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Why type 2 LSA is needed?

gornication
Level 1
Level 1

Hello to all!

How do additional LSA from DR help reduce the amount of LSDB?

There was not enough space in the LSA type1 for 4 bytes of the mask?
Why is there no need to describe the topology from the point of view of the network in the case of a point-to-point link?

1 Accepted Solution

Accepted Solutions

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello @gornication ,

the purpose of the network LSA is to list the OSPF routers that are connected to the common segment of the DR and it actually lists the OSPF RIDs and not the LAN IP addresses of the nodes connected to the DR pseudonode.

The network LSA allows to each router in the same area to know what OSPF routers are connected to a broadcast network even if they are far from it.

The combination of Router LSAs generated by each router that will signal the DR IP address in their link section and of the network LSA with LSA-Id = DR IP address allows each router in the area to create a complete and updated topology of the area.

This is the big difference between link state protocols like OSPF and IS-IS and distance vector protocols like RIPv2 or hybrid protocol like EIGRP that lack this capability.

 

Hope to help

Giuseppe

 

View solution in original post

10 Replies 10

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello @gornication ,

the purpose of the network LSA is to list the OSPF routers that are connected to the common segment of the DR and it actually lists the OSPF RIDs and not the LAN IP addresses of the nodes connected to the DR pseudonode.

The network LSA allows to each router in the same area to know what OSPF routers are connected to a broadcast network even if they are far from it.

The combination of Router LSAs generated by each router that will signal the DR IP address in their link section and of the network LSA with LSA-Id = DR IP address allows each router in the area to create a complete and updated topology of the area.

This is the big difference between link state protocols like OSPF and IS-IS and distance vector protocols like RIPv2 or hybrid protocol like EIGRP that lack this capability.

 

Hope to help

Giuseppe

 

Thanks for the reply, Giuseppe.

LSA's type 1 does not list RID that are connected to the common segment?

Why is it not enough to know which networks are connected to the router, but you also need to know which routers are connected to the network?

I did not understand how the DR ip-address in RouterLSA differs from the DR ip-address in NetworkLSA. Why do you need a combination of them?

Hello @gornication ,

>> LSA's type 1 does not list RID that are connected to the common segment?

No, for a broadcast link where a DR is elected the DR IP address is reported in the router LSA of each router connected to the segment.

Then, the DR for the segment acts like a pseudonode and describes the multi access segment as a star with at the center the DR node and all other routers connected to it.

In this way a router that is in the same area even if it is far from the segment can know what routers are connected to the MA segment.

This provides an exact map of the area with exact topology and this is not possible in RIPv2 or EIGRP as noted in my previous post.

OSPF used objects called graphs that represent the topology within an area. For this reason the Network LSA is needed it allows to correctly describe multi access segments.

 

The Network LSA is originated by the OSPF RID of the DR but has LSA ID = DR IP address this allows to each router in the area to draw a star graph with at the center the DR node and all the router connected to the LAN around.

So the DR IP address is used as a pointer or search key to correlated different data structures in OSPF LS DB the network LSA and the Router LSA.

 

Hope to help

Giuseppe

 

Why not?
This is Router LSA(LSA type 1) and I see RID(10.129.24.251) and DR ip-address(10.129.24.250):

         OSPF Router with ID (10.129.24.251) (Process ID 820)

               Router Link States (Area 10.129.24.248)
 LS age: 681
 Options: (No TOS-capability, DC)
 LS Type: Router Links
 Link State ID: 10.129.24.251
 Advertising Router: 10.129.24.251
 LS Seq Number: 8000EED5
 Checksum: 0x2FBE
 Length: 36
 AS Boundary Router
 Number of Links: 1
   Link connected to: a Transit Network
    (Link ID) Designated Router address: 10.129.24.250
    (Link Data) Router Interface address: 10.129.24.251
     Number of MTID metrics: 0
      TOS 0 Metrics: 1

Yes, not all RID are in one LSA, as in the second type LSA, but all RID are in LSDB.

Hello @gornication ,

>> Yes, not all RID are in one LSA, as in the second type LSA, but all RID are in LSDB.

Let's try to see it in another way:

how to know what routers are members of a LAN segment in an OSPF area if Network LSA would not exist?

Let us suppose there are 200 routers in the area so there 200 Router LSA and that 30 routers are on the segment, each router in the area not connected to the segment should process all the other 199 Router LSAs to see if there is a link id = DR IP address.

 

Do you understand the difference now ?

with Network LSA the list of nodes connected to the LAN segment is already created and ready to be used.

The Network LSA saves on computation efforts on each node of the area.

 

In the past having 30 routers in the same VLAN in a POP was quite common, nowdays most of router links are point to point but when OSPF was designed the networks could have such scenarios.

 

Also the Network LSA combined with the Router LSAs provide a double check on what device is now connected to the LAN segment as both the DR node (originator of the Network LSA) and the other router (originator of the router LSA) agree on the fact that a node is an active member of the LAN segment.

 

Hope to help

Giuseppe

 

 

 

"how to know what routers are members of a LAN segment in an OSPF area if Network LSA would not exist?"
Each member of the LAN segment will tell about themselves and the networks connected to them using the RouterLSA.

"Do you understand the difference now ?"
Difference between types of LSA? Probably yes, but I don't quite understand why both types are needed.

  Each router in the area must receive all LSA1 of this area and receive them regardless of what segment it is and whether someone sends it to the LSA2. It's right?

  It is clear to me that it is more convenient to have one NetworkLSA if we want to know the network and routers connected to it, than some amount RouterLSA  from each router. But this does not exclude RouterLSA, which duplicate this information.

 

 

 

Hello @gornication ,

OSPF is a link state protocol and we can describe it as oriented to create a complete map of an area.

For each link OSPF tracks the link state up/down  , for p2p links it tracks what node is connected to the other end of the link.

 

For a LAN segment OSPF tracks what devices are connected to the segment this is why Network LSA is so useful.

 

The redundancy in information between Router LSA  and Network LSA is wanted to provide this double check.

 

If a router that is connected to a LAN segment has its interface to be shut or to fall down the following events happen:

the affected router wiill create and flood a new Router LSA with that link in down state

the DR for the segment will create and flood a new Network LSA removing that neighbor from the list.

 

Both LSAs are fllooded in the area and will trigger a new SPF computation on each router in the area.

To be noted in IS-IS the DIS that is the equivalent of the OSPF DR of a segment creates an LSP on behalf of the pseudonode and lists what IS-IS nodes are connected to it.

As both protocols use the same SPF algorithm I guess that the information about what nodes are in a multi access segment in a given moment is a useful info for SPF calculations.

 

Hope to help

Giuseppe

 

That is, Network LSA exists to improve the reliability of information about the network?

LSA1 are distributed within an entire area, which can contain more than one broadcast domain.

LSA1 are distributed within the entire area through the DRs, which is selected for each broadcast domain.

LSA2 gives each router an understanding of what their broadcast domain looks like, which helps to build the topology of the area where all these domains are located.

 

It seems now I understand why LSA is needed.

And still unconvincing. There is an idea that the algorithm for computing a tree is more economical with the presence of LSA2, since the understanding that the links of different RouterLSA belong to the same network/broadcast_domain requires additional resources.

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