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OSPF point to point Router LSA

Answered Question
Jul 21st, 2012
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Hi,


I established a point-point adjacency between routers. When I checked the ospf database i found something new,



R1(config-if)#do sho ip os int br

Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C

Fa1/0        10    5               5.1.1.2/24         1     P2P   1/1

Fa1/1        10    5               3.1.1.2/24         1     BDR   1/1




R1(config-if)#do sho ip os da rou 11.11.11.11



            OSPF Router with ID (11.11.11.11) (Process ID 10)



                Router Link States (Area 5)



  LS age: 1807

  Options: (No TOS-capability, DC)

  LS Type: Router Links

  Link State ID: 11.11.11.11

  Advertising Router: 11.11.11.11

  LS Seq Number: 80000006

  Checksum: 0x53EC

  Length: 60

  Number of Links: 3



    Link connected to: another Router (point-to-point)

     (Link ID) Neighboring Router ID: 22.22.22.22

     (Link Data) Router Interface address: 5.1.1.2

      Number of TOS metrics: 0

       TOS 0 Metrics: 1



    Link connected to: a Stub Network

     (Link ID) Network/subnet number: 5.1.1.0

     (Link Data) Network Mask: 255.255.255.0

      Number of TOS metrics: 0

       TOS 0 Metrics: 1



    Link connected to: a Transit Network

     (Link ID) Designated Router address: 3.1.1.1

     (Link Data) Router Interface address: 3.1.1.2

      Number of TOS metrics: 0

       TOS 0 Metrics: 1



What is this new kind of router LSA which is not there for a router in transit network. What is it exactly used for. I did read about this in RFC. They have mentioned for a point to point interface we have to add a stub lsa. But can you explain me why is that and how it helps in calcualting the best routes.


Thanks

Chidambaram

Correct Answer by Peter Paluch about 5 years 4 weeks ago

Hello Chidambaram,


What you see here is not a new kind of LSA. This is an ordinary LSA-1 that describes, in particular, your router and its connections to surrounding objects. From this LSA-1, you can tell that:


  • your router is connected to another router whose RID is 22.22.22.22 via a point-to-point link (the first link entry)
  • the IP network on this point-to-point link is 5.1.1.0/24 (the second link entry)
  • there is a multiaccess network on another interface, and this multiaccess network is represented by an LSA-2 originated by the Designated Router in that network, in this case, 3.1.1.1. Note that the IP address of this network is not recorded here. You must look up the LSA-2 with the Link State ID of 3.1.1.1 to see what is the IP network address of this multiaccess network. (the third link entry)


What is probably confusing here that you have two links to different routers, however, each of them is described differently in the LSA-1. This is by design. For each point-to-point link, there will always be two entries in LSA-1. One of these entries will describe the point-to-point interconnection with another router, while the second of these entries will describe the IP network on this interconnection (labeled as stub network although it is in fact a network connected to another router, not a true stub network). For each multiaccess network, there will be a single entry in LSA-1, pointing to a LSA-2 that represents the multiaccess network including its IP address and attached routers.


This way of representing topological and addressing details in LSA-1 is arguably confusing, yet it is the way OSPF works.


Please feel welcome to ask further!


Best regards,

Peter

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Correct Answer
Peter Paluch Sun, 07/22/2012 - 00:28
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  • Cisco Employee,

Hello Chidambaram,


What you see here is not a new kind of LSA. This is an ordinary LSA-1 that describes, in particular, your router and its connections to surrounding objects. From this LSA-1, you can tell that:


  • your router is connected to another router whose RID is 22.22.22.22 via a point-to-point link (the first link entry)
  • the IP network on this point-to-point link is 5.1.1.0/24 (the second link entry)
  • there is a multiaccess network on another interface, and this multiaccess network is represented by an LSA-2 originated by the Designated Router in that network, in this case, 3.1.1.1. Note that the IP address of this network is not recorded here. You must look up the LSA-2 with the Link State ID of 3.1.1.1 to see what is the IP network address of this multiaccess network. (the third link entry)


What is probably confusing here that you have two links to different routers, however, each of them is described differently in the LSA-1. This is by design. For each point-to-point link, there will always be two entries in LSA-1. One of these entries will describe the point-to-point interconnection with another router, while the second of these entries will describe the IP network on this interconnection (labeled as stub network although it is in fact a network connected to another router, not a true stub network). For each multiaccess network, there will be a single entry in LSA-1, pointing to a LSA-2 that represents the multiaccess network including its IP address and attached routers.


This way of representing topological and addressing details in LSA-1 is arguably confusing, yet it is the way OSPF works.


Please feel welcome to ask further!


Best regards,

Peter

chid_1989 Sun, 07/22/2012 - 01:02
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Thanks much Peter.


Ya, I agree that this is the design. I would like to take this oppurtunity to get some more of my doubts clarified.


Still i am not able to get what is the motive behind this. Assuming the case which i stated, from my 1st entry i can decipher what is the RID of the neighboring router, what is my interface connecting to that remote router and what is the mask of the network ( from the i/f IP ). If you see the second entry, it just states what is the network address and mask which seems to be redundant for me. Yeah, as you said this is the design, in what way this is used by the neihboring router.


Also can you explain me how a router lsa is connected to a type 2 lsa for a multi access network. You mentioned, the IP address of the network is not recorded in the router lsa. My doubt is, since it is a broadcast network, out interface's mask should be the network mask right. Apart from attached routers, what else does a network lsa indicate to a router lsa.


Sorry if my doubts are very naive. I am learning OSPF protocol.


Thanks

Chidambaram

Peter Paluch Sun, 07/22/2012 - 01:37
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  • Cisco Employee,

Hi Chidambaram,


You are welcome.


from my 1st entry i can decipher what is the RID of the neighboring  router, what is my interface connecting to that remote router and what  is the mask of the network ( from the i/f IP ).


From the first entry, you can indeed learn to which neighboring router you are connected and what is the IP address of your interface connecting you to the neighbor. However, using just this single entry, you can not know the mask of the network between you and the neighbor because it is not indicated in the first entry. Remember that this LSA-1 has to be processed by all other routers in the same OSPF area and without further information, they have no means of learning this netmask.


If you see the second entry, it just states what is the network address and mask which seems to be redundant for me.


Well, it is not redundant at all Without this second entry, other routers would not be able to know what is the netmask of the network that interconnects you with the neighbor whose RID is indicated in the first entry.


Also can you explain me how a router lsa is connected to a type 2 lsa for a multi access network.


As this is a topic that requires larger explanation and has been discussed here before, I suggest you first read the following threads. They may partially answer your doubts.


https://supportforums.cisco.com/message/3444089#3444089

https://supportforums.cisco.com/message/3450640#3450640 (here, pseudonodes in IS-IS stand for LSA-2 in OSPF)

https://supportforums.cisco.com/message/831143#831143

https://supportforums.cisco.com/message/834501#834501

https://supportforums.cisco.com/message/3572253#3572253

https://supportforums.cisco.com/message/828595#828595


Sorry if my doubts are very naive. I am learning OSPF protocol.


Not at all. You are asking very good questions and you are very much welcome to ask more and more!


Best regards,

Peter

chid_1989 Sun, 07/22/2012 - 02:03
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That was astute I will definitely go through the links you suggested will get back to you with my doubts.

chid_1989 Sun, 07/22/2012 - 08:16
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Hi Peter,


I do not know what to say. In short, i was worried, i dint have an expertised person to guide me. You had put an end to all that. I now have more insight in OSPF. With that ( hopefully ) i will ask a doubt wrto your reply in the post


https://supportforums.cisco.com/message/3444089


 Instead of each router on a common segment declaring links to each remaining router, the common network itself is represented as a

 separate standalone object in the link-state database. Each router would need to indicate only one link to the network, irrespective of the
 total number of the routers in that network.


So assume there are 10 routers in a network and one DR and BDR has been elected. Now each routers T1 and T2 LSA will be pointing to the DR. If a packet is destined to one of the attached network as specified by the network LSA, how will the router decide to send the packet directly without sending the packet to the DR .


Also another doubt


 With the LSA2 representing a particular multiaccess segment, the memory footprint grows linearly with the number of routers on the
 segment, a significant saving especially with networks with larger counts of routers on common segments.


So, if 10 routers are connected to the same network ( some topology ), i think that there will be only 1 network LSA with the attached router field alone differing in each router. Please feel free to correct me.


Thanks

Chidambaram

Peter Paluch Sun, 07/22/2012 - 14:18
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Hello Chidambaram,


Thank you for your kind words!


So assume there are 10 routers in a network and one DR and BDR has been elected.


Alright. Please note here that the DR/BDR are elected for each multiaccess segment in the network. Sometimes it is incorrectly assumed that the DR/BDR are elected for the entire area or a larger portion of the network. That assumption would be incorrect. Therefore, what you really assume here is that you have 10 routers connected to the same network segment (say, a single switch), and of course, DR/BDR are elected from among these 10 routers.


For simplicity, let's assume further that this is the entire network, i.e. a single switch with 10 routers attached to it directly. Each of these routers may have an arbitrary number of LANs but there are no more routers nor any other interconnections between these routers.


Now each routers T1 and T2 LSA will be pointing to the DR.


More precisely, there will be 10 LSA-1 and 1 LSA-2 in this network. Each router will generate one LSA-1 representing itself, and for each multiaccess segment with multiple routers, the attached DR will generate one LSA-2.


Each router's LSA-1 will point towards the single LSA-2 generated by the DR. The LSA-2 generated by the DR will point to the 10 LSA-1 (one LSA-2 can hold a list of multiple routers attached to the same segment).


If a packet is destined to one of the attached network as specified by  the network LSA, how will the router decide to send the packet directly  without sending the packet to the DR . 


I feel that there is a contradiction in this question: "one of the attached networks as specified by the network LSA". Note that in your network, there is only a single transit multiaccess network - the one interconnecting all 10 routers. Therefore, there will be only one LSA-2, and this one LSA-2 will carry only the IP address of this particular network. If the routers have any LAN interfaces with no other OSPF routers in those LANs, these LAN interfaces will be described in LSA-1 as stub networks including their addresses.


Nevertheless, even if the packet's destination would be located in a network described by a LSA-2: the fact that the LSA-2 was generated by the DR does not mean in any way that the packets should flow to the DR. It is the information content of the LSA-2 that is important. In the case of LSA-2, the information content describes the network IP address of the transit multiaccess network and it also lists which routers are directly connected to this network. If a router indicates in its LSA-1 that it is connected to this transit multiaccess network by referencing its LSA-2, it merely says that it is capable of delivering packets into and through this network. Note that this explanation does not imply in any way that the actual routed packets must in any way be sent to the DR.


So, if 10 routers are connected to the same network ( some topology ), i think that there will be only 1 network LSA with the attached router field alone differing in each router. Please feel free to correct me.


If the entire topology consists only of a single multiaccess network with 10 routers attached to it, there will indeed be only a single LSA-2 present. However, this LSA-2 would be known identically to all routers and each router will see the same contents in this LSA-2. Remember: once a LSA is generated by a router, no other router may modify it in any way. Something like that is absolutely prohibited. That means that once a DR generates a LSA-2, all routers will merely flood it and store it unchanged in their LSDB, but it is not possible to have a LSA-2 "with the attached router field alone differing in each router".


Please feel welcome to ask further! These are very important issues that need to be crystal clear if you want to understand OSPF in depth.


Best regards,

Peter

chid_1989 Tue, 07/24/2012 - 09:44
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Hi Peter,


I had another doubt in OSPF. Posted a new thread. Kindly look into it.


Thanks

Chidambaram

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