LSA packet types?

Well, i have been trying to go deep in OSPF. However, something looks like blurry about LSA packet types. As far as i know,
if there is a multi-access network either broadcast or non-broadcast, DR and BDR is selected in order to reduce number
of LSAs exchanged. Therefore, LSA exchange is performed between DROther and DR or BDR.

However, LSA type 1 definition doesn't say so. According to its definition, these types of LSA packets are exchanged between
all routers in that area.

So, what am i missing?

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pham0329

Maybe you read the text out of order? Every book I've read had the LSA section before the DR/BDR section. The network type that OSPF runs on top of decide how that process is handled. In a broadcast network, the router will only go to the full state with the DR/BDR, and will not exchange LSAs with the DROther.

If you have each router exchanges router lsa between itself and the DR/BDR, but also exchange them with other routers, there would be no point in having a DR/BDR.

thedrama

pham0329 wrote: »

Maybe you read the text out of order? Every book I've read had the LSA section before the DR/BDR section. The network type that OSPF runs on top of decide how that process is handled. In a broadcast network, the router will only go to the full state with the DR/BDR, and will not exchange LSAs with the DROther.

If you have each router exchanges router lsa between itself and the DR/BDR, but also exchange them with other routers, there would be no point in having a DR/BDR.

The first paragraph you explained is the thing what i said in my first paragraph.

As far as i have read, LSA type 1(router LSA) are exchanged between DROther routers. On the contrary to this, DR and/or BDR is elected on multi-access networks to reduce LSAs exchanged. Based on that, how could this be possible to reduce the traffic if DROther routers exchange router LSA between each other? In this way what is the meaning of DR and/or BDR election?

I found something here : http://www.enterprisenetworkingplanet.com/netsp/article.php/3612166/Networking-101-Understanding-OSPF-Routing-Part-2.htm

Notice that we have both a router and a network LSA. The reason a router LSA exists is because in the absence of a DR, there is no network LSA sent. The router LSA would include a list of all links to the other routers on a network. So OSPF can work in the absence of a DR or BDR, albeit with increased complexity due to the fact that the DR is no longer providing nice summaries

So, LSA type 1(router LSA) is sent "only" when there is no DR/BDR?

pham0329

thedrama wrote: »

The first paragraph you explained is the thing what i said in my first paragraph.

Because that's how it works. Router LSA are always exchanged, regardless of whether or not there's a DR/BDR. The DR/BDR only change how they're exchanged. If there's a DR present, DROther only exchange LSAs with the DR/BDR

Eildor

thedrama wrote: »

The first paragraph you explained is the thing what i said in my first paragraph.

As far as i have read, LSA type 1(router LSA) are exchanged between DROther routers. On the contrary to this, DR and/or BDR is elected on multi-access networks to reduce LSAs exchanged. Based on that, how could this be possible to reduce the traffic if DROther routers exchange router LSA between each other? In this way what is the meaning of DR and/or BDR election?

I found something here : Networking 101: Understanding OSPF Routing (Part 2)

Notice that we have both a router and a network LSA. The reason a router LSA exists is because in the absence of a DR, there is no network LSA sent. The router LSA would include a list of all links to the other routers on a network. So OSPF can work in the absence of a DR or BDR, albeit with increased complexity due to the fact that the DR is no longer providing nice summaries

So, LSA type 1(router LSA) is sent "only" when there is no DR/BDR?

My understanding is that in a broadcast network the DR will forward LSA's to the DROthers. A DROther will send an LSU to the DR using multicast address 224.0.0.6. The DR will then forward that LSU to the DROthers using multicast address 224.0.0.5.

thedrama

pham0329 wrote: »

Because that's how it works. Router LSA are always exchanged, regardless of whether or not there's a DR/BDR. The DR/BDR only change how they're exchanged. If there's a DR present, DROther only exchange LSAs with the DR/BDR

As i can figure out from what you have said, if there is a DR/BDR, router LSA packets are exchanged DROther and/or BDR along with network LSAs
if there is not DR/BDR, we can't mention any network LSAs and that time, router LSAs are exchanged between DROthers ?

instant000

OSPF LSA Types, part 1 - YouTube

Try this one out, about 25 minutes.

Eildor

instant000 wrote: »

OSPF LSA Types, part 1 - YouTube

Try this one out, about 25 minutes.

Excellent video, helped me get my head around the different LSA types and their functions.

Eildor

thedrama wrote: »

As i can figure out from what you have said, if there is a DR/BDR, router LSA packets are exchanged DROther and/or BDR along with network LSAs
if there is not DR/BDR, we can't mention any network LSAs and that time, router LSAs are exchanged between DROthers ?

I'd advise you not to take what I say as fact unless someone verifies it, but here is what I understand:

In a broadcast network with 5 OSPF routers there would be 5 * (5-1) / 2 neighbours adjacencies (formula is n * (n-1) / 2). If all routers were to synchronise their databases there would be a large number of LSU's (Link State Updates) and LSAck's (Link State Acknowledgments) traversing between the routers. This problem is solved via the use of Designated Routers. Non-designated routers (DROthers) will form FULL adjacencies with Designated Routers, whilst maintaining a 2-way adjacency with other DROthers. If a DROther has a LSA to send it will send it to the DR using multicast address 224.0.0.6, the DR on recieving that LSA will send it to the other routers (DROthers) in the network using multicast address 224.0.0.5. So, multicast address 224.0.0.6 is used by DROthers to send LSA's to the DR, and the DR uses multicast address 224.0.0.5 to send LSA's to the DROthers. There is no need for the DROthers to exchange LSA's with one another, rather it's done through the DR.

Hopefully someone will correct my mistakes.