OSPF LSA Types Explained | Top Q&A

OSPF uses LSDB (link state database) and populates this with LSA (link state advertising). Instead of using 1 LSA package, OSPF has many different types of LSAs and in this lesson I will introduce all of them to you. Let’s start with an overview:

  • LSA Type 1: Router LSA
  • LSA Type 2: Network LSA
  • LSA Type 3: Summary LSA
  • LSA Type 4: Summary ASBR LSA
  • LSA Type 5: External LSA Autonomous System
  • LSA Type 6: Multicast OSPF LSA
  • LSA Type 7: Regional LSA is not too vague
  • LSA Type 8: External Attribute LSA for BGP

Read: what is lsa in ospf For many people it helps to visualize things to understand and remember. I like to think of OSPF LSAs as pieces of a jigsaw puzzle. A jigsaw is nothing but all of them together give us the big picture… for OSPF, this is the LSDB. Here is the first Type of LSA:OSPF LSA Class 1Each router in the area will be flooded with a type 1 router LSA in the area. In this LSA you will find a list with all the directly connected links of this router. How do we define a link?

  • IP prefix on an interface.
  • Link type. There are 4 different types of links:

Don’t worry too much about link types for now, we’ll see them later. Remember that the LSA . router always in the areaThe second type of LSA (network LSA) is created for multiple access networks:OSPF LSA Type 2The LSA . network or type 2 created for each multiple access network. Remember the OSPF network types? Broadcast and non-broadcast network types require DR/BDR. If that’s the case, you’ll see these network LSAs created by DR. In this LSA we will find all the routers connected to the multiple access network, the DR and of course the prefix and subnet mask. In my example above, we will find R1, R2 and DR in the LSA network. We will also see the 192.168.123.0/24 prefix in this LSA. Last thing to mention: LSA network always in the area.Read more: Midwest Industrial Supply | Top Q&A see the third type of LSA:OSPF LSA Type 3LSA of class 1 . router always in the area. However, OSPF works with multiple regions and you may want full connectivity in all areas. R1 is flooding an LSA router in the area so R2 will store this in its LSDB. R3 and R4 also need to know about the networks in Area 2.R2 will create a Summary LSA type 3 and flood it into area 0. This LSA will flood all other areas of our OSPF network. This way all routers in other regions will know about prefixes from other areas.The name “summary” LSA is very misleading. By default, OSPF is will not summarize anything for you. However, there is a command that allows you to summarize routes between regions. Take a look at my OSPF summary for less if you’re interested. If you are looking at your OSPF router’s routing table and see some O IA the item you are viewing LSA type 3 summarizes the LSA. Those are your inter-regional prefixes! Time for the fourth type of LSA:OSPF LSA Type 4In this example, we have R1 redistributing information from the RIP router into OSPF. This makes R1 a ASBR (Automatic System Border Router). What happens is that R1 flips a bit in the router’s LSA to identify itself as an ASBR. When R2 as ABR receives this router’s LSA, it generates a summary type 4 ASBR LSA and flood it into zone 0. This LSA will also flood all other zones and is required so that all OSPF routers know where to find the ASBR. What about a Category 5 LSA? Let’s check it out:OSPF LSA Class 5Same topology but I added prefix (5.5.5.0/24) at our RIP router. This prefix will be redistributed into OSPF. R1 (our ASBR) will handle this and generate a type 5 external LSA for this. Don’t forget we still need the ASBR LSA summary type 4 to locate R1. If you’ve ever tried redistributing with OSPF, you’ve probably seen O E1 or E2 item. Those are the external prefixes and our Type 5 LSA. What about OSPF LSA class 6? Type 6 ospf LSA omnidirectional I can ignore it because it’s not used. It’s not even supported by Cisco. We use PIM (Protocol Independent Multicast) for multicast configurations. Take a look at the last type of LSA, #7: Read more: What spam tastes likeOSPF LSA Class 7The last kind of LSA… promised! NSSA regions do not allow class 5 external LSAs. In my figure, R1 is still our ASBR redistributing information from RIP into OSPF. Since type 5 is not allowed we have to think about something else. That’s why we have a type 7 external LSA carries the exact same information but is not blocked in the NSSA area. R2 will translate this category 7 into category 5 and feed it into other areas. Let me summarize the types of LSAs for you:

  • Type 1 – LSA router: Router LSAs are generated by each router for each area in which it is located. Under Link State ID you will find the ID of the original router.
  • Type 2 – LSA Network: Network LSAs created by DR. The link state ID will be the interface IP address of the DR.
  • Type 3 – LSA Summary: Summary LSAs were generated by ABR and spilled into other regions.
  • Category 4 – ASBR LSA Summary: Other routers need to know where to find the ASBR. This is why the ABR will generate a summary ASBR LSA that will include the ASBR’s router ID in the link state ID field.
  • Category 5 – External LSA: also known as autonomous system external LSAs: External LSAs are generated by ASBR.
  • Category 6 – Multicast LSA: Unsupported and unused.
  • Type 7 – External LSA: also known as LSA (NSSA): As you can see, zone 2 is an NSSA (not a stout area) that doesn’t allow outside LSAs (category 5). To work around this, we’ll create a Class 7 LSA instead.
See Also  What Animal Has Two Feet But Cant Walk

The only thing left to do is to see these LSAs in action… time to configure some routers!

Verification

We can see all OSPF types in LSDB, to demonstrate this I will use the following topology:OSPF Two Areas 3 . RouterIt’s a simple setup with 3 routers and 2 zones. I added a few loops so we have the prefixes to look at. Here is the config: R1 (config) #router ospf 1 R1 (config-router) #network 192.168.12.0 0334390000 area 0 R1 (config-router) #network 1.1.1.0 0334390000 area 0 R2 (configuration) #router ospf 1 R2 (router configuration) #network 192.168.12.0 0334390000 area 0 R2 (router configuration) #network 192.168.23.0 0334390000 area 1 R3 (configuration) #router ospf 1 R3 (configured-router) #network 192.168.23.0 0334390000 area 1 R3 (configured-router) #network 3.3.3.0 0334390000 area 1Let’s start by looking at the LSDB of R1: R1 #show database ip ospf OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link Status (Area 0) Link ID Router ADV Age Seq # Link Check Number 1.1.1.1 1.1.1.1 30 0x80000003 0x004CD9 2 2.2.2.2 2.2.2.2 31 0x80000002 0x0048E9 1 Net Link Status (Area 0) Link ID ADV Router Age Seq # Check 192.168.12.2 2.2.2.2 31 0x80000001 0x008F1F Summary of Statuses net link (Area 0) Link ID ADV Router Age Seq # Checksum 3.3.3.3 2.2.2.2 17 0x80000001 0x00D650 192.168.23.0 2.2.2.2 66 0x80000001 0x00A70CB Using ip ospf display database, we can see LSDB and we can see class 1 router LSA, class 2 network LSA and type 3 summary LSA here. What else do we find here?

  • Associate ID: This is what identifies each LSA.
  • Router ADV: the router is advertising this LSA.
  • Age: Maximum age counter in seconds. Maximum is 3600 seconds or 1 hour.
  • Seq #: Here you will see the sequence number starting from 0x80000001 and will increment by 1 for each update.
  • Checksum: There is a checksum for each LSA.
  • Link Number: This will show the total number of directly connected links and is only used for LSA router.
See Also  What Does 5 Pounds Of Fat Look Like

So those are LSAs of classes 1,2 and 3. To show you numbers 4 and 5, I had to make some changes:OSPF Two Area 3 Routers ABR ASBRTo accomplish this, I would redistribute something on R1 into OSPF. R1 (configuration) #loop repeat interface 1 R1 (config-if) #IP address 11.11.11.11 255.255.255.0 R1 (config-if) #exit R1 (configuration) #router ospf 1 R1 (config-router) #redistribution of connected subnetsI created an additional repeater interface and configured the IP address. Then I’m asking OSPF to redistribute interfaces that are connected directly into OSPF. Take a look at R2 and R3’s LSDB: Further reading: What does it mean when a guy fasts

Last, Wallx.net sent you details about the topic “OSPF LSA Types Explained | Top Q&A❤️️”.Hope with useful information that the article “OSPF LSA Types Explained | Top Q&A” It will help readers to be more interested in “OSPF LSA Types Explained | Top Q&A [ ❤️️❤️️ ]”.

Posts “OSPF LSA Types Explained | Top Q&A” posted by on 2021-09-06 12:09:09. Thank you for reading the article at wallx.net

Rate this post
Back to top button