OSPF LSA Types Explained

Hi,
On a multi-access network with a DR. What happens to type 1 LSAs are they still flooded or sent only to the DR?

Stuart

Stuart,
In a multi-access network with an elected DR, Type 1 LSAs are sent (assuming IPv4) to 224.0.0.6. Only routers that have been elected as DR or BDR will join this multicast group and receive the traffic. So, technically, you could say that every router actually gets the Type 1 bits, but it is dropped immediately (and obviously not processed) by DROTHERs who aren’t subscribed to the 224.0.0.6.

The BDR and DR will listen to, collect, and process all the Type 1 LSAs sent to them. The DR will then send a Type 2 Network LSA to 224.0.0.5 which is the multicast group that ALL OSPF routers are subscribed to. This Type 2 LSA is essentially a summary of everything the DR has learned from all the Type 1 LSAs it received.

1 Like

Hello sims!

If I understand your question correctly, you are asking why you can’t see the Type 4 LSAs in the ospf database of R3. The Type 4 LSAs are those found under the Summary ASB Link States heading. You can see that these LSAs came from ADV Router 2.2.2.2 which is the ABR. Type 4 LSAs are exactly that, LSAs that come from the ABR that inform the router of subnet information in other areas.

I hope this has been helpful!

Laz

Hi,
You are correct .Why I can’t see type 4 LSA if I type sh ip ospf database on router R3 .
?
Sorry to say that I did not understand the below part
“Type 4 LSAs are exactly that, LSAs that come from the ABR that inform the router of subnet information in other areas.”

Thanks

Hello sims.

On Rene’s example, he shows the output of the command show ip ospf database for R3. Here it is below as well:

R3#show ip ospf database | begin Summary
		Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         2.2.2.2         149         0x80000001 0x0033FB
192.168.12.0    2.2.2.2         195         0x80000001 0x00219D

		Summary ASB Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         2.2.2.2         62          0x80000001 0x004DB9

		Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
11.11.11.0      1.1.1.1         68          0x80000001 0x000F44 0

In the above output, the Summary ASB Link States (Area 1) section displays the LSA type 4. A type for LSA has been received from 2.2.2.2 which is R2, which is indeed an ABR. With my last statement in the previous message, I wanted to confirm to you that type 4 LSAs come from ABRs, and R2 with an ID of 2.2.2.2 is indeed an ABR.

If you have created a topology of your own and you cannot see any type 4 LSAs on R3, check to make sure the neighbour relationships are being created correctly between routers and make sure that R2 is indeed an ABR, that is, it should have at least one interface in each OSPF area you have created.

I hope this has been helpful!

Laz

I had configured 3 routers mirroring configuration as noted in this section. I am noting that R3 does loses network 11.11.11.11 (R1 loop 1 interface) once R2 & R3 are configured area 1 part of nssa. Network 11.11.11.11 is showing up in R2 but not R3. Output below is R3… any ideas?

                    Router Link States (Area 1)

    Link ID         ADV Router      Age         Seq#       Checksum Link count
    2.2.2.2         2.2.2.2         81          0x80000009 0x00E229 1
    3.3.3.3         3.3.3.3         23          0x8000000B 0x00D90B 2

                    Net Link States (Area 1)

    Link ID         ADV Router      Age         Seq#       Checksum
    192.168.23.3    3.3.3.3         23          0x80000005 0x00DFAC

                    Summary Net Link States (Area 1)

    Link ID         ADV Router      Age         Seq#       Checksum
    1.1.1.1         2.2.2.2         81          0x80000004 0x00D253
    2.2.2.2         2.2.2.2         81          0x80000004 0x009A88
    192.168.12.0    2.2.2.2         81          0x80000004 0x00C0F4

                    Type-7 AS External Link States (Area 1)

    Link ID         ADV Router      Age         Seq#       Checksum Tag
    33.33.33.0      3.3.3.3         308         0x80000001 0x00E634 0

19 posts were merged into an existing topic: OSPF LSA Types Explained

Hi Laz,

For the below output, T3 and T4 LSA looks exactly the same (I have marked with arrow below) Therefore if we don’t have T4 LSA, R3 should be able to find R1 with the help of T3 LSA?

R3#show ip ospf database | begin Summary
		Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         2.2.2.2         149         0x80000001 0x0033FB <<<<<<<<<
192.168.12.0    2.2.2.2         195         0x80000001 0x00219D

		Summary ASB Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         2.2.2.2         62          0x80000001 0x004DB9  <<<<<<<

		Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
11.11.11.0      1.1.1.1         68          0x80000001 0x000F44 0

Best Regards,
Neeraj

Hello Neeraj

You are correct that T3 and T4 show the same information as far as the Link ID of R1 goes, however, that information is interpreted differently depending on the LSA type.

In both cases you have the Link ID of R1. However, T3 LSA information will tell you how to reach networks in OSPF area 0 in which the R1 Link ID is included. It is included simply as a router that exists in area 0.

T4 LSAs will essentially tell other routers where to find the ASBR. This LSA specifically identifies R1 as the ASBR.

Routers that get these two types of LSAs will use each one in a specific manner.

I hope this has been helpful!

Laz

Hi Rene,

1.Can you please explain the difference between implicit and explicit LSA Acknowledgement types…?
2.Why OSPF can’t do the MTU Fragmentation when IP can do the fragmentation.Why there is the difference even though both belong to the Network layer…?

Regards
Ravi

The short answer:

Implicit: this happens when a router sends a duplicate of the LSA that it receives. When the sender sees its own LSA, it knows that the other side has received it.

Explicit: this happens when you send an ACK packet in response to an LSA that you have received.

RFC2328 explains this in more detail:

_Each newly received LSA must be acknowledged. This is usually done by sending Link State Acknowledgment packets. However, acknowledgments can also be accomplished implicitly by sending Link State Update packets (see step 7a of Section 13)._

_If the LSA is listed in the Link state retransmission list for the receiving adjacency, the router itself is expecting an acknowledgment for this LSA. The router should treat the received LSA as an acknowledgment by removing the LSA from the Link state retransmission list. This is termed an “implied acknowledgment”. Its occurrence should be noted for later use by the acknowledgment process (Section 13.5).

OSPF can’t do fragmentation but IP does. By default, OSPF will check for MTU mismatches but it’s possible to disable this with the ip ospf mtu-ignore command. In this case, OSPF will work even with MTU mismatches and you can use IP fragmentation.

Thank you Rene for your support.I am expecting Basic Lessons in this portal for wireless,Nexus,Juniper boxes to work on Enterprise Network.

Ravi

I am guessing this is not CCNP Route? Where is this talked about CCIE Route?

P.S I try to read all the forums as an extra learning tool I understood the lessons completely but this forum thing I don’t remember seeing in my reading.

jugn78Jaime G

Jun '16

Hi August. As far as I know, when you redistribute into ospf you can specify the external type (E1 or E2 - E2 is the default).

E2 will have a default cost of 20 and, when advertised to other routers inside ospf , the cost will keep in 20, so type E2 dose not consider the additional cost between other routers inside OSPF.

E1 will add the cost when advertised inside ospf. So, if you have 3 hops between the ASBR and other router with links running at 100 Mbps, you will have to add 3 to the cost, so you will end up with a cost of 23 at the far end.

Hello Brian

Redistribution of E1 and E2 types is actually found within the realm of CCNP Route. Take a look at this Cisco link to see more.

I hope this has been helpful!

Laz

1 Like

2 posts were merged into an existing topic: How to configure Redistribution between OSPF and RIP

Actually I believe Type 9’s can also be used in IPv4, although it’s confusing because they use the terminology “link-local” it just means “applies to a segment” rather than the IPv6 link local addresses.

Hi Rene,

In type 2 LSA , why it is showing 192.168.12.2 in link id and why not 192.168.12.1
the explanation shows The link-state ID will be the router ID of the DR. then why it is showing 192.168.12.2

Hello Ananth

Here is the output in question for reference:
image

In the OSPF database output, there are two columns that we must examine: The Link ID and the ADV Router.

The ADV router is the router ID of the router that SENT or advertised the LSA. The router ID can be one of three things in this order: 1) the manually configured router id 2) the highest loopback address configured on the router 3) the highest IP address on an ACTIVE interface of the router. In our case, Rene has configured the router to have an router ID of 2.2.2.2. This is the R2 router which is the DR of the segment.

Now the Link ID column actually refers to the actual IP address of the DR on that segment. R2 is the DR and this IP address is indeed that of the interface on the segment in question.

I hope this has been helpful!

Laz

Hi Rene,

I would like to know if I’ll be able to see in the show ip ospf database the type 4 LSA? I can’t seem to see it? All I can see is the router link, network link, summary link for inter-area and type 5 for redistributed routes into OSPF…

Cheers,

Hi @bluephoenix71,

The type 4 LSA is advertised by the ASBR. Here’s a quick example, look at this picture:
ospf-r1-r2-r3-loopback-33-33-33-33
R1 and R2 are in area 0, R2 and R3 in area 1. The loopback interface is redistributed in OSPF, making R3 an ASBR:

R3(config)#interface Loopback 1
R3(config-if)#ip address 33.33.33.33 255.255.255.255

R3(config)#route-map R3_L1 permit 10
R3(config-route-map)#match interface loopback 1

R3(config)#router ospf 1
R3(config-router)#redistribute connected route-map R3_L1 subnets

R1 sees the route:

R1#show ip route | include 33.33.33.33
O E2     33.33.33.33 [110/20] via 192.168.12.2, 00:00:32, GigabitEthernet0/1

Here’s how to see it in the LSDB:

R1#show ip ospf database asbr-summary 

            OSPF Router with ID (11.11.11.11) (Process ID 1)

                Summary ASB Link States (Area 0)

  LS age: 67
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 3.3.3.3 (AS Boundary Router address)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0xBE68
  Length: 28
  Network Mask: /0
        MTID: 0         Metric: 1 

You can see above that R2 advertises this LSA.