS in Nokia SR OS is 15 for internal routes; external IS-IS routes are also 15. Compare this against OSPF (110) and static (5) when planning redistribution.

SPF runs on the local L1 and L2 databases independently. The Dijkstra calculation uses the link metric; the router adds its own interface metric to reach a neighbor and selects the lowest total cost path to each destination prefix.

Equal-cost multipath (ECMP) is supported: up to the configured maximum (often 16 or 32 paths) are installed when total costs tie. The SPF tie-breaker prefers lower System ID for the designated path when ECMP is not enabled.

The PRC (Partial Route Calculation) is triggered when only reachability TLVs change, avoiding a full SPF. A full SPF runs when the topology (adjacency or pseudonode) changes; PRC is much cheaper and is a key IS-IS scalability feature.

LSP generation is throttled: the initial wait is typically 50 ms, the secondary wait 200 ms, and the maximum wait 5 s in Nokia SR OS. SPF itself is throttled similarly (initial 100 ms, secondary 200 ms, max 5 s).

The LSP lifetime is refreshed at 75% of max age by default; if a router receives its own LSP with the remaining lifetime at zero, it re-originates it with a new sequence number. A purge LSP has lifetime zero and sequence number incremented.

Authentication is per-level and per-PDU: hello, CSNP/PSNP, and LSP authentication can be configured independently. Nokia SR OS supports cleartext, HMAC-MD5, and HMAC-SHA-1/256/384/512 with key chains.

The IS-IS interface metric is configured per level (L1 and L2 separately) and per interface. The default metric is 10 on Nokia SR OS; a metric of 0 is valid and makes the link preferred.

Route leaking allows selected L2 prefixes to be advertised into L1 (typically via a route policy on the L1/L2 router) so L1 routers can reach them without a default route. Without leaking, L1 routers only see their own area plus a default.

Graceful restart (NSF/GR) uses the restart TLV (TLV 211) and the restart-capability TLV (TLV 211/212) so a restarting router can keep forwarding while its adjacencies and database are rebuilt; helper routers must support the capability.

Nokia SR OS enables IS-IS under the router instance: configure router isis, then interface "system" with level capability and passive, then interface "to-core" with interface-type point-to-point and level 2. The NET is set with the area-id and system-id under router isis.

On Nokia SR OS the interface-type options are point-to-point, broadcast, and non-broadcast (NBMA). The default is broadcast; explicitly setting point-to-point avoids DIS election and pseudonode LSPs on core links.

The level capability is configured per interface with level 1, level 2, or level 1/2 (both). The router-level default is level 1/2; a pure L1 router should set level-capability level-1 to avoid forming L2 adjacencies.

The passive command marks an interface as advertised in the LSP but not used to form adjacencies; it is used for loopbacks and stub networks so their prefixes are reachable.

The reference bandwidth for wide metrics on Nokia SR OS defaults to 10 Gbps; the interface metric is derived as reference-bandwidth / interface speed unless overridden with the metric command. Set reference-bandwidth 100000000 (100 Gbps) for modern cores.

The L1/L2 router's default behavior is to set the ATT bit and advertise itself as an exit. Disable with suppress-attached-bit if you want L1 routers to rely only on leaked routes or explicit defaults.

The route redistribution policy uses route-policy statements applied under router isis export. Only external routes (from other protocols or static) are exported; internal IS-IS routes are never redistributed back into IS-IS.

The show router isis adjacency command lists neighbor System ID, interface, level, state (Up/Init), hold time, and uptime. The show router isis database detail command shows LSP ID, sequence number, checksum, lifetime, and TLVs.

The show router isis routes command displays the IS-IS routing table with the level, metric, next-hop, and outgoing interface. The show router isis status command shows the NET, system ID, and level capabilities.

Adjacency formation requires matching levels (L1 with L1, L2 with L2), matching area addresses for L1, matching authentication, and matching MTU. L2 adjacencies do not require matching area addresses.

The three-way handshake uses the point-to-point adjacency TLV (TLV 240) on P2P links to confirm bidirectional connectivity; without it, a one-way link can form an adjacency that black-holes traffic.

The hello PDU carries the holding time; if no hello is received within the hold time, the adjacency is torn down. The hold time is hello interval times the multiplier; Nokia SR OS defaults are 10 s and 3.

L1 routers in different areas cannot form an L1 adjacency; they can form an L2 adjacency if both are L1/L2. A mismatched area address is the most common cause of an L1 adjacency stuck in Init.

The DIS election on a broadcast link is non-preemptive in the sense that the current DIS stays until it fails; a new router with higher priority does not preempt immediately in all implementations, but Nokia SR OS re-elects on the next hello. Verify with show router isis interface.

The pseudonode LSP is generated by the DIS with the pseudonode ID set to a nonzero value (the DIS's own circuit ID). The pseudonode LSP lists all routers on the link as neighbors, which is how SPF sees the broadcast link as a node.

The CSNP is sent by the DIS every 10 s on broadcast links and every 5 s on P2P links in some implementations; it allows routers to detect missing or stale LSPs and request them with a PSNP.

The overload bit is set manually with set-overload on Nokia SR OS, or automatically during startup with the overload-on-boot timer. It is cleared when the timer expires or the operator clears it.

The maximum LSP size is bounded by the interface MTU minus the IS-IS header; Nokia SR OS supports LSP fragmentation via the LSP MTU setting (default 1492). Large domains may need larger MTU or more fragments.

IPv6 support in IS-IS uses TLVs 232 (IPv6 interface address), 236 (IPv6 reachability), and 237 (IPv6 global address). A single IS-IS instance can route both IPv4 and IPv6; no separate protocol instance is required.

Multi-topology IS-IS (MT-ISIS) uses TLV 229 (MT supported) and TLV 222 (MT IS reachability) to run separate topologies for IPv4 and IPv6. This allows different paths per address family and avoids IPv4-only islands breaking IPv6 reachability.

The single-topology model requires all routers to support both address families and have congruent topologies; MT-ISIS is preferred in dual-stack networks with different IPv4 and IPv6 designs.

Segment routing with IS-IS uses TLV 22 sub-TLVs for SR capabilities (SR-Capability TLV 2, SR-Algorithm, SID/Label) and TLV 135 sub-TLVs for prefix SIDs. Nokia SR OS supports SR-ISIS with node SIDs and adjacency SIDs.

The SRGB (Segment Routing Global Block) must be consistent across the SR domain for node SIDs to be interpreted correctly. Nokia SR OS default SRGB is 8000-8999 in some releases; verify with show router mpls-labels label-range.

TI-LFA (Topology Independent Loop-Free Alternate) uses IS-IS SR extensions to compute backup paths; it requires segment routing enabled and the remote LFA capability in the IGP.

The IS-IS LSP purge is triggered when a router's own LSP is received with a higher sequence number or when the lifetime expires; the purge LSP has lifetime zero and is flooded to remove the LSP from all databases.

The sequence number space is 32-bit; when it wraps, the LSP must be purged and re-originated with a new sequence number starting at 1. This is rare but must be planned for in long-lived networks.

The IS-IS administrative distance in Nokia SR OS is 15 for both internal and external routes; this makes IS-IS preferred over OSPF (110) and RIP (100) for the same prefix.

The IS-IS PDU header is 8 bytes: 1 byte discriminator (0x83), 1 byte length indicator, 1 byte version/protocol ID extension, 1 byte ID length, 1 byte PDU type, 1 byte version, 1 byte reserved, 1 byte max area addresses. The fixed header follows with the PDU-specific fields.

The LSP fixed header includes the PDU length, remaining lifetime, LSP ID, sequence number, checksum, and flags (P, ATT, OL, IS type). The checksum is a Fletcher checksum over the LSP contents.

The hello PDU includes the circuit type (L1, L2, or L1/L2), the source ID (System ID), the holding time, and the TLV list. The circuit type must match for an adjacency to form.

The CSNP/PSNP includes the source ID, the start LSP ID, the end LSP ID, and a list of LSP entries (LSP ID, sequence number, checksum, remaining lifetime). The PSNP can also request a specific LSP.

The IS-IS metric is added per link; the total path cost is the sum of interface metrics along the path. The router does not add a cost for itself, only for the outgoing interface to each neighbor.

The default metric on Nokia SR OS is 10 per interface; a 10 Gbps link with reference bandwidth 100 Gbps gets a metric of 10, while a 1 Gbps link gets 100. The metric can be overridden per level.

The L1 default route is installed only if the ATT bit is set in an L1 LSP; the route points to the nearest L1/L2 router. If no ATT bit is seen, L1 routers have no default and rely on leaked routes.

The route preference for IS-IS in Nokia SR OS is 15; when multiple protocols offer the same prefix, the lower preference wins. IS-IS beats OSPF and RIP but loses to static and direct routes.

The IS-IS graceful restart helper mode is enabled by default on Nokia SR OS; the restarting router sends a restart TLV in its hello and LSPs, and helpers suppress SPF until the restart completes or the timer expires.

Troubleshooting an adjacency stuck in Init: check level mismatch, area address mismatch for L1, authentication mismatch, MTU mismatch, and one-way link. The three-way handshake TLV 240 helps detect one-way links on P2P.

Troubleshooting missing routes: verify the LSP is in the database (show router isis database), the prefix TLV is present, SPF has run, and the route is not filtered by a route policy. Check the ATT bit for L1 default routes.

Troubleshooting duplicate System ID: two routers with the same System ID cause LSP sequence number conflicts and flapping. Use show router isis database to see the LSP ID and compare with the local system ID.

Troubleshooting authentication: the hello, CSNP/PSNP, and LSP authentication are independent; a mismatch in any one prevents adjacency or database synchronization. Verify the key chain and algorithm match on both sides.

Troubleshooting metric mismatch: narrow metrics cap at 63; if one router uses wide metrics and another narrow, the metric interpretation differs and paths may be suboptimal. Ensure consistent wide-metrics configuration.

Troubleshooting LSP fragmentation: if the LSP exceeds the interface MTU, it is fragmented; too many fragments can slow convergence. Increase the LSP MTU or reduce the number of TLVs (e.g., summarize prefixes).

Troubleshooting DIS election: verify interface priority and MAC address; the highest priority wins, then the highest MAC. The DIS is re-elected on the next hello if a higher priority appears.

Troubleshooting SPF throttling: if SPF runs too often, increase the throttle timers; if convergence is slow, decrease them. The default Nokia SR OS values are initial 100 ms, secondary 200 ms, max 5 s.

Troubleshooting route leaking: verify the route policy on the L1/L2 router exports the L2 prefixes into L1 and that the L1 routers receive them in their L1 database. Without leaking, L1 routers only see their area plus a default.

The IS-IS area address is 1 to 13 bytes; the area ID in Nokia SR OS is configured as area-id 49.0001, which is 3 bytes. The area address must match for L1 adjacencies; L2 adjacencies ignore it.

The System ID is exactly 6 bytes; Nokia SR OS derives it from the system interface loopback address if not explicitly configured. The NET is the area ID plus the System ID plus the NSEL (00).

The NSEL must be 0x00 for a router; a nonzero NSEL is used for other OSI applications (e.g., IS-IS over OSI). A nonzero NSEL on a router NET prevents IS-IS from starting.

The L1/L2 router advertises both L1 and L2 LSPs; the L1 LSP has the ATT bit set, and the L2 LSP has the L2 topology. The router maintains separate L1 and L2 databases and SPF trees.

The L1 router's default route points to the nearest L1/L2 router by L1 metric; if multiple L1/L2 routers exist, the one with the lowest L1 metric to the L1 router is chosen. The ATT bit does not carry a metric.

The L2 router's backbone topology is built from L2 LSPs; L2 routers do not need to be in the same area. The L2 backbone must be contiguous for inter-area routing to work.

The IS-IS hello interval and multiplier are configured per interface and per level; the hold time is the product. A shorter hello interval speeds failure detection but increases overhead.

The IS-IS LSP lifetime default is 1200 s (20 minutes); the refresh interval is 75% of that (900 s). The max age can be tuned; a shorter lifetime increases LSP refresh overhead.

The IS-IS CSNP interval default is 10 s on broadcast links; the PSNP is sent on demand for acknowledgement and requests. The CSNP allows routers to detect missing LSPs without waiting for the LSP to expire.

Nokia SR OS IS-IS configuration example: router isis area-id 49.0001 system-id 1921.6800.1001 level-capability level-2 interface "system" passive interface "to-core" interface-type point-to-point level 2. The NET is formed as 49.0001.1921.6800.1001.00.

The interface "system" is the loopback; it is passive and its prefix is advertised in the LSP. The system interface address is used to derive the System ID if not explicitly set.

The interface-type point-to-point avoids DIS election and pseudonode LSPs; use it on core links between routers. Broadcast is the default and is appropriate for LAN segments with multiple routers.

The level-capability level-2 on a core interface means only L2 adjacencies form; the router-level level-capability must include L2 for the interface to form L2 adjacencies. A mismatch leaves the adjacency down.

The metric command sets the interface metric per level; the default is 10. The reference-bandwidth command scales the metric by interface speed for wide metrics.

The export policy under router isis redistributes external routes; the policy must match the route and set the external route type. Internal IS-IS routes are never exported back into IS-IS.

The show router isis adjacency detail command shows the neighbor System ID, level, state, hold time, and the interface. The state must be Up for the adjacency to be usable.

The show router isis database detail command shows the LSP ID, sequence number, checksum, lifetime, and TLVs; use it to verify the prefix TLVs and the ATT/OL bits.

The show router isis routes command shows the IS-IS routes with level, metric, next-hop, and interface; use it to verify the SPF result and the default route from the ATT bit.

IS-IS and OSPF comparison: IS-IS runs on Layer 2, OSPF on IP; IS-IS uses TLV extensibility, OSPF uses fixed LSA types; IS-IS has L1/L2 areas, OSPF has areas with ABRs; IS-IS DIS preempts, OSPF DR does not.

IS-IS scalability: the TLV structure and PRC make IS-IS efficient for large domains; the LSP fragmentation and wide metrics support large topologies. OSPF's LSA types and area design are more restrictive.

IS-IS authentication: Nokia SR OS supports HMAC-SHA-256 and key chains; OSPFv2 supports MD5 and SHA. IS-IS authentication is per-PDU type, giving finer control.

IS-IS and MPLS: IS-IS is the IGP for many MPLS networks; it advertises TE TLVs (22 sub-TLVs) for RSVP-TE and SR. The TE metric can differ from the IGP metric.

IS-IS and segment routing: SR-ISIS uses node SIDs and adjacency SIDs; the SRGB must be consistent. TI-LFA uses SR to compute backup paths.

IS-IS and IPv6: MT-ISIS runs separate topologies for IPv4 and IPv6; single-topology requires congruent topologies. Nokia SR OS supports both models.

IS-IS and graceful restart: the restart TLV and helper mode allow a router to restart without tearing down adjacencies; helpers suppress SPF until the restart completes.

IS-IS and route leaking: L2 prefixes can be leaked into L1 to avoid a default route; the route policy on the L1/L2 router controls which prefixes are leaked.

IS-IS and the overload bit: set during startup or maintenance to avoid transit traffic; the router remains reachable as a directly attached destination.

The IS-IS PDU type values: 15 = LAN IIH, 16 = P2P IIH (some references), 17 = P2P IIH, 18 = LSP, 20 = LSP (some), 24 = CSNP, 26 = PSNP. Verify the exact values in the Nokia SR OS documentation; the common values are 15, 17, 18, 24, 26.

The IS-IS discriminator is 0x83 (NLPID for IS-IS); the frame is an 802.3 frame with the LLC header 0xFE 0xFE 0x03 in some encapsulations. The 0x83 NLPID is the key identifier.

The IS-IS checksum is a Fletcher checksum over the LSP from the LSP ID to the end; it detects corruption. The checksum is recalculated on each LSP refresh.

The IS-IS sequence number starts at 1 and increments; the LSP ID includes the fragment number. A router can have multiple LSP fragments, each with its own sequence number.

The IS-IS remaining lifetime counts down from max age; at zero the LSP is purged. The refresh at 75% of max age prevents expiration.

The IS-IS CSNP lists all LSPs in the database; the PSNP lists a subset for acknowledgement or request. The CSNP is sent by the DIS; the PSNP by any router.

The IS-IS hello PDU includes the circuit type, source ID, holding time, and TLVs; the circuit type must match for adjacency. The holding time is the hello interval times the multiplier.

The IS-IS LSP flags: P (partition repair), ATT (attached), OL (overload), IS type (L1/L2). The ATT bit is set by L1/L2 routers; the OL bit by overloaded routers.

The IS-IS TLV 22 (extended IS reachability) includes the neighbor System ID, the metric, and sub-TLVs for TE and SR. TLV 135 (extended IP reachability) includes the prefix and metric.

The IS-IS area design: a contiguous L2 backbone connects areas; L1 routers stay within an area; L1/L2 routers connect L1 to L2. The L2 backbone must be contiguous for inter-area routing.

The IS-IS area address: 1 to 13 bytes; the area ID in Nokia SR OS is configured as area-id. All L1 routers in an area must share the same area address.

The IS-IS System ID: 6 bytes; unique per router. The NET is area ID + System ID + NSEL (00). Duplicate System IDs cause LSP conflicts.

The IS-IS DIS: elected per broadcast link by priority then MAC; preempts immediately; generates the pseudonode LSP. No backup DIS.

The IS-IS pseudonode: a virtual node representing the broadcast link; the DIS generates its LSP; SPF sees the link as a node.

The IS-IS ATT bit: set by L1/L2 routers in L1 LSPs; L1 routers install a default route to the nearest attached router. The ATT bit does not carry a metric.

The IS-IS OL bit: set by overloaded routers; other routers avoid transit through them but still reach directly attached destinations.

The IS-IS PRC: partial route calculation when only reachability changes; full SPF when topology changes. PRC is cheaper and improves scalability.

The IS-IS graceful restart: restart TLV and helper mode; helpers suppress SPF until the restart completes; the restarting router keeps forwarding.

Nokia SR OS IS-IS show commands: show router isis status, show router isis adjacency, show router isis database, show router isis routes, show router isis interface. Use these to verify NET, adjacencies, LSPs, and routes.

The show router isis status command shows the NET, system ID, level capability, and the administrative state. Verify the NET is correct and the levels match the design.

The show router isis adjacency command shows neighbor System ID, interface, level, state, hold time, and uptime. The state must be Up; Init indicates a mismatch or one-way link.

The show router isis database command shows LSP ID, sequence number, checksum, lifetime, and TLVs. Use detail to see the prefix TLVs and the ATT/OL bits.

The show router isis routes command shows the IS-IS routes with level, metric, next-hop, and interface. Verify the default route from the ATT bit and the leaked routes.

The show router isis interface command shows the interface type, level capability, metric, hello interval, and multiplier. Verify the point-to-point type on core links.

The clear router isis adjacency command resets an adjacency; use it after fixing a mismatch to force re-formation. The clear router isis database command clears the database (use with caution).

The debug router isis command enables IS-IS debugging; use it to trace hello, LSP, and SPF events. Limit the debug scope to avoid CPU impact.

The monitor router isis command shows real-time IS-IS events; use it to observe adjacency formation and LSP flooding.

The IS-IS metric type: narrow (6-bit, max 63) and wide (24-bit, max 16,777,215). Wide metrics are required for TE and SR; narrow metrics are legacy.

The IS-IS reference bandwidth: Nokia SR OS default 10 Gbps; the metric is reference-bandwidth / interface speed. Set reference-bandwidth to the highest link speed for accurate metrics.

The IS-IS ECMP: up to the configured maximum paths; equal-cost paths are installed. The SPF tie-breaker prefers lower System ID when ECMP is disabled.

The IS-IS route preference: 15 in Nokia SR OS; lower is preferred. IS-IS beats OSPF (110) and RIP (100) but loses to static (5) and direct (0).

The IS-IS LSP MTU: default 1492; larger MTU reduces fragmentation. The LSP is fragmented if it exceeds the MTU; each fragment has its own LSP ID.

The IS-IS hello padding: hellos are padded to the interface MTU to detect MTU mismatches; disable padding if overhead is a concern, but MTU mismatches may go undetected.

The IS-IS authentication: cleartext, HMAC-MD5, HMAC-SHA-1/256/384/512; key chains supported. Configure per level and per PDU type.

The IS-IS graceful restart: restart TLV and helper mode; helpers suppress SPF; the restarting router keeps forwarding. The restart timer bounds the helper's wait.

The IS-IS overload: set-overload or overload-on-boot; the OL bit is set in the LSP; other routers avoid transit. Clear the overload when maintenance is complete.

IS-IS adjacency states: Down, Init, Up. Init means hellos are received but the three-way handshake is incomplete; Up means bidirectional communication is confirmed.

The IS-IS three-way handshake: TLV 240 on P2P links confirms bidirectional connectivity; without it, a one-way link can form an adjacency that black-holes traffic.

The IS-IS hold time: hello interval times multiplier; if no hello is received within the hold time, the adjacency is torn down. Defaults: 10 s and 3.

The IS-IS DIS election: highest priority, then highest MAC; preempts immediately; no backup. The DIS generates the pseudonode LSP.

The IS-IS CSNP: sent by the DIS every 10 s; lists all LSPs; allows routers to detect missing LSPs. The PSNP acknowledges and requests LSPs.

The IS-IS LSP flooding: reliable; acknowledged by PSNP; retransmitted after the retransmit interval (default 5 s). The SRM/SSN flags track state per circuit.

The IS-IS LSP sequence number: 32-bit; starts at 1; increments; wraps require purge and re-origination. A higher sequence number is installed.

The IS-IS purge: lifetime zero; flooded to remove the LSP; triggered by expiration or a higher sequence number from the originator.

The IS-IS route leaking: L2 prefixes leaked into L1 via route policy; avoids a default route; controlled by the L1/L2 router.

The IS-IS NET format: area ID (1-13 bytes) + System ID (6 bytes) + NSEL (1 byte, 00). Example: 49.0001.1921.6800.1001.00. The NET is configured with area-id and system-id in Nokia SR OS.

The IS-IS area ID: 1-13 bytes; the area address must match for L1 adjacencies. L2 adjacencies ignore the area address.

The IS-IS System ID: 6 bytes; unique; derived from the system interface address if not set. Duplicate System IDs cause LSP conflicts.

The IS-IS NSEL: must be 00 for a router; nonzero for other OSI applications. A nonzero NSEL prevents IS-IS from starting.

The IS-IS L1/L2 router: maintains both databases; sets the ATT bit in L1 LSPs; connects L1 to L2. The L2 backbone must be contiguous.

The IS-IS L1 router: stays within an area; uses the default route from the ATT bit; does not see L2 topology. Leaked routes can provide specific L2 prefixes.

The IS-IS L2 router: backbone; does not need to be in the same area; carries inter-area traffic. The L2 backbone must be contiguous.

The IS-IS area design: a contiguous L2 backbone; L1 areas at the edges; L1/L2 routers at the boundaries. Avoid discontiguous L2 backbones.

The IS-IS route preference: 15; IS-IS beats OSPF and RIP; loses to static and direct. Verify with show router route-table.

The IS-IS PDU header: 8 bytes; discriminator 0x83; length indicator; version/protocol ID extension; ID length; PDU type; version; reserved; max area addresses. The fixed header follows.

The IS-IS LSP fixed header: PDU length; remaining lifetime; LSP ID; sequence number; checksum; flags (P, ATT, OL, IS type). The checksum is Fletcher.

The IS-IS hello PDU: circuit type; source ID; holding time; TLVs. The circuit type must match for adjacency.

The IS-IS CSNP/PSNP: source ID; start LSP ID; end LSP ID; LSP entries (LSP ID, sequence number, checksum, remaining lifetime). The PSNP can request a specific LSP.

The IS-IS metric: added per link; total path cost is the sum. The router does not add a cost for itself.

The IS-IS default metric: 10 on Nokia SR OS; scaled by reference bandwidth for wide metrics. The metric can be overridden per level.

The IS-IS L1 default route: installed only if the ATT bit is set; points to the nearest L1/L2 router. Without ATT, L1 routers rely on leaked routes.

The IS-IS route preference: 15; lower is preferred. IS-IS beats OSPF (110) and RIP (100); loses to static (5) and direct (0).

The IS-IS graceful restart helper: enabled by default; suppresses SPF until the restart completes or the timer expires. The restarting router sends the restart TLV.

Troubleshooting adjacency Init: check level mismatch, area address mismatch for L1, authentication mismatch, MTU mismatch, and one-way link. The three-way handshake TLV 240 detects one-way links on P2P.

Troubleshooting missing routes: verify the LSP is in the database, the prefix TLV is present, SPF has run, and the route is not filtered. Check the ATT bit for L1 default routes.

Troubleshooting duplicate System ID: two routers with the same System ID cause LSP conflicts and flapping. Use show router isis database to compare LSP IDs.

Troubleshooting authentication: hello, CSNP/PSNP, and LSP authentication are independent; a mismatch prevents adjacency or database sync. Verify the key chain and algorithm.

Troubleshooting metric mismatch: narrow metrics cap at 63; wide metrics are 24-bit. Ensure consistent wide-metrics configuration across the domain.

Troubleshooting LSP fragmentation: if the LSP exceeds the MTU, it is fragmented; too many fragments slow convergence. Increase the LSP MTU or summarize prefixes.

Troubleshooting DIS election: verify priority and MAC; the highest priority wins, then the highest MAC. The DIS is re-elected on the next hello if a higher priority appears.

Troubleshooting SPF throttling: adjust the throttle timers; defaults are initial 100 ms, secondary 200 ms, max 5 s. Slow convergence may need lower values.

Troubleshooting route leaking: verify the route policy on the L1/L2 router exports L2 prefixes into L1 and that L1 routers receive them. Without leaking, L1 routers see only their area plus a default.

The IS-IS area address: 1-13 bytes; the area ID in Nokia SR OS is configured as area-id 49.0001. The area address must match for L1 adjacencies.

The IS-IS System ID: 6 bytes; unique; derived from the system interface address if not set. The NET is area ID + System ID + NSEL (00).

The IS-IS NSEL: must be 00 for a router; nonzero for other OSI applications. A nonzero NSEL prevents IS-IS from starting.

The IS-IS L1/L2 router: maintains both databases; sets the ATT bit in L1 LSPs; connects L1 to L2. The L2 backbone must be contiguous.

The IS-IS L1 router: stays within an area; uses the default route from the ATT bit; does not see L2 topology. Leaked routes provide specific L2 prefixes.

The IS-IS L2 router: backbone; does not need to be in the same area; carries inter-area traffic. The L2 backbone must be contiguous.

The IS-IS area design: contiguous L2 backbone; L1 areas at the edges; L1/L2 routers at boundaries. Avoid discontiguous L2 backbones.

The IS-IS route preference: 15; IS-IS beats OSPF and RIP; loses to static and direct. Verify with show router route-table.

The IS-IS graceful restart: restart TLV and helper mode; helpers suppress SPF; the restarting router keeps forwarding. The restart timer bounds the helper's wait.

The IS-IS PDU type values: 15 = LAN IIH, 17 = P2P IIH, 18 = LSP, 24 = CSNP, 26 = PSNP. Verify the exact values in the Nokia SR OS documentation; the common values are 15, 17, 18, 24, 26.

The IS-IS discriminator: 0x83 (NLPID for IS-IS); the frame is an 802.3 frame with the LLC header in some encapsulations. The 0x83 NLPID is the key identifier.

The IS-IS checksum: Fletcher checksum over the LSP from the LSP ID to the end; detects corruption. The checksum is recalculated on each LSP refresh.

The IS-IS sequence number: 32-bit; starts at 1; increments; wraps require purge and re-origination. A higher sequence number is installed.

The IS-IS remaining lifetime: counts down from max age; at zero the LSP is purged. The refresh at 75% of max age prevents expiration.

The IS-IS CSNP: lists all LSPs in the database; sent by the DIS. The PSNP lists a subset for acknowledgement or request; sent by any router.

The IS-IS hello PDU: circuit type; source ID; holding time; TLVs. The circuit type must match for adjacency. The holding time is hello interval times multiplier.

The IS-IS LSP flags: P (partition repair), ATT (attached), OL (overload), IS type (L1/L2). The ATT bit is set by L1/L2 routers; the OL bit by overloaded routers.

The IS-IS TLV 22: extended IS reachability; neighbor System ID; metric; sub-TLVs for TE and SR. TLV 135: extended IP reachability; prefix and metric.

The IS-IS area design: a contiguous L2 backbone connects areas; L1 routers stay within an area; L1/L2 routers connect L1 to L2. The L2 backbone must be contiguous for inter-area routing.

The IS-IS area address: 1-13 bytes; the area ID in Nokia SR OS is configured as area-id. All L1 routers in an area must share the same area address.

The IS-IS System ID: 6 bytes; unique per router. The NET is area ID + System ID + NSEL (00). Duplicate System IDs cause LSP conflicts.

The IS-IS DIS: elected per broadcast link by priority then MAC; preempts immediately; generates the pseudonode LSP. No backup DIS.

The IS-IS pseudonode: a virtual node representing the broadcast link; the DIS generates its LSP; SPF sees the link as a node.

The IS-IS ATT bit: set by L1/L2 routers in L1 LSPs; L1 routers install a default route to the nearest attached router. The ATT bit does not carry a metric.

The IS-IS OL bit: set by overloaded routers; other routers avoid transit through them but still reach directly attached destinations.

The IS-IS PRC: partial route calculation when only reachability changes; full SPF when topology changes. PRC is cheaper and improves scalability.

The IS-IS graceful restart: restart TLV and helper mode; helpers suppress SPF until the restart completes; the restarting router keeps forwarding.

Nokia SR OS IS-IS show commands: show router isis status, show router isis adjacency, show router isis database, show router isis routes, show router isis interface. Use these to verify NET, adjacencies, LSPs, and routes.

The show router isis status command shows the NET, system ID, level capability, and administrative state. Verify the NET is correct and the levels match the design.

The show router isis adjacency command shows neighbor System ID, interface, level, state, hold time, and uptime. The state must be Up; Init indicates a mismatch or one-way link.

The show router isis database command shows LSP ID, sequence number, checksum, lifetime, and TLVs. Use detail to see the prefix TLVs and the ATT/OL bits.

The show router isis routes command shows the IS-IS routes with level, metric, next-hop, and interface. Verify the default route from the ATT bit and the leaked routes.

The show router isis interface command shows the interface type, level capability, metric, hello interval, and multiplier. Verify the point-to-point type on core links.

The clear router isis adjacency command resets an adjacency; use it after fixing a mismatch to force re-formation. The clear router isis database command clears the database (use with caution).

The debug router isis command enables IS-IS debugging; use it to trace hello, LSP, and SPF events. Limit the debug scope to avoid CPU impact.

The monitor router isis command shows real-time IS-IS events; use it to observe adjacency formation and LSP flooding.

The IS-IS metric type: narrow (6-bit, max 63) and wide (24-bit, max 16,777,215). Wide metrics are required for TE and SR; narrow metrics are legacy.

The IS-IS reference bandwidth: Nokia SR OS default 10 Gbps; the metric is reference-bandwidth / interface speed. Set reference-bandwidth to the highest link speed for accurate metrics.

The IS-IS ECMP: up to the configured maximum paths; equal-cost paths are installed. The SPF tie-breaker prefers lower System ID when ECMP is disabled.

The IS-IS route preference: 15 in Nokia SR OS; lower is preferred. IS-IS beats OSPF (110) and RIP (100) but loses to static (5) and direct (0).

The IS-IS LSP MTU: default 1492; larger MTU reduces fragmentation. The LSP is fragmented if it exceeds the MTU; each fragment has its own LSP ID.

The IS-IS hello padding: hellos are padded to the interface MTU to detect MTU mismatches; disable padding if overhead is a concern, but MTU mismatches may go undetected.

The IS-IS authentication: cleartext, HMAC-MD5, HMAC-SHA-1/256/384/512; key chains supported. Configure per level and per PDU type.

The IS-IS graceful restart: restart TLV and helper mode; helpers suppress SPF; the restarting router keeps forwarding. The restart timer bounds the helper's wait.

The IS-IS overload: set-overload or overload-on-boot; the OL bit is set in the LSP; other routers avoid transit. Clear the overload when maintenance is complete.

IS-IS adjacency states: Down, Init, Up. Init means hellos are received but the three-way handshake is incomplete; Up means bidirectional communication is confirmed.

The IS-IS three-way handshake: TLV 240 on P2P links confirms bidirectional connectivity; without it, a one-way link can form an adjacency that black-holes traffic.

The IS-IS hold time: hello interval times multiplier; if no hello is received within the hold time, the adjacency is torn down. Defaults: 10 s and 3.

The IS-IS DIS election: highest priority, then highest MAC; preempts immediately; no backup. The DIS generates the pseudonode LSP.

The IS-IS CSNP: sent by the DIS every 10 s; lists all LSPs; allows routers to detect missing LSPs. The PSNP acknowledges and requests LSPs.

The IS-IS LSP flooding: reliable; acknowledged by PSNP; retransmitted after the retransmit interval (default 5 s). The SRM/SSN flags track state per circuit.

The IS-IS LSP sequence number: 32-bit; starts at 1; increments; wraps require purge and re-origination. A higher sequence number is installed.

The IS-IS purge: lifetime zero; flooded to remove the LSP; triggered by expiration or a higher sequence number from the originator.

The IS-IS route leaking: L2 prefixes leaked into L1 via route policy; avoids a default route; controlled by the L1/L2 router.