Packet types in OSPF

[thangaRasu]

Hi,

     After reading about the OSPF packet types i am having some doubts,

1. What is the difference between DD,LSRequest,LSUpdate,LSAck packets and Router LSA

2. Instead of using the above four packets may i use only router LSA packets because it will send the update to all neighbors

3.Initially for making adjacency with the neighbors the four(DD,LSR,LSU,LSA) packets are used ,anyother purpose

Thanks in advance

[antonio.guirado]

Hello,

OSPF is a little complicated. First of all, you need the LSA concept. I do not know if you have managed some SQL

database. There is a table a each table has entries. An entry is defined by fields. Fields storages information and

are numerical, boolean and so on. Ok. The SQL table is the LSDB. An LSA is an entry in the table. An LSA have

fields: LS ID, LS Age, LSA Type, Advertisement Router, LS Sequence number and so on.

The main objetive in OSPF is that all routers in the same area have the same LSDB, they must be syncronized.

So, OSPF Packets Type are:

Hello Packets. Discovers neighbors and builds adjacencies between them.

Database Despcription Packets(BDB). It includes only a LSA header. It is not include all fields about the LSA. Only

some fields (as LS sequence number LS checksum) that let know if the LSA has change. A BDB is also multicast.

by a Link-State ACK packet.

Link-state request (LSR) Packet. If a router receive a BDB packet from a neighbor and have older information about the LSA, the it sends a LSR to receive all fields about the LSA.

LSUpdate packet. A router that receives a LSR send LSUpdate with all information about the LSA request. A LSUpdate must be ACK by a LS Ack packet.

LS ACK. It only a packet to ACK other packets (LS Updates).

So, now your questions:

1- You are mistaken Router LSA is not a packet type. It is a type of LSA.

2- Yoy need all types of packets.

3- Only Hello packets.

Regards.

[thangaRasu]

Thank you

The below lines i referred from RFC 2328

LSRefreshTime

The maximum time between distinct originations of any particular

LSA. If the LS age field of one of the router’s self-originated

LSAs reaches the value LSRefreshTime, a new instance of the LSA

is originated, even though the contents of the LSA (apart from

the LSA header) will be the same. The value of LSRefreshTime is

set to 30 minutes.

MinLSInterval

The minimum time between distinct originations of any particular

LSA. The value of MinLSInterval is set to 5 seconds.

After reading this only i thought Router LSA also one packet it will be transmitted periodically

Router LSA is a packet or just one entry in LSDB

[antonio.guirado · Accepted solution]

Hello,

there is no Router LSA packet. Router LSA is a LSA type (type-1) that travels inside a packet (LS Update packet).

Regards.

[thangaRasu]

Thank you so much

Finally what is the use of LSRefreshtime - 30 minutes, MinLSInterval - 5 seconds, MinLSArrival - 1 second and Maxage - 1 hour (RFC 2328)

[antonio.guirado · Accepted solution]

Hello,

each LSA have a LS age field. If this timer arrives to 60 min the LSA is delete. So each LSA have to be refresh by

its advertisement router before this timer expired. LSRefreshtime refers how often a router refresh the LSA's that

are originated by itself.

The other timers is to avoid traffic flooding. A router, after generating a LSA, must wait MinLSInterval to generate the

same LSA. And a route drop the received LSA if the same LSA was received in the last MinLSArrival seconds.

Regards.

[thangaRasu]

what is the interval for sending database description packet

My assumption is if LSRefresh is expired means start to send database description packets after that LSR, LSU then LSA

Thanks in advance

[Firuz Azimov]

Hi

OSPF has five Message types

Every OSPF Messages have some function. When neighborship is forming first time or exchanging routes

1. Hello - Used to discover neighbors, supply information used to confirm two routers should be allowed to become neighbors (for example hello packet consists: Hello interval, dead interval, authentication, area ....), for becoming neighbor two routers must have the same parametrs.

And to monitor a neighbor's responsiveness in case it fails.

2. DD      - Database Description Used to exchange brief version of each LSA, typical on initial topology exchage, so that a router knows a list of that neighbor's known LSAs

3. Link State Request(LSR) - A packest is used to request LSA from neighbor who has information about requested LSA

4. Link State Update (LSU) - A packet that contains fully detailed LSAs, typically sent in response to an LSR message

5. Link-State Acknowledgment (LSAck) - Sent to confirm receipt of an LSU, LSR, message. I think only hello packet is not reliable.

From the above briefly: When two routers enabled OSPF Protocol first the exchange Hello Packest, Hello packest consists some parametrs that must match to become a neighbor (Parametrs: area, authentication, Hello dead interval...). Next Routers exchange DD briefly two routers send brief information about networks thats it knows.

Received DD message router, compare information in DD with his own. If neighbor router has a new route. Router sends LSR. LSR message requests a new route from a neighbor. Then Neighbor advertise this route with LSU. Remember LSU may contain many LSAs.

I am not native english )

Hope this was helpfull