Nokia 4A0-113 OSPF cheat sheet

OSPF is a link-state IGP using IP protocol number 89; it runs directly over IP with no TCP/UDP transport and uses multicast 224.0.0.5 (AllSPFRouters) and 224.0.0.6 (AllDRouters).

Five packet types: Hello (1), Database Description (2), Link State Request (3), Link State Update (4), Link State Acknowledgment (5).

Hello timer defaults: 10 s on broadcast/point-to-point, 30 s on NBMA/point-to-multipoint; dead (router-dead) interval defaults to 4x hello. Mismatched hello/dead values prevent adjacency.

Adjacency progression: Down -> Init -> 2-Way -> ExStart -> Exchange -> Loading -> Full. 2-Way is the stable state between non-DR/non-BDR routers on a broadcast segment.

Router ID selection order: explicit router-id, then highest loopback address, then highest non-loopback interface address; changing it requires an OSPF restart to take effect.

DR/BDR election is per broadcast/NBMA segment, decided by highest interface DR priority (default 1, 0 = ineligible), then highest router ID; election is not preemptive.

Cost = reference-bandwidth / interface bandwidth; Nokia SR OS default reference bandwidth is 100 Gbps, so costs are 1 for links of 100G and above.

LSA types: 1 Router, 2 Network, 3 Summary, 4 ASBR Summary, 5 AS External, 7 NSSA External; Type 1/2 stay within an area, Type 3/4 are generated by ABRs.

Hello contents that must match for adjacency: area ID, authentication type and key, hello/dead intervals, stub area flag, MTU (for DBD exchange), and matching network type expectations.

Network types in SR OS: point-to-point, broadcast, non-broadcast (NBMA), point-to-multipoint; broadcast and point-to-point form full adjacencies without manual neighbor configuration.

On NBMA, neighbors must be statically defined with neighbor and DR election still applies; point-to-multipoint treats the segment as a set of point-to-point links and needs no DR.

MTU mismatch causes the adjacency to stall in ExStart/Exchange; SR OS can ignore MTU in DBD with the ignore-mtu-mismatch option on the interface.

Passive interfaces suppress Hellos so no adjacency forms but the connected subnet is still advertised; use for stub-facing or user-facing interfaces.

Interface parameters that must match on both sides include area-id, hello-interval, dead-interval, auth-key, and network-type (or the adjacency will not reach Full).

A router with priority 0 never becomes DR or BDR; if all priorities are 0, no DR/BDR is elected and the segment stays at 2-Way.

Stuck-in-Init means the local router hears the neighbor but the neighbor does not hear it back - typically one-way reachability, ACL, or multicast filtering.

Type 1 Router LSA is flooded within one area and lists the router's links, interface types, and costs; it never crosses an ABR.

Type 2 Network LSA is generated by the DR on a broadcast/NBMA segment and lists all attached routers; it does not exist on point-to-point links.

Type 3 Summary LSA is generated by an ABR to advertise inter-area prefixes; it carries the ABR's cost to the destination, not the originating router's cost.

Type 4 ASBR Summary LSA tells other areas how to reach an ASBR; Type 5 AS External LSA carries external prefixes and is flooded domain-wide except into stub/NSSA areas.

Type 7 NSSA External LSA is used inside an NSSA and translated to Type 5 by the NSSA ABR (the translator) when it is redistributed out of the area.

Stub area blocks Type 5 and Type 4 LSAs and uses a default route injected by the ABR; totally stubby additionally blocks Type 3 LSAs except the default.

NSSA blocks Type 5 but allows Type 7 so external routes can be originated inside; totally NSSA also blocks Type 3 except the default.

The LSDB must be identical among all routers within the same area; Type 1 and Type 2 LSAs are area-scoped and are not flooded across area boundaries.

SPF (Dijkstra) runs per area over Type 1 and Type 2 LSAs to build the intra-area tree; inter-area and external routes are computed from Type 3/5/7 LSAs afterward.

Route preference in SR OS OSPF: intra-area (preference 10) beats inter-area (preference 10 on Nokia with distinct tie-break), which beats external Type 5/7 (preference 150 by default).

External Type 1 cost = external metric + internal cost to the ASBR; External Type 2 cost = external metric only (default), with internal cost used only as a tie-breaker.

For multiple equal-cost paths, SR OS installs up to ECMP paths controlled by the ecmp setting; SPF keeps all equal-cost next hops in the routing table.

A router only runs full SPF when its own Type 1/2 LSAs change; receiving a changed Type 3/5 LSA triggers a partial SPF (PRC) rather than a full SPF.

The forwarding address in a Type 5 LSA, when nonzero, is used as the next hop instead of the ASBR address, allowing traffic to bypass the ASBR.

Inter-area route selection prefers the lowest cost to the destination ABR plus the ABR's advertised cost; the ABR closest to the destination is preferred.

SPF throttling in SR OS uses spf-wait (initial, secondary, maximum) to limit CPU churn from repeated topology changes.

Base configuration: configure router ospf, then area interface ; the interface must already exist under configure router interface.

Router ID is set with configure router router-id or inherited; OSPF uses it as the identity in all LSAs and Hellos.

Enable OSPF on an interface with area interface ; the interface's IP address and mask determine the attached subnet advertised in the Type 1 LSA.

Interface-level knobs: hello-interval, dead-interval, priority, metric, passive, authentication-type, authentication-key, network-type.

Redistribution: configure router ospf export ; the policy must be defined with configure router policy-options and match the desired routes.

SR OS supports OSPFv2 (IPv4) and OSPFv3 (IPv6); OSPFv3 uses link-local addresses and a per-interface instance ID.

Verify with show router ospf neighbor, show router ospf interface, show router ospf database, and show router route-table protocol ospf.

Changing the router ID or area configuration requires clearing OSPF (clear router ospf) to force re-adjacency and a fresh SPF.

Area summarization is configured on the ABR with area area-range [advertise|not-advertise]; it aggregates Type 3 LSAs and reduces LSDB size in other areas.

External summarization is configured with configure router ospf export policy using a prefix-list/aggregate; Type 5 LSAs are summarized at the ASBR.

Stub and NSSA areas reduce LSDB and SPF scope; totally stubby (SR OS: stub with no-summary) blocks Type 3 LSAs except the default route.

NSSA is used when an area needs to originate external routes but should not carry Type 5 LSAs; the ABR translates Type 7 to Type 5.

Use virtual links (area virtual-link ) to connect a backbone-discontiguous area to area 0; the transit area must be non-stub.

Graceful restart (NSR/GR) preserves forwarding during OSPF process restart; configure with graceful-restart under router ospf.

Limit LSA churn with spf-wait and lsa-arrival/lsa-generate timers; dampening repeated changes reduces CPU load in large topologies.

Keep area 0 contiguous and place ABRs at area borders; every non-backbone area must touch area 0 directly or via a virtual link.

Authentication types: Null (0), Simple password (1), MD5 (2), and SHA/HMAC (SR OS supports MD5 and SHA-1/SHA-256 via authentication-key and authentication-type).

Authentication must match on both sides; a mismatch causes Hellos to be discarded and the adjacency never leaves Down/Init.

MD5 uses key-id and key; SR OS supports multiple keys with key-id for hitless key rollover.

Passive interfaces and interface-level authentication are the primary hardening controls; also filter OSPF (protocol 89) at network edges.

Common adjacency faults: area mismatch, hello/dead mismatch, auth mismatch, MTU mismatch, duplicate router ID, and one-way multicast reachability.

show router ospf neighbor displays state and dead timer; a neighbor stuck in ExStart points to MTU mismatch, stuck in Init points to one-way hello.

show router ospf database detail shows LSA age, sequence number, checksum, and advertising router; use it to detect stale or flapping LSAs.

Debug with debug router ospf and clear router ospf to force re-adjacency; capture with monitor or mirror to confirm Hellos are sent and received.