§ 4 Static, RIP, OSPF, EIGRP, IS-IS, Redistribution, and Fast Convergence

1. Overview

An IGP owns reachability inside one administrative domain. Static routes and PBR handle precise edge policy, RIP teaches distance-vector failure modes, OSPF and IS-IS flood topology state, and EIGRP precomputes safe backups with DUAL.

2. § 4.1 — Static and Floating Static Routes

Administrative distance is the first tie-breaker between route sources. A connected route beats a static route, static beats OSPF, OSPF beats RIP, and floating static works because its AD is deliberately worse until the primary path disappears.

A tracked floating static should depend on real reachability, not just interface carrier. IP SLA probes a target beyond the next hop; when the probe fails, the track object changes state and the backup route can enter the RIB.

3. § 4.2 — RIPv2 and RIPng

RIP is routing by rumor: every router periodically advertises its table to neighbors. The 15-hop ceiling, 30-second update rhythm, and count-to-infinity problem make it mostly historical, but its loop-prevention tools are still foundational.

• RIPv2 is classless, uses multicast 224.0.0.9, and can authenticate updates.
• RIPng carries IPv6 routes over UDP/521 and uses link-local next hops.
• Metric 16 means unreachable; split horizon and poison reverse reduce bad rumor propagation.

4. § 4.3 — OSPFv2

OSPF divides the topology into areas so every router does not have to hold every detail. Area 0 is the backbone; stub and NSSA variants reduce external or inter-area LSA pressure where full visibility is not needed.

LSAs have scope and authorship. Routers create Type 1, a DR creates Type 2 on broadcast networks, ABRs create Type 3 and Type 4 summaries, ASBRs create Type 5 externals, and NSSA ASBRs create Type 7 that an ABR converts.

OSPF adjacency formation is a packet-by-packet state machine. On Ethernet, DR/BDR election limits full adjacencies: DROthers stay 2-Way with each other and go Full only with the DR and BDR.

Minimal C Demo — OSPF Neighbor State Machine

Minimal C Demo — LSA Flood and SPF Reroute

5. § 4.4 — OSPFv3

OSPFv3 separates topology from addressing. Router LSAs describe links and neighbors; Link LSAs and Intra-Area Prefix LSAs carry IPv6 or address-family prefixes. Adjacencies use IPv6 link-local addresses.

6. § 4.5 — OSPF Optimization

Fast convergence is a chain: detect failure quickly, avoid excessive LSA churn, schedule SPF sanely, then update the FIB. BFD generally beats aggressive Hello timers because it is purpose-built for failure detection.

• timers throttle lsa all 0 1000 5000 dampens repeated LSA generation.
• timers throttle spf 5 500 10000 avoids CPU spikes during flaps.
• Prefix suppression removes unnecessary transit-link prefixes from the LSDB while loopbacks remain reachable.

7. § 4.6 — EIGRP and DUAL

EIGRP's Feasibility Condition is the loop-free shortcut: if a neighbor's reported distance is lower than our current feasible distance, that neighbor cannot be routing back through us and can be used as a feasible successor.

If no feasible successor exists, DUAL makes the route Active and sends Queries. Large query domains create Stuck In Active risk; summarization and EIGRP stub boundaries keep the blast radius small.

Minimal C Demo — EIGRP DUAL Walk-through

8. § 4.7 — IS-IS

IS-IS runs directly over Layer 2 and uses NET addresses rather than IP subnets for adjacency identity. L1 routers know their local area; L2 routers form the backbone. This makes flat L2-only DC or SP cores easier than forcing every non-backbone area through OSPF area 0.

• IIH packets discover neighbors; LSPs carry topology; CSNP/PSNP packets synchronize missing database entries.
• metric-style wide is required for modern traffic engineering and Segment Routing.
• Multi-topology IS-IS can keep IPv4 and IPv6 reachability in separate SPF topologies.

9. § 4.8 — Route Redistribution

Redistribution imports routes between protocols, but mutual redistribution can feed a route back into its origin with a different metric or AD. Tags make route origin explicit and allow the opposite edge to block re-entry.

10. § 4.9 — Policy-Based Routing

PBR overrides pure destination routing for selected packets. Match source, protocol, port, or DSCP, then set a next hop only if the tracked path is alive; otherwise fall back to the normal FIB.

11. § 4.10 — BFD, LFA, rLFA, and TI-LFA

BFD is a protocol-independent failure detector. It moves through Down, Init, and Up, then notifies OSPF, IS-IS, EIGRP, BGP, static tracking, or first-hop HA when control packets are missed.

LFA precomputes local loop-free backups when the topology has a safe neighbor. TI-LFA uses Segment Routing to encode a repair path through a P/Q node, covering topologies where plain LFA has no valid local backup.

12. Interview Prep

• Why does a floating static route work? Its AD is worse than the primary route, so it enters the RIB only when the primary source disappears.
• What must match for OSPF neighbors? Area, subnet on v2 broadcast links, authentication, timers, options, and compatible network type.
• What is the EIGRP Feasibility Condition? A neighbor's reported distance must be lower than our feasible distance to be a loop-free backup.
• Why tag redistributed routes? Tags prevent routes from being redistributed back into the protocol they came from.
• Why use BFD instead of only OSPF fast hellos? BFD is lightweight, protocol-independent, and tuned for fast failure detection under load.