Uploaded by Bar Levi

OSPF overview

Why should I use OSPF?
The main reason is Summarization and Fault Containment (link updates stay within the
ABR- Area Border Router - The router that connects a few areas.
An ABR can summarize and fault contain.
ASBR- Autonomous System Boundary Router - The boundary that represents the OSPF
network as a whole. The router that connects another network to one of your own.
This could be another network that runs EIGRP/RIP, but most of the time it will be the
An ABR and an ASBR are the ONLY routers that can summarize.
Area 0 is the backbone area and all areas have to have a link to area 0.
You shouldn't design a network with an area that doesn't connect directly to area 0, but if
that's the case, you can use Virtual-Link, which is like a tunnel, you use it to tunnel
information THROUGH areas that are not 0, but it will think that it is connected to area 0.
You should only use a Virtual Link as a temporary solution, until you find a way to connect it
directly to area 0.
You do not want to use multi-area OSPF unless you need to summarize your networks.
OSPF Neighbor Relationship:
)1The router picks its router-ID. You can use either priority, hard-code the IP address, it can
pick the highest ACTIVE interface for an ID (Loopbacks beats physical)
but most of the time it will pick the highest Loopback address that is configured.
If you add a higher Loopback after the routers had established a neighbor relationship, it will
not update the Router-ID. It will, only after you reload the router.
You configure Virtual Link by the Router-ID.
)2Use "network" command to add an interface to the data-link database (sh ip ospf
)3Send Hellos on the chosen interfaces (an hello every 10 second for Broadcast/P2P
network, an hello every 30 seconds for an MBMA network[Frame Relay.)]
The Hello packet contains: Router-ID, Hello/Dead timers, Network Mask, Area ID, Neighbors,
Router Priority, DR/BDR IP address, Authentication Password.
In order to form a relationship the routers have to have identical Hello/Dead timers, Mask,
Area ID and Password.
)4Receive Hello and check if compatible to be a neighbor (by the conditions made above)
)5Send reply Hello, if the replying router isn't a neighbor, it will add it to it's neighbors. If it
is, it will reset the Dead Timer.
)6Master-Slave relationship, the master sends his DBD (database description, the cliff-notes
of the link-state DB) first (master chosen by priority and router-id.)
)7Slave requests the details that he's missing (LSR-Link State Request). Master sends update
(LSU-Update). Master requests details that slave has (LSR). Slave sends
update (LSU). LSA-Acknowledge.
)8Neighbors are in sync!
DR and BDR - Designated Router and Backup DR.
Their purpose is to limit the amount of updates sent in a network to a minimum. How do
they do that?
Two routers are elected to be DR and BDR (the highest Router-IDs). Every router in the AREA
are in full relationship status with them, so every time a network goes
down, the router that the network is connected to sends the update only to the DR and BDR,
and they are the ones that inform all the other routers of that update.
Instead of everyone being in a full relationship and informing everyone once a network goes
down, and by that causing a massive sending of unnecessary updates.