Nokia NRS II Composite (OSPF) flashcards

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

OSPF protocol number

OSPFv2 runs directly over IP using protocol number 89, not TCP or UDP. Firewall rules must permit IP protocol 89 for adjacencies to form.

OSPF administrative distance

On Nokia SR OS, OSPF internal routes default to administrative distance 10 and OSPF external routes to 150. Lower values are preferred in the routing table.

Router ID selection order

Explicit router-id wins; otherwise the highest loopback IPv4 address; otherwise the highest interface IPv4 address. Changing it requires an OSPF restart to take effect.

Hello and dead timers

Defaults are 10 s hello and 40 s dead on broadcast and point-to-point, and 30 s hello and 120 s dead on NBMA and point-to-multipoint. Timers must match on both ends.

OSPF packet types

Type 1 Hello, type 2 Database Description, type 3 Link State Request, type 4 Link State Update, type 5 Link State Acknowledgment. Each serves a distinct phase of adjacency formation.

Neighbor state order

Down, Attempt (NBMA only), Init, 2-Way, ExStart, Exchange, Loading, Full. Full means the LSDBs are synchronized and the adjacency can carry traffic.

DR/BDR election rule

Election uses interface priority (default 1, 0 means ineligible), then highest router ID. It is not preemptive: a new higher-ID router does not displace the current DR.

Type 1 router LSA

Originated by every router, flooded only within its area. It lists the router's links, interface types, costs, and neighbor router IDs used to build the SPF tree.

Type 2 network LSA

Generated by the DR of a broadcast or NBMA segment and lists all routers attached that reached at least 2-Way. It represents the pseudonode in the SPF tree.

LSA MaxAge

An LSA reaching 3600 seconds is reflooded at MaxAge and removed from the LSDB unless the originator refreshes it. The originator refreshes every 1800 seconds by default.

Type 3 summary LSA

Generated by area border routers to advertise prefixes from one area into another. It is flooded across areas but never into a totally stubby area.

Type 4 ASBR summary

Generated by ABRs to advertise reachability to an ASBR located in another area. Without it, routers in other areas cannot resolve type 5 external routes.

Type 7 NSSA LSA

Carries external routes originated inside an NSSA. The translating ABR converts it to a type 5 LSA when exporting to the backbone, preserving the forwarding address.

E1 versus E2 metrics

E1 cost equals external cost plus internal cost to the ASBR. E2 cost equals external cost only and is the SR OS default; a forwarding metric breaks ties.

Partial SPF trigger

A change to a type 3, 4, 5, or 7 LSA triggers only a partial SPF. A change to a type 1 or type 2 LSA triggers a full SPF in that area.

SPF root

Each router places its own type 1 LSA at cost 0 as the tree root. Stub networks are added as leaves using the advertising router's interface cost.

Unresolvable external route

A type 5 LSA with no route to its ASBR cannot be installed. The external appears in the LSDB but stays out of the routing table until the ASBR is reachable.

OSPF interface cost formula

Cost equals reference bandwidth divided by interface bandwidth, rounded down, minimum 1. SR OS default reference is 100000 Mbps, so 10G and faster links cost 1.

OSPF ECMP limit

SR OS supports equal-cost multipath up to the configured ecmp value, default 16 and maximum 32 next hops per prefix. OSPF does not support unequal-cost multipath.

E2 ECMP behavior

Because E2 cost ignores internal cost, ECMP across multiple ASBRs compares only the external metric. Use E1 when internal distance to the ASBR should influence path choice.

Virtual link requirement

Both endpoints must be ABRs with an interface in the transit area, and the remote router ID must be reachable through it. The transit area cannot be stub or NSSA.

Virtual link behavior

The virtual link acts as an unnumbered point-to-point link in area 0 and carries type 1 LSAs for the backbone. It is a workaround for a partitioned backbone.

Stub area behavior

Blocks type 4 and type 5 LSAs and cannot carry virtual links. The ABR injects a type 3 default route so internal routers still reach external destinations.

NSSA P-bit

The propagate bit in a type 7 LSA tells the translating ABR to convert it to type 5. The ABR clears it when the forwarding address is unreachable.

SR OS redistribution method

SR OS uses policy-based export rather than a redistribute command. An export policy must explicitly match connected, static, or other protocol routes for them to be advertised.

Enabling OSPF on SR OS

Configure router ospf with a router-id, then add interfaces under an area. OSPFv3 uses a separate ospf3 instance and IPv6 link-local addressing.

OSPFv3 authentication

OSPFv3 has no native authentication field. It relies on IPv6 IPsec AH or ESP, or on upper-layer checksum protection, unlike OSPFv2 MD5.

Passive OSPF interface

A passive interface advertises its connected network into OSPF but forms no adjacencies. It is used on access ports to prevent rogue OSPF speakers.

LFA inequality

A neighbor N is a loop-free alternate for destination D if distance(N,D) is less than distance(N,S) plus distance(S,D). SR OS enables it with loopfree-alternate.

TI-LFA requirement

TI-LFA computes a repair path using segment routing label stacks from the post-convergence SPF. Segment routing must be enabled on the OSPF interfaces.

Neighbor stuck in ExStart

Usually an MTU mismatch, duplicate router ID, or filtered DD packets. The DD packet carries the interface MTU, so mismatched MTUs stall the exchange.

LSDB comparison command

show router ospf database detail lists LSA age, sequence number, checksum, and advertising router. Identical checksums across routers confirm a synchronized area.