Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements
Webcast-Catalyst9k
New Member

OSPF's LSA, LSU & BMA topology

Hi

I'm not very clear about this LSA and LSU. When router is sending an update it sends a LSU packet or a LSA packet? If what it sends is called a LSA, then LSU what you call that update LSA packet, is it?

In a normal scenario, when a router is pluged to the network, it will send a Hello packet to 224.0.0.5 to be adjacent with it's neighbors. But in a BMA topology, where there is a DR and a BDR this new router should send Hello packet to the multicast address 224.0.0.6 not 224.0.0.5 It should adjacent with the DR and BDR. So my question is, does it detect the BMA topology automatically and sends the first Hello packet to 224.0.0.6 If not can you please explain how it happens?

Thanks in advance!

1 ACCEPTED SOLUTION

Accepted Solutions
Purple

Re: OSPF's LSA, LSU & BMA topology

Hi again,

Yes. There are 5 different packet types in OSPF and they all perform different functions. The thing you should understand is that they all have a common header (with a different packet type) but all have different contents, depending on the function performed.

Here are the 5 packet types and their functions:

1 -- Hello -- Discover/maintain neighbors

2 -- Database Description -- Summarize database contents

3 -- Link State Request -- Database download

4 -- Link State Update -- Database update

5 -- Link State Ack -- Flooding acknowledgment

Now, the LSU can have multiple LSAs... As you know, there are 5 types of LSAs... LSA types 1-5 (as per the base protocol, but there are other LSAs too for things such as NSSA areas and opaque types):

1 - Router LSA

2 - Network LSA

3 - Summary LSA

4 - ASBR Summary LSA

5 - AS-external LSA

The LSU can have one or more of the above LSA types. Depending on the type of LSA, there could be one more more links or networks described by that LSA. A single type-1 LSA, for example, could advertise multple links. It's not true to say that there is one link per LSA.

Paresh.

PS> Pls do remember to rate posts.

6 REPLIES
New Member

Re: OSPF's LSA, LSU & BMA topology

LSA is abberivated as Link state advertisement as well as Link state acknowledgement. linl state advertisement is same as Link state update (lsu).but link state acknowledgement is ack packet.So it depedns on the information required and information provided not in the sort forms like LSA /LSU.

BMA is detected automatically.

Purple

Re: OSPF's LSA, LSU & BMA topology

Hi,

The actual packet that OSPF uses to transmit link-state information is called a Link State Update. Within the packet, there can be one or more Link State Advertisements. Therefore, a LSU is actually a collection of LSAs.

Typically, different implementations will have default behaviour where actually multi-access segments will automatically operate as broadcast or NBMA. However, Cisco IOS allows you to operate a broadcast segment as point-to-point if you only have two routers on it.

Paresh

PS> Pls do remember to rate posts.

Re: OSPF's LSA, LSU & BMA topology

Hi,

hope this URL will answer to your queries.

http://www.cisco.com/warp/public/104/1.html

rate if it helps ....

New Member

Re: OSPF's LSA, LSU & BMA topology

Hi Paresh

Thanks for the post. The first paragraph is not so clear to me, because even in a book I read it the same way. So you mean the actual packet is called LSU? Then if it's a LSR, you mean the router sends a totally different packet, is it? I mean then it has to be a LSR packet, is it? So you mean to say that there are five packet types in OSPF, such as Hello, Database Description, LSU, LSR and LSAck? I was under the impression it is the same packet, but depends type field in the packet it varies slightly. Was my understanding wrong? Please explain.

Also what do you mean by, in one LSU there can be many LSAs? You mean one LSA is one advertisement about one link, is it? Then there are three LSAs in a LSU means, there are three advertisements about three links, is it?

Many thanks in advance!

Purple

Re: OSPF's LSA, LSU & BMA topology

Hi again,

Yes. There are 5 different packet types in OSPF and they all perform different functions. The thing you should understand is that they all have a common header (with a different packet type) but all have different contents, depending on the function performed.

Here are the 5 packet types and their functions:

1 -- Hello -- Discover/maintain neighbors

2 -- Database Description -- Summarize database contents

3 -- Link State Request -- Database download

4 -- Link State Update -- Database update

5 -- Link State Ack -- Flooding acknowledgment

Now, the LSU can have multiple LSAs... As you know, there are 5 types of LSAs... LSA types 1-5 (as per the base protocol, but there are other LSAs too for things such as NSSA areas and opaque types):

1 - Router LSA

2 - Network LSA

3 - Summary LSA

4 - ASBR Summary LSA

5 - AS-external LSA

The LSU can have one or more of the above LSA types. Depending on the type of LSA, there could be one more more links or networks described by that LSA. A single type-1 LSA, for example, could advertise multple links. It's not true to say that there is one link per LSA.

Paresh.

PS> Pls do remember to rate posts.

New Member

Re: OSPF's LSA, LSU & BMA topology

Hi Paresh

Many many thanks to you for explaining it very clearly. You have a good teaching ability. It is very clear to me.

I also thank everyone else also who spent their time on clarifying my doubt.

Thank you soooo much everyone!

1481
Views
24
Helpful
6
Replies
CreatePlease to create content