IS IS practice questions

Configure 'set protocols isis interface <name> priority <value>' where a higher value wins.

Configuring `set protocols isis interface <name> priority <value>` directly satisfies the requirement to guarantee a specific router wins the DIS election: on broadcast multi-access segments, Junos selects the highest-priority router as DIS, with the highest MAC address breaking ties. Raising this interface-level priority above all peers ensures deterministic DIS selection.

set level 1-2 wide-metrics-only

Configuring `level 1-2` enables the interface to form both Level 1 and Level 2 adjacencies, satisfying the boundary-router requirement. The `wide-metrics-only` keyword merely restricts metric advertisement to wide TLVs; it does not itself establish dual-level operation, so the level statement carries the required behaviour.

The metric-style configuration must be compatible (such as enabling wide metrics on both sides or using transition modes) to ensure LSAs containing TLVs 128/130 or 22/135 are understood.

IS-IS encodes reachability in TLVs: narrow metrics use TLVs 128/130, wide metrics use TLVs 22/135. A router only advertising narrow metrics cannot interpret wide TLVs, so metric-style must match or use transition mode before link-state PDUs are processed.

The IS-IS DIS election is non-preemptive; if a router with a higher priority comes online later, it does not take over until the current DIS fails or is restarted.

Unlike OSPF DR election, which allows a higher-priority router to preempt, IS-IS DIS election is non-preemptive. A newly arriving router with greater priority waits until the existing DIS fails or restarts, avoiding disruptive re-election on stable broadcast links.

In the event of a priority tie during DIS election, the router with the higher interface MAC address (or higher system ID depending on media) wins the tie-breaker.

IS-IS breaks priority ties by comparing the higher MAC address or system ID, unlike OSPF's DR election, which uses Router ID and never re-elects a preemptive DR. This satisfies the tie-breaker behaviour described in the stem.

Unlike OSPF, IS-IS does not have a separate Backup Designated Router (BDR) role; instead, routers keep track of priorities for a backup DIS implicitly.

IS-IS has no dedicated backup DIS; every router on the broadcast segment maintains a priority list, so a failed DIS is replaced immediately by the next-highest priority router without a pre-elected standby. This satisfies the stem's requirement to identify an operational difference from OSPF's explicit BDR election.

Configure the interface as point-to-point and enable IS-IS on both routers.

IS-IS defaults to broadcast on Ethernet, so a point-to-point link needs the interface explicitly set to point-to-point plus IS-IS enabled on both routers to form the adjacency. This satisfies the stem's requirement for a working adjacency over the point-to-point link.

The interface level is set to Level 1, but the neighbor has no Level 1 area address match.

IS-IS Initializing means hellos are received but no adjacency forms. A Level 1 interface requires matching area addresses; if the neighbour advertises a different area, the Level 1 adjacency never completes, leaving it stuck in Initializing.

The highest MAC address.

Equal DIS priorities are broken by the highest MAC address, so the router with the numerically greatest interface MAC becomes DIS. This satisfies the stem's tie-breaker constraint after priority comparison, since IS-IS uses MAC address as the final deterministic arbiter rather than system ID or loopback address.

TLV 22

TLV 22, the extended IS reachability TLV, carries wide metric values for IS-IS neighbour reachability, replacing the narrow 6-bit metric field. Changing the metric to influence path selection uses this TLV when wide metrics are configured.

10

Junos assigns every IS-IS interface a default metric of 10, so no explicit configuration is needed for the interface to participate in shortest-path calculation. This satisfies the stem's request for the default value, distinguishing it from the wide-metrics range where values reach into the millions.

To provide a summary of the entire database to neighbors.

On a broadcast segment, the DIS multicasts Complete Sequence Number PDUs summarising its entire link-state database, letting neighbours detect and request any missing or outdated link-state entries. This database synchronisation mechanism is why CSNP settings affect adjacency formation.

Link State PDU

Link State PDU is a valid IS-IS PDU type, used at Level 1 and Level 2 to advertise link-state information between intermediate systems. Other listed options, such as Hello and Sequence Numbers, are not valid IS-IS PDU types.

It is manually configured.

The System ID is a manually configured value, typically derived from the loopback interface address, forming part of the ISO NSAP address used by IS-IS. It is not auto-derived from a chassis serial number or router ID, satisfying the stem's determination question.

set protocols isis interface <name> point-to-point

Setting `point-to-point` under the IS-IS interface hierarchy forces the interface to form an adjacency using the point-to-point Hello PDU rather than the LAN IIH, satisfying the stem's requirement to configure point-to-point operation. Junos applies this per-interface, so the broadcast-to-point-to-point transition occurs without altering the global IS-IS configuration.

show isis database

`show isis database` displays the link-state PDU database, listing each LSP with its sequence number, checksum, holdtime and ATT/P/OL bits. This satisfies the stem's requirement for a database summary, giving the IS-IS link-state view rather than interface or adjacency detail.

Under the 'protocols isis export' hierarchy.

Redistribution into IS-IS is configured under the `protocols isis export` hierarchy, where you reference a policy that matches OSPF routes and sets the appropriate IS-IS metric. This satisfies the stem's requirement to inject OSPF-learned prefixes into the IS-IS link-state database, unlike import, which controls routes entering the routing table.

TLV 128

TLV 128, the IP Internal Reachability Information TLV, carries IPv4 prefixes and their metrics inside IS-IS Link State PDUs. It is the standard encoding for advertising internal IP reachability, distinguishing it from TLV 130 for external prefixes.

They maintain both a Level 1 and a Level 2 database.

A Level 1/Level 2 router participates in both areas, so it holds a Level 1 link-state database for its area and a separate Level 2 database for the backbone. This dual database is what lets it route intra-area and inter-area traffic.

They set the ATT bit in their Level 1 LSPs.

A Level 1/Level 2 router sets the ATT (Attached) bit in its Level 1 LSPs, signalling to Level 1-only routers that it offers a path out of the area toward the Level 2 backbone. This enables default route reachability.

It discards the LSP if the sequence number is lower.

IS-IS compares sequence numbers on receipt; an LSP with a lower sequence number than the stored copy is discarded as stale, satisfying the default behaviour described. Higher sequence numbers replace the entry and trigger flooding.

SNPA (MAC) address

DIS election on a broadcast link is decided first by interface priority, then by the highest SNPA (MAC) address as the tie-breaker when priorities are equal. The SNPA address therefore influences the outcome whenever priorities match.

Interface priority

Interface priority directly determines DIS election on a broadcast link, with the highest priority winning. It satisfies the stem's requirement by being one of the two influencing factors, alongside the highest 64-bit system ID acting as tiebreaker when priorities match.

Partial reachability issues.

Narrow and wide metric TLVs are not mutually understood, so routers only install reachability for prefixes whose metric type they can interpret. This yields partial reachability rather than total adjacency loss, matching the mismatch consequence.

SPF calculation fails to find all paths.

A narrow-versus-wide metric mismatch causes TLVs to be misinterpreted, so the receiving router computes incorrect path costs rather than the intended values. Because SPF relies on accurate link metrics to build the shortest-path tree, corrupted costs mean some valid paths are never selected, so SPF fails to find all paths.

Adjacency may form, but routes will not be accepted.

IS-IS adjacency formation depends on hello parameters, not metric style, so neighbours still form. However, mismatched narrow/wide metric TLVs mean the received routes cannot be installed, so routes are not accepted into the routing table.