CCNA IS IS Questions

On Juniper routers, when you enable IS-IS on an interface, the router operates as a Level 1/2 router by default, participating in both Level 1 and Level 2 routing. No additional configuration is needed to enable both levels. The 'level 1 disable' or 'level 2 disable' commands are used to restrict to a single level. Therefore, the correct answer is that both levels are enabled by default.

On point-to-point links, IS-IS establishes adjacencies using the three-way handshake TLV carried in IIH PDUs, confirming bidirectional connectivity before the adjacency reaches Up. Because the link is point-to-point, no DIS election occurs — that process applies only to broadcast links, satisfying the stem's comparison of the two link types.

An area address mismatch prevents Level 1 adjacency formation because IS-IS requires both routers to share at least one common area address before establishing a Level 1 link. Without this overlap, the routers cannot exchange Level 1 link-state PDUs, so the adjacency remains down, satisfying the stem's constraint.

The `lsp-generation-interval` timer throttles how often a router regenerates and floods its own link-state PDUs, batching rapid topology changes into a single update. This satisfies the stem's requirement by limiting update frequency, reducing CPU load and flooding overhead while still propagating changes within the configured delay.

The hello-interval timer defines the periodic rate at which IS-IS Hello PDUs are transmitted on an interface. Shortening or lengthening it directly changes how frequently neighbours hear from the router, matching the statement that it determines Hello PDU send frequency.

TLV 135 carries IPv4 internal and external reachability information as extended IP reachability entries, supporting wide metric values beyond the 6-bit limit. It is the correct TLV for advertising IPv4 prefixes when wide metrics are enabled in IS-IS.

The DIS multicasts Complete Sequence Numbers PDUs every ten seconds on a multi-access LAN, listing every link-state PDU in its database so neighbours can compare and request anything missing. This satisfies the stem's requirement for the PDU that maintains synchronised link-state databases, whereas partial sequence numbers PDUs only describe a subset.

IS-IS authentication applies a cryptographic checksum to Hello, LSP, CSNP and PSNP PDUs, so a receiving router verifies each PDU originated from a trusted neighbour. This secures the PDUs exchanged between neighbours, satisfying the question's requirement.

`show isis adjacency` displays each IS-IS neighbour with its interface, state (Up, Initialising, Down), hold time and remaining lifetime, directly satisfying the stem's requirement to verify adjacency state on a Junos device. It reads the adjacency database maintained by the IS-IS process, unlike route or interface commands.

A Level 1/Level 2 router sets the ATT (Attached) bit in its Level 1 LSPs. Level 1 routers interpret this bit as indicating the router is attached to another area and install a default route towards it, avoiding a full Level 2 database.

The 'Holdtime' field in the 'show isis adjacency' output indicates the remaining time before the adjacency is declared down if no hello is received. A decreasing holdtime means that hellos are being received and the timer is being refreshed. As long as hellos continue to arrive, the holdtime will reset to its maximum value. Therefore, a decreasing holdtime is normal and indicates a stable adjacency.

Point-to-point is a valid IS-IS interface type, satisfying the stem's requirement for genuine interface types. IS-IS recognises broadcast, point-to-point, and passive interfaces; point-to-point forms adjacencies between exactly two routers without electing a Designated Intermediate System, unlike broadcast links. This makes it a legitimate answer alongside broadcast.

Multi-topology IS-IS runs independent link-state topologies per address family, so IPv4 and IPv6 can use different paths. This satisfies the scenario's requirement to separate topologies for different address families, rather than forcing both to share one common topology.

The `show isis statistics` command on Junos devices reports per-PDU-type counters, including the number of received and transmitted IS-IS protocol data units such as hellos, link-state PDUs and sequence-number PDUs. This directly satisfies the stem's requirement, revealing PDU receive and transmit counts rather than adjacency state or routing table entries.

When an LSP's remaining lifetime reaches zero, the router purges it: the LSP is removed from the link-state database rather than retained. This satisfies the stem's scenario of an expired lifetime, distinguishing purge from refresh or re-origination.

SPF runs Dijkstra's algorithm over the link-state database to compute the lowest-cost path from the local router to every reachable node and prefix, producing the loop-free topology used for forwarding. This directly satisfies the question's requirement that shortest-path computation occurs during the IS-IS SPF calculation.

IS-IS does not use a reference bandwidth to calculate interface metrics; metric values are configured manually per interface. Unlike OSPF, changing the reference bandwidth has no effect on IS-IS metric calculation, so wide or narrow metric values remain unchanged.

Passive mode advertises the interface's prefix into the IS-IS link-state PDU so remote routers can reach the network, yet suppresses Hello PDUs, preventing adjacency formation. This satisfies the requirement of including the network in the LSP without forming an adjacency.

Junos enables both Level 1 and Level 2 by default when IS-IS is configured on an interface, satisfying the stem's default-level question. The interface therefore forms Level 1 adjacencies within its area and Level 2 adjacencies across areas unless the level is explicitly restricted.

IIH PDUs carry the IS-IS hello and DIS election fields, so they are exchanged on multi-access links to elect the Designated Intermediate System. The DIS election is triggered by hello adjacency formation, not by link-state flooding, making the IIH PDU the specific carrier for this process.

Level 2 routers form the IS-IS backbone and route between areas, using Level 2 link-state databases to reach destinations outside their local area. This satisfies the stem's requirement for a characteristic of Level 2 routing behaviour.

The DIS is identified in IS-IS hello packets by a 7-byte LAN ID: the 6-byte System ID of the DIS router followed by a non-zero pseudonode ID, distinguishing the broadcast LAN's pseudonode from the router itself.

A Level 2 adjacency requires both routers to be configured for Level 2 operation, since the hello PDU's circuit type must match on both sides. If one router only supports Level 1, the Level 2 adjacency cannot form regardless of other parameters.

The csnp-interval controls how often Complete Sequence Number PDUs are sent on a broadcast link. Increasing it lengthens the gap between CSNPs, reducing control traffic overhead on slow links where frequent flooding would otherwise consume scarce bandwidth.

`show isis interface` directly queries the IS-IS interface database, returning per-interface parameters such as circuit type, metric, hello interval, and DIS priority. This satisfies the stem's requirement for interface-level IS-IS parameters, unlike commands that display adjacency or link-state database information.

IIH (IS-IS Hello) PDUs are the valid IS-IS PDU type used to establish and maintain adjacencies on broadcast and point-to-point circuits, carrying neighbour and circuit information. Alongside LSP and SNP types, IIH forms the complete set of IS-IS PDU categories defined by the protocol.

The detail keyword expands each LSP, displaying its TLVs, including extended IP reachability entries carrying wide metrics. The summary form omits TLV contents, so only the detailed database output satisfies the requirement to inspect those specific TLVs.

IS-IS hello packets are padded to the interface MTU by default, so if link MTU sizes differ, oversized IIHs are silently dropped and adjacency never forms. Matching MTU satisfies the link-level constraint required for reliable IIH exchange between neighbours.

IS-IS authentication passwords can be configured at both the interface level, controlling hello PDUs on a specific link, and the global level, securing link-state PDUs across the routing domain. This dual placement satisfies the stem's requirement by allowing authentication at either scope, matching how Junos applies IS-IS authentication.

On broadcast networks the DIS advertises a pseudonode LSP on behalf of the LAN, representing the multi-access segment as a single virtual node. This reduces adjacency and LSP flooding overhead, which is precisely the DIS function the question targets.

The 'lsp-lifetime' command defines how long a router's own IS-IS link-state PDUs remain valid before expiring from the link-state database. This satisfies the stem's requirement by setting the LSP's remaining lifetime field, which the originating router refreshes before expiry to prevent neighbours discarding the LSP.

Level 2 routers form the backbone, carrying inter-area traffic between Level 1 areas via Level 1-2 routers. This backbone role is precisely what connects separate Level 1 areas, matching the statement's claim about Level 2's function.