Nokia NRS II Composite (IS-IS) flashcards

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

IS-IS PDU encapsulation

IS-IS PDUs are carried directly in OSI CLNS data-link frames rather than in IP packets, so the protocol can form adjacencies and route before any IP address is configured on the link.

Pseudonode

On a broadcast network the DIS creates a virtual node called a pseudonode, so the LAN appears as a single node in the SPF tree and LSP and adjacency counts stay small.

LSP ID format

An LSP ID consists of the 6-byte System ID, a 1-byte pseudonode ID, and a 1-byte fragment number, uniquely identifying each link-state PDU in the database.

IS-IS default hello timer

The default IS-IS hello interval is 10 seconds and the hold time is three times the hello, so an adjacency is declared down after roughly 30 seconds without a hello.

NSAP address structure

An NSAP is composed of the AFI byte, a variable-length area ID, a 6-byte System ID, and a 1-byte NSEL; the value 49 is the private AFI.

NET definition

A Network Entity Title is an NSAP whose NSEL equals 00, identifying an IS-IS router; a router may hold several NETs but they must share one System ID.

System ID uniqueness

The 6-byte System ID must be unique within the IS-IS routing domain; deriving it from a loopback address or chassis MAC is a common way to guarantee uniqueness.

L1 area design rule

Level 1 areas should remain small and stub-like, with all inter-area traffic crossing a contiguous Level 2 backbone formed by the L1/L2 routers.

Point-to-point adjacency states

A point-to-point IS-IS adjacency moves through Down, Initializing, and Up; there is no DIS election, but both ends must agree on circuit type and authentication.

DIS election order

The Designated Intermediate System on a broadcast link is chosen by highest interface priority, then highest SNPA or MAC address, then highest System ID, and preemption is immediate.

L1 adjacency area check

A Level 1 adjacency forms only when both routers advertise the same area ID in their hellos; differing area IDs leave the adjacency stuck in Initializing.

Hello padding and MTU

IS-IS pads hellos to the interface MTU to detect MTU mismatches; if the MTU differs between neighbors the IIH is discarded and the adjacency never reaches Up.

CSNP purpose

The Complete Sequence Number PDU lists every LSP ID and sequence number in the database; the DIS sends it periodically on broadcast links so neighbors can detect missing LSPs.

PSNP purpose

The Partial Sequence Number PDU acknowledges received LSPs on point-to-point links and requests missing LSPs after a CSNP comparison, keeping databases synchronized.

LSP comparison rules

A received LSP replaces the stored one if it has a higher sequence number, or an equal sequence number with a lower remaining lifetime; checksum failures cause discard.

Overload bit

Setting the overload bit in a router's LSP excludes it from transit paths while still advertising its own prefixes, commonly used during convergence or maintenance.

LSP purge lifetime

Purging an LSP sets its remaining lifetime to zero and floods it; the entry is retained in the database for the zero-age lifetime of 60 seconds before removal.

SPF per level

SPF runs independently for Level 1 and Level 2; each router computes a shortest-path tree rooted at itself using only the link-state database of that level.

IS-IS route preference

Nokia SR OS prefers Level 1 intra-area routes (preference 15) over Level 2 intra-area routes (preference 18), with inter-area routes ranked below their intra-area counterparts.

Prefix cost calculation

The cost to reach a prefix equals the SPF cost to the advertising router plus the metric the router advertises for that prefix in its LSP.

ATT bit

An L1/L2 router sets the Attachment bit in its Level 1 LSP to signal that an exit to the Level 2 backbone exists, letting L1 routers install a default route.

L1/L2 router databases

An L1/L2 router maintains two separate link-state databases, one per level, and sets the ATT bit in its L1 LSP to advertise the Level 2 exit point.

Route leaking L2 to L1

Route leaking injects selected Level 2 prefixes into a Level 1 area so intra-area routers can choose optimal paths instead of always using the default route.

Level 2 backbone contiguity

The Level 2 backbone must be contiguous across the domain; a partition isolates Level 1 areas from each other because inter-area traffic must traverse Level 2.

Narrow metric limit

The narrow metric is a 6-bit field with a maximum link cost of 63 and a total path cost limit of 1023, which is too small for large service provider topologies.

Wide metric range

The wide metric is a 24-bit field ranging from 0 to 16,777,215, carried in TLVs 22 and 135, and is a prerequisite for traffic engineering extensions.

Metric-style consistency

All routers in a level must use the same metric style (narrow, wide, or transition); a mismatch causes adjacency or SPF inconsistency and unreachable prefixes.

IS-IS TE sub-TLVs

TLV 22 carries traffic engineering sub-TLVs such as maximum link bandwidth, reservable bandwidth, and administrative group, which RSVP-TE CSPF uses to compute constrained paths.

Single-topology IPv6

Under RFC 5308, IPv4 and IPv6 share one SPF tree, so every link in the path must support IPv6 or IPv6 end-to-end reachability breaks.

Multi-topology IS-IS

RFC 5120 multi-topology uses MT TLVs to build separate topologies; MT ID 0 is IPv4 and MT ID 2 is IPv6, allowing different paths per address family.

IPv6 reachability TLVs

IPv6 prefixes are advertised in TLV 236 for interface addresses and TLV 237 for reachability, with MT variants used when multi-topology is enabled.

SR OS adjacency verification

On Nokia SR OS, show router isis adjacency lists neighbor state, level, hold time, and last transition, making it the first command for adjacency troubleshooting.

SR OS level capability

The command configure router isis level-capability sets the router to level-1, level-2, or level-1/2, and show router isis status confirms the active capability.