10 Replies Latest reply: Mar 16, 2019 11:21 PM by Deepak Kumar RSS

    Is there real need for Type 4 LSA ?

    Guru

      Hello Guys,

       

      Posing two basic question on LSAs with reference to Ethernet i.e broadcast network. Apologies if this is not a forum for such basic questions.

       

      1: ASBR in a area also sends Type-1 LSA with E bit set. So all routers in the area apparently know about ASBR.

          Then why would an ABR again needs to send Type 4 LSA ?

       

      2. I read at few places that in stub areas, routers include subnet mask in Type 1 LSA ? Is it true. Sorry that i dont have any setups with me

      to verify this behaviour. So kindly someone help is this true. Because, if type 1 LSAs contain subnet mask, there is no need for Type 2 LSA ?

       

      Appreciate detailed explanation please, so that i understand the real need of these type of LSA..

       

      Thanks,

      Guru

        • 1. Re: Is there real need for Type 4 LSA ?
          Ing_Percy

          Hi!

           

          In these discussions of CLN you can see about OSPF LSA type 4

          OSPF LSA TYPE 4

           

          Type-4 ASBR summary LSA

           

          Best regards!

          • 2. Re: Is there real need for Type 4 LSA ?
            jh
            • 3. Re: Is there real need for Type 4 LSA ?
              Guru

              Thank you for the quick update. Could you please throw some light on one more point pleas -->

               

              I read at few places that in stub areas, routers include subnet mask in Type 1 LSA ? Is it true. Sorry that i dont have any setups with me to verify this behaviour. So kindly someone help is this true. Because, if type 1 LSAs contain subnet mask, there is no need for Type 2 LSA ?

              • 4. Re: Is there real need for Type 4 LSA ?
                Ing_Percy

                Hi!

                 

                • Type 1 - Represents a router
                • Type 2 - Represents the pseudonode (designated router) for a multiaccess link

                 

                LSA types 1 and 2 are found in all areas, and are never flooded outside of an area. Whether the other types of LSAs are advertised within an area depends on the area type, and there are many:

                 

                -Backbone area (area 0)

                -Standard area

                -Stub area

                -Totally stubby area

                -Not-so-stubby area (NSSA)

                 

                More details: OSPF area types - PacketLife.net

                 

                Best regards!

                • 5. Re: Is there real need for Type 4 LSA ?
                  Sergey

                  Guru,

                   

                  Type 2 LSA is not a hard necessity. It is an efficiency mechanism. At the time, OSPF was developed, routers had limited CPU and memory resources and spending these resources just on maintaining neighbour relationships on a broadcast segment seemed a bit wasteful, so the powers that be came up with the mechanism of reducing the need for full mesh of neighbour relationships on such a segment. So, if you have 10 routers that are all connected to the segment, you don't have to maintain 9 full neighbour relationships on each router. Instead, routers on that segment elect a DR and a BDR and only establish full peering with them two. Then, DR sends out type 2 LSA for every router on the segment on behalf of every router on the segment. So, you are kind of correct, there is no NEED as such for type 2 LSA. It is just a way to make things more efficient and to conserve router's memory and CPU resources.

                  • 6. Re: Is there real need for Type 4 LSA ?
                    eddylynx

                    Hello Guru,

                     

                    Type 1 LSAs describes the router WITHIN the area, and lists the router's links (to other routers or networks) in the same area. So the Type 1 LSAs that a router acting as an ASBR generates will describe the router and its links within the same area. Key point to note here is that the Type 1 LSAs have area scope (are kept within that area and not flooded to other areas). But a router is an ASBR  because it has routes to other "routing domains". These routes are advertised via Type 5 LSAs which are flooded to all areas. For routers in areas other than the resident area of the ASBR to know the router that originated the external (and as such how to reach those external routes), the ABR within the area of the ASBR will advertise the presence of the ASBR as a Type 4 into other areas. Other ABRs will do same into their respective areas. So Type 4 is needed.

                     

                     

                    Type 1 and 2 LSAs are two very different LSAs so the presence of one, or the information it carries, does not render the other unneeded.  Type 2 LSA is needed on multia-ccess networks more of to make the operation of OSPF more efficient; its purpose is to describe routers connected directly to that multiaccess network / segment. This is only generated by the designated router, whereas every router generates a Type 1 LSA  to describe itself WITHIN the area, and lists its links (to other routers or networks) in the same area.


                    Regards.

                    • 7. Re: Is there real need for Type 4 LSA ?
                      Guru

                      Thank you much @Eddylynx..  this makes sense. Another follow up on point 1 just to make sure that my understanding is correct. Please comment..

                       

                      If i have a topo like    R1(ASBR) is Area 1<--------->R2(ABR)<----->Area 0<------> R3(ABR in Area 2) and R4 another internal router in Area 2.

                       

                      - So R1 sends a Type 1 LSA with E bit set.

                      - R2 which is a ABR floods Type 4 and Type 5 LSA in Area 1 as well as Area 0 ?

                      - R3 picks up Type 4 and Type 5 LSA from Area 0 and Floods in Area 2

                      - R4 gets to know how to reach R1

                       

                      If there is another ABR along with R3 in Area 2, whether that would also perform similar operation ? Basically there would be two ABRs sitting between Area 0 and Area 2. If either one of them does the ABR job, how would the other ABR knows to sit silent ?

                       

                      Any other important points to be noted in such scenarios ?

                       

                      Thanks ,

                      Guru

                      • 8. Re: Is there real need for Type 4 LSA ?
                        eddylynx

                        Hello Guru,

                         

                        Consider this design. Here, R4 (Router id 4.4.4.4) is the ASBR which is redistributing the two loopbacks indicated.

                         

                        ospf 1.JPG

                         

                        #R3 receives Type 1 from R4 and R4 tells R3 it is an ASBR.

                         

                        R3#sh ip ospf 1 2 database router adv-router 4.4.4.4

                                    OSPF Router with ID (3.3.3.3) (Process ID 1)

                                        Router Link States (Area 2)

                         

                          Routing Bit Set on this LSA in topology Base with MTID 0

                          LS age: 711

                          Options: (No TOS-capability, DC)

                          LS Type: Router Links

                          Link State ID: 4.4.4.4

                          Advertising Router: 4.4.4.4

                          LS Seq Number: 80000003

                          Checksum: 0x1AF6

                          Length: 48

                          AS Boundary Router

                        ----- some output ommitted-----

                         

                         

                        #R3 (ABR btn Area 2 and 0), originates Type 4 LSA and send into Area 0.

                         

                        R3#sh ip ospf  1 0 database asbr-summary self-originate

                                    OSPF Router with ID (3.3.3.3) (Process ID 1)

                                        Summary ASB Link States (Area 0)

                          LS age: 907

                          Options: (No TOS-capability, DC, Upward)

                          LS Type: Summary Links(AS Boundary Router)

                          Link State ID: 4.4.4.4 (AS Boundary Router address)

                          Advertising Router: 3.3.3.3

                          LS Seq Number: 80000001

                          Checksum: 0xCC49

                          Length: 28

                          Network Mask: /0

                                MTID: 0        Metric: 10

                         

                         

                        #R2 in Area 0 sees this Type 4 LSA (as coming from R3)

                         

                        R2#sh ip ospf database asbr-summary

                                    OSPF Router with ID (2.2.2.2) (Process ID 1)

                                        Summary ASB Link States (Area 0)

                         

                          Routing Bit Set on this LSA in topology Base with MTID 0

                          LS age: 1002

                          Options: (No TOS-capability, DC, Upward)

                          LS Type: Summary Links(AS Boundary Router)

                          Link State ID: 4.4.4.4 (AS Boundary Router address)

                          Advertising Router: 3.3.3.3

                          LS Seq Number: 80000001

                          Checksum: 0xCC49

                          Length: 28

                          Network Mask: /0

                                MTID: 0        Metric: 10

                         

                         

                        #R1 and R7 (ABRs btn area 1 and 0) will both originate the Type 4 LSA and send into Area 1

                         

                        R1#sh ip ospf 1 1 database asbr-summary self-originate

                         

                                    OSPF Router with ID (1.1.1.1) (Process ID 1)

                                        Summary ASB Link States (Area 1)

                          LS age: 1154

                          Options: (No TOS-capability, DC, Upward)

                          LS Type: Summary Links(AS Boundary Router)

                          Link State ID: 4.4.4.4 (AS Boundary Router address)

                          Advertising Router: 1.1.1.1

                          LS Seq Number: 80000001

                          Checksum: 0x6DA6

                          Length: 28

                          Network Mask: /0

                                MTID: 0        Metric: 20

                         

                        R7#sh ip ospf 1 1 database asbr-summary self-originate

                                    OSPF Router with ID (10.10.10.7) (Process ID 1)

                                        Summary ASB Link States (Area 1)

                         

                          LS age: 1173

                          Options: (No TOS-capability, DC, Upward)

                          LS Type: Summary Links(AS Boundary Router)

                          Link State ID: 4.4.4.4 (AS Boundary Router address)

                          Advertising Router: 10.10.10.7

                          LS Seq Number: 80000001

                          Checksum: 0x7082

                          Length: 28

                          Network Mask: /0

                                MTID: 0        Metric: 20

                         

                        #R6 in area 1 receives both LSAs and makes a decision based on the metrics.

                         

                        R6#sh ip ospf 1 1 database asbr-summary

                         

                         

                                    OSPF Router with ID (6.6.6.6) (Process ID 1)

                                        Summary ASB Link States (Area 1)

                         

                          Routing Bit Set on this LSA in topology Base with MTID 0

                          LS age: 1301

                          Options: (No TOS-capability, DC, Upward)

                          LS Type: Summary Links(AS Boundary Router)

                          Link State ID: 4.4.4.4 (AS Boundary Router address)

                          Advertising Router: 1.1.1.1

                          LS Seq Number: 80000001

                          Checksum: 0x6DA6

                          Length: 28

                          Network Mask: /0

                                MTID: 0        Metric: 20

                         

                         

                          Routing Bit Set on this LSA in topology Base with MTID 0

                          LS age: 1233

                          Options: (No TOS-capability, DC, Upward)

                          LS Type: Summary Links(AS Boundary Router)

                          Link State ID: 4.4.4.4 (AS Boundary Router address)

                          Advertising Router: 10.10.10.7

                          LS Seq Number: 80000001

                          Checksum: 0x7082

                          Length: 28

                          Network Mask: /0

                                MTID: 0       Metric: 20


                        R6#sh ip ospf border-routers

                                    OSPF Router with ID (6.6.6.6) (Process ID 1)

                         

                                        Base Topology (MTID 0)

                        Internal Router Routing Table

                        Codes: i - Intra-area route, I - Inter-area route

                         

                         

                        I 4.4.4.4 [30] via 67.67.67.7, Ethernet0/1, ASBR, Area 1, SPF 4

                        I 4.4.4.4 [30] via 16.16.16.1, Ethernet0/0, ASBR, Area 1, SPF 4

                        i 1.1.1.1 [10] via 16.16.16.1, Ethernet0/0, ABR, Area 1, SPF 4

                        i 10.10.10.7 [10] via 67.67.67.7, Ethernet0/1, ABR, Area 1, SPF 4

                         

                        #Same metrics so it load balances.

                         

                        R6#sh ip route 4.4.4.3

                        Routing entry for 4.4.4.3/32

                          Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 30

                          Last update from 67.67.67.7 on Ethernet0/1, 00:21:31 ago

                          Routing Descriptor Blocks:

                            67.67.67.7, from 4.4.4.4, 00:21:31 ago, via Ethernet0/1

                              Route metric is 20, traffic share count is 1

                          * 16.16.16.1, from 4.4.4.4, 00:23:52 ago, via Ethernet0/0

                              Route metric is 20, traffic share count is 1

                         

                        #If I change the ospf interface cost on R6's e0/0 to 1, R6 still receives both Type 4 LSA's from R1 and R7 but chooses R1 because of the lower forward metric.

                         

                        R6#sh ip ospf 1 1 database asbr-summary

                                    OSPF Router with ID (6.6.6.6) (Process ID 1)

                         

                                        Summary ASB Link States (Area 1)

                          Routing Bit Set on this LSA in topology Base with MTID 0

                          LS age: 1639

                          Options: (No TOS-capability, DC, Upward)

                          LS Type: Summary Links(AS Boundary Router)

                          Link State ID: 4.4.4.4 (AS Boundary Router address)

                          Advertising Router: 1.1.1.1

                          LS Seq Number: 80000001

                          Checksum: 0x6DA6

                          Length: 28

                          Network Mask: /0

                                MTID: 0        Metric: 20

                         

                         

                          LS age: 1572

                          Options: (No TOS-capability, DC, Upward)

                          LS Type: Summary Links(AS Boundary Router)

                          Link State ID: 4.4.4.4 (AS Boundary Router address)

                          Advertising Router: 10.10.10.7

                          LS Seq Number: 80000001

                          Checksum: 0x7082

                          Length: 28

                          Network Mask: /0

                                MTID: 0        Metric: 20

                         

                         

                        R6#sh ip route 4.4.4.3

                        Routing entry for 4.4.4.3/32

                          Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 21

                          Last update from 16.16.16.1 on Ethernet0/0, 00:24:19 ago

                          Routing Descriptor Blocks:

                          * 16.16.16.1, from 4.4.4.4, 00:26:40 ago, via Ethernet0/0

                              Route metric is 20, traffic share count is 1

                         

                        Regards.

                        • 9. Re: Is there real need for Type 4 LSA ?
                          Guru

                          @eddylynx Wonderful explanation... Thank you very much for your time..

                          • 10. Re: Is there real need for Type 4 LSA ?
                            Deepak Kumar

                            Hi,

                            The LSA 4 is like a type 3 LSA, except it advertises a host route used to reach an ASBR. Flooded only within its area of origin; reoriginated on ABRs.

                            Note that a type 4 LSA concerning a particular ASBR is not required in the area where the ASBR resides. It is therefore never flooded into it. Only other areas require the type 4 LSA to be able to compute their metrics toward ASBRs and external networks behind them.


                            Regards,

                            Deepak Kumar