Nokia NRS II Composite (IS-IS) cheat sheet

IS-IS is a link-state IGP that runs directly on the data link layer (OSI CLNS), not on IP; PDUs are encapsulated in L2 frames, so it can route before IP is configured.

Three PDU categories: Hello PDUs (IIH) for adjacency, LSPs (Link State PDUs) carrying topology and prefixes, and SNPs (CSNP/PSNP) for database synchronization.

IS-IS uses a fixed 8-bit cost field in narrow metrics; the original ISO 10589 metric is unidirectional and typically set to 10 per link by convention.

Two subnetwork-dependent types: Level 1 (intra-area) and Level 2 (inter-area/backbone); a router can be L1, L2, or L1/L2.

Pseudonode: on broadcast links the DIS creates a virtual node so the LAN is represented as a single node in the SPF tree, reducing LSP and adjacency count.

LSP ID = System ID + pseudonode ID + fragment number; LSPs are identified by (LSP ID, sequence number, remaining lifetime, checksum).

Timers: Hello 10 s (default), hold time 3x Hello; LSP refresh 900 s, max age 1200 s, zero-age lifetime 60 s for purging.

IS-IS runs over MPLS/IP without IP reachability dependency, which is why it is preferred as the IGP for SP networks and for BGP-free core designs.

NSAP address components: AFI (1 byte, 49 for private), Area ID (1-2 bytes typical), System ID (6 bytes), NSEL (1 byte, 0x00 for routers).

A NET is an NSAP with NSEL = 00; a router may have multiple NETs but they must share the same System ID.

System ID is 6 bytes (12 hex digits) and must be unique within the routing domain; common practice is to derive it from a loopback IP or MAC.

Area ID length is variable; all routers in the same area must use the same area portion and System ID length must be consistent.

Example NET: 49.0001.1921.6800.1001.00 = AFI 49, area 0001, system ID 1921.6800.1001, NSEL 00.

The NSEL is not part of routing; routers forward to the System ID, and NSEL 00 identifies the router itself.

Area design rule: keep L1 areas small and stub-like; the L2 backbone should be contiguous and carry all inter-area traffic.

Changing a router's area requires changing its NET; a router with an L1 NET and L2 NET in different areas acts as an L1/L2 boundary.

Hello PDU types: Level 1 LAN IIH, Level 2 LAN IIH, Point-to-Point IIH; L1 and L2 adjacencies are formed and maintained independently.

Point-to-point adjacency states: Down → Initializing → Up; broadcast adds the DIS election and uses the same three states in ISO 10589.

Adjacency requirements: matching level, matching area ID for L1, matching authentication, compatible MTU, and matching protocol (narrow/wide metric) configuration.

On broadcast networks the DIS is elected by highest interface priority, then highest SNPA (MAC), then highest System ID; preemption is immediate.

On point-to-point links both routers form an adjacency; there is no DIS election, and both must agree on the circuit type (L1, L2, or both).

Authentication mismatch (plain text, MD5, or HMAC-SHA) prevents adjacency and is a common cause of stuck 'Initializing' state.

MTU mismatch causes IIH discard; the L2 MTU must be at least 1492 bytes for standard IIH, and larger for padding/hello padding.

Nokia SR OS: show router isis adjacency and show router isis interface display state, level, hold time, and last transition.

Each router originates its own LSP(s); LSP fragments are limited by the PDU size (narrow metric LSP ~1492 bytes, wide metric larger).

Flooding: a received LSP with a higher sequence number is installed and flooded out all other interfaces in the same level.

CSNP is sent periodically on broadcast links by the DIS (default 10 s) and on point-to-point at adjacency bring-up; it lists LSP IDs and sequence numbers.

PSNP is used to acknowledge LSPs on point-to-point links and to request missing LSPs after comparing CSNPs.

LSP comparison: higher sequence number wins; equal sequence number with lower remaining lifetime wins; checksum mismatch discards the LSP.

Overload bit (OL) set in LSP prevents transit traffic; the router still originates its own prefixes but is not used as a transit node.

LSP purge: setting remaining lifetime to zero floods a purge; the LSP is held in the database for the zero-age lifetime (60 s).

Nokia SR OS: show router isis database detail, show router isis database [lsp-id] detail, and clear router isis database to reset.

SPF runs independently per level: L1 SPF builds the intra-area tree; L2 SPF builds the backbone tree including inter-area reachability.

Dijkstra: each router computes shortest path to all nodes in its level's LSDB; the result is a shortest-path tree rooted at itself.

Route preference order: L1 intra-area > L2 intra-area > L1 inter-area > L2 inter-area (Nokia default preferences: L1 15, L2 18).

Prefixes are attached to the SPF tree via the advertising router; the cost to a prefix = cost to the advertising node + the prefix metric.

Equal-cost multipath (ECMP) is supported when multiple next hops have the same total cost; Nokia SR OS supports up to 32 paths by default configurable.

L1 routers use a default route (0/0) toward the nearest L2 router (ATT bit set in LSP) unless a more specific route exists.

The ATT (attachment) bit is set by L1/L2 routers and signals L1 routers that an exit point exists; it is cleared if the L2 adjacency is lost.

Nokia SR OS: show router isis routes, show router isis spf-log, and tools dump router isis spf to verify SPF behavior.

L1 routers only know their own area's topology; they forward all inter-area traffic to the nearest L1/L2 router.

L1/L2 routers maintain two separate LSDBs (L1 and L2) and set the ATT bit in their L1 LSP to advertise the L2 exit.

L2 routers form the backbone; they do not leak L1 topology into L2, and L1 areas are stub-like by design.

Inter-area prefix propagation: L2 LSPs carry prefixes from all areas; L1 routers receive them only via the default route or via route leaking.

Route leaking (L2→L1) is used to inject specific L2 prefixes into L1 for optimal routing; Nokia SR OS uses export policies on the L1/L2 router.

An L1 area must be contiguous; a partitioned area breaks intra-area reachability, and L1 routers cannot transit between partitions.

Level 2 must be contiguous across the domain; a broken L2 backbone isolates areas from each other.

Nokia SR OS: configure level capability with configure router isis level-capability [level-1|level-2|level-1/2] and verify with show router isis status.

Narrow metric: 6-bit field, 0-63 per link; total path cost is limited to 1023, which is insufficient for large networks.

Wide metric: 24-bit field (0-16,777,215), carried in TLV 22 (Extended IS Reachability) and TLV 135 (Extended IP Reachability); required for TE.

Metric styles: narrow, wide, and narrow/wide transition; all routers in a level must agree, or adjacency/SPF inconsistencies occur.

Default Nokia SR OS metric-style is wide; verify with show router isis interface and configure with configure router isis level-capability plus metric-style.

Traffic engineering: TLV 22 carries the TE default metric and sub-TLVs for max bandwidth, reservable bandwidth, and admin group (link colors).

TE extensions require wide metrics and MPLS-TE; IS-IS TE is used by RSVP-TE CSPF to compute constrained paths.

Overload bit and TE: a router with OL set is excluded from TE paths unless the overload is configured with a high metric for prefix reachability.

Nokia SR OS: configure router isis traffic-engineering and interface level-1/level-2 metric to set wide metrics explicitly.

IPv6 reachability is carried in TLV 236 (IPv6 Interface Address) and TLV 237 (IPv6 Reachability); original RFC 5308 single-topology mode.

Single-topology (RFC 5308): IPv4 and IPv6 share the same SPF tree; all links must support both, and any link without IPv6 breaks IPv6 end-to-end.

Multi-topology (RFC 5120): MT TLVs (MT IS Reachability TLV 222, MT IPv6 Reachability TLV 237 with MT ID) allow separate topologies for IPv4 and IPv6.

MT ID 0 = standard IPv4 topology; MT ID 2 = IPv6 topology; MT IDs 1-3995 are user-defined.

MT requires consistent configuration on all routers in the level; mismatched MT IDs cause adjacency or reachability failures.

Nokia SR OS: configure router isis multi-topology ipv6 and address-family ipv6 unicast under the IS-IS instance.

IPv6 adjacencies use the same IIH PDUs; the IPv6 interface address is advertised via TLV 232 (IPv6 Interface Address) in MT mode.

Verification: show router isis database detail shows MT TLVs; show router isis routes ipv6 confirms IPv6 prefix installation.

Base config: configure router isis area-id , level-capability, interface with interface-type point-to-point or broadcast.

Enable IS-IS on an interface with configure router isis interface and set level 1/2 metrics; the interface must have an IP address.

Authentication: configure router isis authentication key-chain or authentication-type md5/hmac-sha; both sides must match exactly.

Verification commands: show router isis adjacency, show router isis interface, show router isis database, show router isis routes, show router isis status.

Troubleshooting: check area mismatch (L1 only), level mismatch, authentication, MTU, and metric-style mismatch; use debug router isis and show log isis.

Clear commands: clear router isis adjacency, clear router isis database, clear router isis spf; use with care in production.

SR OS supports IS-IS over MPLS with LDP/RSVP; verify with show router isis interface and show router mpls lsp.

Logging: configure log filter and log destination to capture IS-IS adjacency state changes for post-incident analysis.