Single-area OSPF

Link-state distance vector routing protocol

distance vector:

copies of routing table neighbor

Bellman-Ford algorithms

not exact topology

link-state:

full knowledge

Dijkstras algorithm (SPF)

exact topoloy

1) hello info + LSA's to build a topological database

2) SPF algorithm tree of the network

3) shortest routes routing table

OSPF operation

Hello protocol

Hellos: every 10s

Multicast address 224.0.0.5

Elects DR and BDR

LSA's

Describe all of the router links

Link-state database

SPF algorithm

To calculate a loop free logical topology to every known network

Routing table

Shortest path with the lowest cost

Hello protocol

same hello and dead intervals

Router ID

highest local active IP address

Loopback interface

to ensure OSPF stability (if there is no active interface, the OSPF process will not start)

if configured, it is used as Router ID (highest loopback IP address)

DR

highest OSPF priority (broadcast default: priority = 1)

OSPF priorities = same DR = highest router ID

Priorities can be set: 0 to 255

BDR

second highest priority

OSPF cost metric

OSPF cost may be calculated using the formula 10⁸/bandwidth