Video 9 : OSPF – The concepts (part1)

In Uncategorized on April 20, 2011 by carlosfvc

Link-State Routing Protocols

– Two types



– Maintain three table

1) Neighbor table

2) Topology table (roadmap for entire area)

3) Routing table

– Use Dijkstra`s shortest path first (spf) algorithm

– Send triggered updates to announce netwrok changes

– Send Periodic updates (LS Refresh) on long intervals

Once every 30 minutes

OSPF Area design and terms

– All areas must connect to area 0

– All routers in an area have the same topology table

– Area border routers (ABR) has one interface in more than areas) and one must be in the area 0. Goal is to localize updates within an area.

– Autonomous System roundary routers (ASBR)

– Only ABR and ASBR

– Requires a hierarchical design

Understanding OSPF Neighbor Relationship

1. Determine your own router ID

– The router ID is simply the router`s name in the OSPF process

– Highest active interface IP address when OSPF starts (loopbacks beat physical interfaces)

2. Add interfaces to the link state database (dictated by the network command)

3. Send a Hello message on chosen interfacas(s)


– Once every 10 sec on broadcast /P2P network

– Once every 30 seconds on NBMA networks

– Contains all sorts of information:

Router id


Hello and dead timers*

Router Priority

Netwrok mask*

DR/BDR ip address

Area ID*

Authentication password*

* Have to be equal between neighbors

4. Receive Hello


– check hello / dead interval

– check netmasks

– check area ID

– check authentication passwords

5. Send reply Hello

***2-WAY STATE***

– Am i listed as a neighbor in your hello packet?

(if yes, reset dead timer) END, no exchaging is done

(If no, add as new neighbor)

6. Master – slave relationship determined

***Exstart State***

– Determines by “priority”, router-id breaks tie

– Master sends database description (DBD) packets

DBD = Cliff noted of link-state database

– Slave sends its DBD packet

7. DBDs are acknowledge and reviewed


– Slave requests details (link state requests – LSR)

– Master sends updates (Link state updates – LSU)

– Master requests details (LSR)

– Slave sends updates (LSU)

8. Neighbors are synchronized


Now it`s time to run the Dijkstra SPF algorith to figure out what to do with all this data


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