OSPF
Link-state routing: neighbours through Hello messages, flooding of announcements and a shared database, shortest-path computation, cost derived from bandwidth, areas and area 0, the designated router and its election.
- 29 cards
- 7 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
- Questions
- Sort and link
- The numbers
- On the diagram
- Scenarios
- Understand
A few cards from the deck
OSPF: what is it?
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An open-standard interior link-state routing protocol that runs directly over IP (protocol 89) and computes its routes from a complete map of its area.
Each router receives the whole map and computes alone: fast convergence after a failure, no counting to infinity. The price is memory and computation, hence the split into areas.
A DR is elected on a segment. Later, a router with priority 255 joins that segment. What happens?
- The DR in place stays DR: the election is not preemptive
- The newcomer becomes DR at once, as its priority is the highest
- A new election takes place among all routers on the segment
- It becomes BDR and replaces the old backup
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The DR in place stays DR: the election is not preemptive
Changing DR would force every router to re-form its adjacencies. The newcomer will only be elected if the DR goes down. Hence the importance of bringing up the intended router first or restarting the process.
Neighbours discover each other through … messages sent by default every … seconds on Ethernet; a neighbour silent for … seconds is declared dead.
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Neighbours discover each other through Hello messages sent by default every 10 seconds on Ethernet; a neighbour silent for 40 seconds is declared dead.
The dead interval is four times the Hello period. Two neighbours must have the same values, or the adjacency does not form.