🗺️ Routing Protocols & WAN
1
Static Routing & Administrative Distance
Static Routes
Manually configured routes. Best for: small networks, stub networks, providing a default gateway (default route: 0.0.0.0/0).
Cisco IOS: ip route [destination] [mask] [next-hop | exit-interface]
Administrative Distance (AD)
When multiple routing sources provide a route to the same destination, the router selects the route with the lowest AD (most trustworthy source).
| Source | AD |
|---|---|
| Connected interface | 0 |
| Static route | 1 |
| OSPF | 110 |
| IS-IS | 115 |
| RIP | 120 |
| EIGRP (external) | 170 |
2
OSPF & Dynamic Routing
OSPF (Open Shortest Path First)
Link-state routing protocol using Dijkstra's Shortest Path First (SPF) algorithm. Classless (supports VLSM and CIDR), fast convergence, no hop count limit.
- Hello packets — discover and maintain neighbour relationships.
- LSA (Link State Advertisement) — carries topology information flooded to all routers in the same OSPF area.
- LSDB (Link State Database) — each router maintains an identical database of all LSAs in the area.
- DR/BDR — on multi-access networks, a Designated Router (DR) and Backup DR (BDR) are elected to reduce OSPF traffic.
- Router ID — unique 32-bit number identifying each OSPF router; typically the highest loopback or interface IP.
OSPF Neighbour States
Down → Init → 2-Way → Exstart → Exchange → Loading → Full
Neighbours must reach the Full state to exchange complete routing information.