Nokia IS-IS Routing Protocol flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

IS-IS encapsulation

IS-IS runs directly on Layer 2 and is identified by NLPID 0x83 in an 802.3 frame or a 0xFE padding byte in PPP; it does not use IP for its own PDUs.

NET format

The Network Entity Title is area address (1-13 bytes) plus 6-byte System ID plus 1-byte NSEL, which must be 0x00 for a router; example 49.0001.1921.6800.1001.00.

System ID

A 6-byte identifier unique to each IS-IS router, usually derived from a loopback or MAC address; duplicate System IDs cause LSP sequence conflicts and database flapping.

Level 1 routing

Intra-area routing using the L1 database; L1 routers reach other areas only through a default route pointing at the nearest L1/L2 router that sets the ATT bit.

Level 2 routing

Inter-area backbone routing using the L2 database; L2 routers form a contiguous backbone and do not need to share area addresses with each other.

ATT bit

The attached bit is set in L1 LSPs by an L1/L2 router to advertise itself as an exit from the area, causing L1 routers to install a default route toward it.

DIS election

On a broadcast link the Designated Intermediate System is chosen by highest interface priority then highest MAC address; it preempts immediately and has no backup.

Pseudonode LSP

An LSP generated by the DIS with a nonzero pseudonode ID that represents the broadcast link as a single node, so routers report one adjacency instead of a full mesh.

LSP ID

The LSP identifier is the 6-byte System ID plus a 1-byte pseudonode ID plus a 1-byte fragment number; fragment zero is the base LSP.

LSP sequence number

A 32-bit counter starting at 1 that increments with each new version of an LSP; a higher value is installed, and wrapping requires a purge and re-origination.

LSP lifetime

The remaining lifetime counts down from the max age (default 1200 seconds); the originator refreshes at 75% of max age and the LSP is purged at zero.

CSNP

The Complete Sequence Number PDU lists every LSP in the database with sequence number, checksum, and lifetime; the DIS sends it periodically so routers can detect missing LSPs.

PSNP

The Partial Sequence Number PDU acknowledges received LSPs and requests missing ones; it lists a subset of LSPs rather than the full database.

Reliable LSP flooding

A received LSP is acknowledged with a PSNP, and the sender retransmits after the retransmit interval (default 5 seconds) if no acknowledgement arrives; SRM and SSN flags track per-circuit state.

Overload bit

The OL flag in an LSP marks a router as unavailable for transit traffic while it remains reachable as a directly attached destination; it is set manually or on boot.

TLV structure

IS-IS carries all optional data in type-length-value fields, such as TLV 22 extended IS reachability, TLV 135 extended IP reachability, and TLV 236 IPv6 reachability.

Wide metrics

A 24-bit metric field carried in TLVs 22 and 135 that supports values up to 16,777,215 and is required for traffic engineering and segment routing.

Narrow metrics

The legacy 6-bit-per-link metric capped at 63; mixing narrow and wide metrics across a domain causes inconsistent path computation and must be avoided.

SPF calculation

Dijkstra runs separately on the L1 and L2 databases; the cost to a neighbor is the local interface metric, and the lowest total cost path is installed.

Partial Route Calculation

PRC recomputes only reachability when prefix TLVs change, avoiding a full SPF; a full SPF runs only when the adjacency or pseudonode topology changes.

LSP and SPF throttling

Timers bound how often LSPs are regenerated and SPF runs; Nokia SR OS defaults are initial 50 ms, secondary 200 ms, maximum 5 s for LSPs and 100/200/5000 ms for SPF.

IS-IS authentication

Hello, CSNP/PSNP, and LSP authentication are configured independently per level; Nokia SR OS supports cleartext, HMAC-MD5, and HMAC-SHA with key chains.

Three-way handshake

TLV 240 on point-to-point links confirms bidirectional connectivity before an adjacency reaches Up, preventing one-way links from black-holing traffic.

Hello and hold timers

The hello interval defaults to 10 seconds and the multiplier to 3, giving a 30-second hold time; the adjacency is torn down when no hello arrives within the hold time.

Adjacency Init state

Init means hellos are being received but the handshake is incomplete; common causes are level mismatch, L1 area mismatch, authentication mismatch, MTU mismatch, or a one-way link.

Route leaking

A route policy on an L1/L2 router exports selected L2 prefixes into L1 so L1 routers can reach them specifically instead of relying only on a default route.

Graceful restart

The restart TLV signals helpers during a control-plane restart; helpers suppress SPF and the restarting router keeps forwarding until adjacencies and the database are rebuilt.

Multi-topology IS-IS

TLV 229 and TLV 222 define separate topologies per address family, allowing IPv4 and IPv6 to use different paths instead of requiring congruent topologies.

Segment routing TLVs

SR-ISIS uses sub-TLVs of TLV 22 for SR capability, algorithm, and SID/label, and sub-TLVs of TLV 135 for prefix SIDs; the SRGB must be consistent domain-wide.

Nokia SR OS IS-IS config

Configure router isis with area-id and system-id, then interfaces with interface-type point-to-point and level 2; mark loopbacks passive so their prefixes are advertised without adjacencies.

show router isis adjacency

Displays neighbor System ID, interface, level, state, hold time, and uptime; the state must be Up, and Init points to a mismatch or one-way link.

IS-IS route preference

Nokia SR OS assigns IS-IS a preference of 15, so it beats OSPF at 110 and RIP at 100 but loses to static routes at 5 and direct routes at 0.