CCNA quiz 4: routing and OSPF

Routers always prefer the longest prefix match. /24 is more specific than /16, so it wins regardless of the source.

OSPF is 110. EIGRP internal is 90, RIP is 120, static routes are 1 and connected networks are 0.

A default route matches everything: network 0.0.0.0 with mask 0.0.0.0.

OSPF is link-state: every router builds the same map and runs Dijkstra's shortest path first algorithm. EIGRP uses DUAL; RIP uses Bellman-Ford.

OSPF uses cost, based by default on interface bandwidth (reference bandwidth divided by interface bandwidth).

Neighbours must agree on hello and dead timers, area ID, subnet, authentication and (for full adjacency) MTU.

Highest priority wins (default 1, 0 means never), and the highest router ID breaks a tie. The election is not pre-emptive.

Area 0 is the backbone. Inter-area traffic passes through it, and every other area must attach to it (normally via an ABR).

A passive interface sends no hellos, so no neighbours form there, but its subnet is still advertised. Use it on user-facing LAN ports.

O is an intra-area OSPF route, O IA is inter-area (learned from another area via an ABR), and O E1/E2 are external routes.

Connected routes have AD 0, static routes 1, EIGRP internal 90 and OSPF 110. Lower is more trusted.

show ip ospf neighbor lists neighbours, their router IDs, states (such as FULL) and the interface used.

FULL means the link-state databases are synchronised. DROTHER routers stay at 2-WAY with each other, which is normal.

OSPF uses the manually set router-id, then the highest loopback IP, then the highest IP on an active interface.

Giving a static route a higher administrative distance than the main route keeps it out of the table until the main route fails.

Hello packets are sent regularly to find neighbours and confirm they are still alive.

Hello is 10 seconds and dead interval 40 seconds on broadcast and point-to-point networks by default.

The default reference is 100 Mbps, so all fast links cost 1. auto-cost reference-bandwidth (in Mbps) fixes this; set it on every router.

Without a matching route the packet is dropped, and the router may send an ICMP destination unreachable message.

HSRP uses an active and a standby router sharing a virtual IP. VRRP is the open standard; GLBP also load-balances.