Nokia 4A0-113 OSPF flashcards

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

OSPF protocol number

OSPF runs directly over IP using IP protocol number 89. It does not use TCP or UDP, and it relies on multicast addresses 224.0.0.5 and 224.0.0.6 for Hello and flooding.

OSPF Hello packet

Type 1 packet sent periodically on each OSPF interface. It carries router ID, area ID, hello/dead timers, authentication data, stub flag, DR/BDR, and neighbor list used to discover and maintain adjacencies.

OSPF adjacency states

Down, Init, 2-Way, ExStart, Exchange, Loading, Full. 2-Way is stable between non-DR/BDR routers on broadcast segments; Full means LSDBs are synchronized and the router is fully adjacent.

DR/BDR election rule

On broadcast and NBMA segments, the highest interface DR priority wins (default 1); ties break on highest router ID. Priority 0 is ineligible. The election is not preemptive once a DR exists.

Hello parameter mismatch

Area ID, authentication type/key, hello and dead intervals, stub flag, and MTU must match for adjacency. Any mismatch causes Hellos to be dropped and the neighbor remains Down or Init.

NBMA versus point-to-multipoint

NBMA requires static neighbor statements and still elects a DR/BDR. Point-to-multipoint treats the segment as multiple point-to-point links, needs no DR, and forms adjacencies without static neighbors.

Passive interface behavior

A passive OSPF interface sends no Hellos and forms no adjacencies, but its connected subnet is still advertised in the router's Type 1 LSA. It is used on stub-facing or user-facing links.

Stuck in ExStart cause

A neighbor stuck in ExStart or Exchange usually indicates an MTU mismatch between the two interfaces, because the Database Description packets are too large to be accepted by the peer.

Type 1 Router LSA

Originated by every OSPF router for each area it belongs to. It lists the router's links, interface types, and costs. It is flooded only within its area and never crosses an ABR.

Type 2 Network LSA

Generated by the DR on a broadcast or NBMA segment. It lists the DR, the attached routers, and the subnet mask. It does not exist on point-to-point or point-to-multipoint links.

Type 3 Summary LSA

Generated by an ABR to advertise inter-area prefixes into another area. The metric is the ABR's cost to the destination, not the originating router's cost.

Type 5 AS External LSA

Carries external prefixes redistributed into OSPF. It is flooded domain-wide except into stub and NSSA areas, and it may carry a forwarding address to bypass the ASBR.

Type 7 NSSA External LSA

Used inside an NSSA to carry redistributed external routes. The NSSA ABR (the translator) converts Type 7 to Type 5 when advertising the prefix out of the area.

Stub area characteristics

A stub area blocks Type 5 and Type 4 LSAs. The ABR injects a default route into the area. A totally stubby area also blocks Type 3 LSAs except the default.

NSSA characteristics

An NSSA blocks Type 5 LSAs but permits Type 7 LSAs so external routes can be originated inside the area. The ABR translates Type 7 to Type 5 on export.

LSDB area scope

Routers within the same area must have identical Type 1 and Type 2 LSAs. Type 1 and Type 2 LSAs are area-scoped and are not flooded across area boundaries.

SPF algorithm scope

Dijkstra SPF runs per area over Type 1 and Type 2 LSAs to build the intra-area tree. Inter-area and external routes are computed afterward from Type 3, 4, 5, and 7 LSAs.

External Type 1 versus Type 2

Type 1 external cost equals the external metric plus the internal cost to the ASBR. Type 2 external cost equals the external metric only, with internal cost used only as a tie-breaker.

OSPF route preference order

Intra-area routes are preferred over inter-area routes, which are preferred over external Type 5 and Type 7 routes. SR OS assigns external routes a default preference of 150.

Partial SPF (PRC)

A router runs full SPF only when its own Type 1 or Type 2 LSAs change. Receiving a changed Type 3 or Type 5 LSA triggers a partial SPF, also called partial route calculation.

Forwarding address in Type 5

When the forwarding address in a Type 5 LSA is nonzero, traffic is sent to that address instead of the ASBR, allowing the external next hop to bypass the ASBR.

SPF throttling timers

SR OS spf-wait uses initial, secondary, and maximum intervals to limit how often SPF runs after topology changes, reducing CPU churn during repeated flaps.

Enabling OSPF on SR OS

Configure router ospf, then area <area-id> interface <interface-name>. The interface must already exist under configure router interface, and its address determines the advertised subnet.

SR OS OSPF verification commands

show router ospf neighbor, show router ospf interface, show router ospf database, and show router route-table protocol ospf display adjacency state, timers, LSAs, and installed routes.

OSPF redistribution on SR OS

Redistribute with configure router ospf export <policy-name>. The policy is defined under configure router policy-options and must match the routes intended for external advertisement.

Area range summarization

On an ABR, area <id> area-range <prefix> advertise aggregates Type 3 LSAs, shrinking the LSDB in other areas. The not-advertise option suppresses the aggregate instead.

Virtual link purpose

A virtual link connects a discontiguous area to area 0 through a transit area. The transit area must be non-stub, and the virtual link is configured with the remote router ID.

Graceful restart in OSPF

Graceful restart preserves forwarding during an OSPF process restart by keeping routes installed while adjacencies re-form. It is enabled with graceful-restart under router ospf.

OSPF authentication types

Null (0), simple password (1), MD5 (2), and SHA/HMAC variants. Authentication must match on both sides or Hellos are discarded and the adjacency never forms.

MD5 key rollover

SR OS supports multiple MD5 keys distinguished by key-id, allowing a new key to be configured and accepted before the old key is removed, avoiding an adjacency drop during rotation.

Common adjacency failure causes

Area mismatch, hello/dead timer mismatch, authentication mismatch, MTU mismatch, duplicate router ID, and one-way multicast reachability are the most frequent reasons an adjacency fails to reach Full.

OSPF troubleshooting commands

Use show router ospf neighbor for state and dead timer, show router ospf database detail for LSA age and checksum, debug router ospf for events, and clear router ospf to force re-adjacency.