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LSA 2 – Network LSA

As router LSA (LSA 1) is used to describe the router links, LSA 2 is used to describe a L2 segment in OSPF domain. So by using LSA 2 we knows where a SWITCH present in OSPF domain. But why do we need to know about the L2 domain/Switch – Because L2 domain broadcast the incoming messages to all other interfaces which will cause unnecessary flooding in OSPF domain.

Network LSA is used to describe all the routers connected to specific segment. How many LSA 2 are generated in a multiaccess segment = only 1 by DR router and it is sent to 224.0.0.5 address (all DROther)


But do we use switches in between routers apart from the labs – In 99 % cases we do not use, but LSA 2 are generated on every link, WHY – because by default a ethernet link is considered as a broadcast segment by a router in OSPF domain. SO even if we have R1—-R2 topology, still a LSA 2is generated for that broadcast segment (link between R1 and R2)
It is worth noting that OSPF selects DR/BDR in multiaccess segments and DR/BDR are always links (NOT ROUTERS AS MOST ENGINEERS CLAIM)…Network LSA is generated by the DR router (Router whose link is selected as DR) in the network).

Now lets understand this using the LAB. We have below scenario :-

Since we have studied in another article how a DR/BDR are selected. SO in our lab, 12.12.12.3 was selected as DR because of configuration i did


R1#
*Nov 18 18:49:09.419: %SYS-5-CONFIG_I: Configured from console by console
R1#sh ip ospf interface
Loopback0 is up, line protocol is up
  Internet Address 1.1.1.1/32, Area 0, Attached via Network Statement
  Process ID 100, Router ID 1.1.1.1, Network Type LOOPBACK, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Loopback interface is treated as a stub Host
GigabitEthernet0/1 is up, line protocol is up
  Internet Address 12.12.12.1/24, Area 0, Attached via Network Statement
  Process ID 100, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State DROTHER, Priority 1
  Designated Router (ID) 3.3.3.3, Interface address 12.12.12.3
  Backup Designated router (ID) 2.2.2.2, Interface address 12.12.12.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Now lets see if the NETWORK LSA IS SHARED or not

R1#sh ip ospf database network

            OSPF Router with ID (1.1.1.1) (Process ID 100)

                Net Link States (Area 0)

  LS age: 1127
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 12.12.12.3 (address of Designated Router)
 - Remember we discussed that Link is DR
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0xE9EB
  Length: 40
  Network Mask: /24
 - The connected routers to the segment
        Attached Router: 3.3.3.3
        Attached Router: 1.1.1.1
        Attached Router: 2.2.2.2
        Attached Router: 4.4.4.4

The same network LSA is shared on other routers.

R2#sh ip ospf database network

            OSPF Router with ID (2.2.2.2) (Process ID 100)

                Net Link States (Area 0)

  LS age: 1262
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 12.12.12.3 (address of Designated Router)
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0xE9EB
  Length: 40
  Network Mask: /24
        Attached Router: 3.3.3.3
        Attached Router: 1.1.1.1
        Attached Router: 2.2.2.2
        Attached Router: 4.4.4.4
**************************************************************************************
R3#sh ip ospf database network

            OSPF Router with ID (3.3.3.3) (Process ID 100)

                Net Link States (Area 0)

  LS age: 1304
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 12.12.12.3 (address of Designated Router)
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0xE9EB
  Length: 40
  Network Mask: /24
        Attached Router: 3.3.3.3
        Attached Router: 1.1.1.1
        Attached Router: 2.2.2.2
        Attached Router: 4.4.4.4
*************************************************************************************
R4#sh ip ospf database network

            OSPF Router with ID (4.4.4.4) (Process ID 100)

                Net Link States (Area 0)

  LS age: 1347
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 12.12.12.3 (address of Designated Router)
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0xE9EB
  Length: 40
  Network Mask: /24
        Attached Router: 3.3.3.3
        Attached Router: 1.1.1.1
        Attached Router: 2.2.2.2
        Attached Router: 4.4.4.4

Now lets see the Wireshark capture for the same :-

SO this was all about the LSA 2. Do comment or contact us for any queries.

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