What is IGP?
Interior Gateway Protocol
An IGP (Interior Gateway Protocol) is a routing protocol, such as OSPF or IS-IS, that finds the best paths between routers inside one organisation's network.
An IGP's job is to know the internal topology: which routers exist, which links connect them, and how to get from any router to any other as quickly as possible. Link-state IGPs such as OSPF and IS-IS have every router flood a description of its links, build the same map, and run a shortest-path calculation over it using link costs. Older distance-vector protocols such as RIP, and Cisco's EIGRP, are also IGPs. Because every router trusts the others in the same administration, IGPs are optimised for fast convergence after a failure rather than for policy: there is no notion of "I'd rather not accept routes from this neighbour" in the way BGP has. The opposite term is EGP, a protocol used between autonomous systems, which today means BGP.
In a typical service-provider or enterprise network the two work together. The IGP carries only infrastructure routes, mainly loopback addresses and the links between routers, which keeps it small and fast. BGP carries everything else, including the full internet table, and relies on the IGP to reach each route's NEXT_HOP and to run iBGP sessions between loopbacks. A frequent beginner mistake is redistributing the full BGP table into the IGP, which overwhelms it; another is forgetting to carry the edge links or loopbacks in the IGP, which leaves BGP routes with unreachable next hops. The OSPF Fundamentals lesson builds a working IGP, and Edge and core shows OSPF underpinning iBGP.
Lessons that use IGP
- OSPF FundamentalsTeaches this term · Beginner · 70 min
The link-state IGP that runs inside most networks: neighbour adjacencies, LSA flooding, SPF, costs, DR/BDR, areas and LSA types, a hands-on FRR lab, and how OSPF underpins BGP.
- BGP FundamentalsIntermediate · 75 min
How the Border Gateway Protocol glues the internet together: autonomous systems, sessions, path attributes, best-path selection, iBGP scaling and a hands-on eBGP lab with real FRR configuration.
- BGP on loopbacks vs interface addressesIntermediate · 45 min
Why BGP sessions between loopbacks survive link failures and use parallel links, and what they need to work: an IGP or static routes, update-source, the eBGP connected check, and BFD. Every failure is reproduced on two real VyOS routers.
- Edge and core: building out a VyOS networkIntermediate · 60 min
Split one BGP router into edge routers that face the outside world and a core that carries traffic inside: loopbacks, OSPF for infrastructure, iBGP between loopbacks, next-hop-self, and what happens when a link fails. Built and broken on three real VyOS routers.
- Route reflectors on VyOSIntermediate · 30 min
Replace an iBGP full mesh with route reflectors: see iBGP refuse to pass routes on, turn on reflection, read ORIGINATOR_ID and CLUSTER_LIST, confirm the reflector stays out of the forwarding path, and survive taking one reflector down.
- OSPF on VyOS: costs and OSPFv3Intermediate · 30 min
OSPF the VyOS way: interface-based configuration, reading the interface and database output, steering traffic with costs (and the asymmetric routing that follows), and OSPFv3 for IPv6 over link-local addresses. Captured on the three-router edge/core lab.
- IPv4 over an IPv6-only core (RFC 8950)Intermediate · 40 min
Carry IPv4 routes over IPv6 BGP sessions with IPv6 next hops, so core routers need no IPv4 addresses at all. Built on VyOS: the missing router ID, what breaks without the extended next hop capability, native IPv4 on the wire, and why traceroute shows 192.0.0.8.
- EVPN and VXLAN: layer 2 over a routed fabricAdvanced · 45 min
Stretch a layer-2 segment across a routed leaf-spine fabric: VXLAN for the data plane, BGP EVPN for the control plane. Built with FRR: the underlay, VNIs, type-2 and type-3 routes, route targets, and the VXLAN packets on the wire.