Five types of OSPF Packets

These may be simple but are likely to come up again and again in studies, practice tests, and perhaps the real thing.

Name the five OSPF packet types:
Hello - Discovers neighbors/builds adjacencies
Database Description - Checks to see if databases are synchronized between neighbor routers.
Link-State request (LSR) - request a specific link state record(s) from another router
LSU - Sends the specific link state record(s) requested by the LSR
LSAck - Acknowledgment of all the above packet types

The reason OSPF needs an Ack packet is because it does not use TCP or UDP, the above packets are encapsulated directly into IP.


What does a Router do when it receives an LSU (link state update)

1. If LSA entree does not already exist: add entry to LSDB, send LSAck, Flood update to other routers, run SPF, updates routing table

2. If LSA already exists with same sequence number: ignores

3. If LSA already exists with but new has later sequence number: add entry to LSDB, send LSAck, Flood update to other routers, run SPF, updates routing table

4. If LSA already exists and new LSA has older Sequence Number: sends LSU to sender with newer update


What is a Drother?
A non-DR and non-BDR on a segment - still gets a two-way adjacency state.

How does a router verify Bi-directional communication?
Sees itself listed in Hello Packet of Neighbor

OSPF's multicast address - (KNOW THIS!!!!!, Not just for the test, but for every desktop support technician who wants to send multicast images over the first few multicast addresses!!!)
224.0.0.5 - used for hello packets


Hello and dead intervals must match to create an adjacency.
By default: hello - 10 sec / Dead - 4x hello

Once a DR and BDR have been selected, any future routers joining that segment will only form adjacencies with the DR and BDR, not any DROTHERS.