Network types, area types

Network types, area types.

Network types

OSPF has 6 different network type

1 - Broadcast

• Uses LSA Type 2
• There is DR and BDR election
• Send hello to multicast 224.0.0.5 and 224.0.0.6
• Hello is 10, Dead is 40

2 - Non-broadcast

• Uses LSA Type 2
• There is DR and BDR election
• Hello 30, Dead 120
• Static Neighbor

3 - Point-to-point

• There is no DR and BDR election
• Send hello to multicast 224.0.0.5
• Hello is 10, Dead is 40
• HDLC, PPP, GRE and etc.
• Send hello to multicast 224.0.0.5 and 224.0.0.6

4 - Point-to-multipoint

• There is no DR and BDR election
• Hello 30, Dead 120
• Send hello to multicast 224.0.0.5 and 224.0.0.6

5 - Point-to-multipoint non-broadcast

• There is no DR and BDR election

6 - Loopback

• Advertised as /32 (even if they are not)
• Can change the network type to allow the prefix to be advertised.
• 
  

Area Types

Below are the LSA types. We will include the examples based on the topology we have created.

Type 1 LSA - Router LSA

• Generated by all the routers in OSPF topology.
• Scope - Intra-area.
• Once they crosse the boarder router, they became type 3.
• They describe a route.

Example: R1, R2, R3 and R4 in area 0 and, R3 and R8 in area 2.

Type 2 LSA - Network LSA

• Generated by the DR.
• Scope: Intra-area.
• They do not describe a route, just the DR.
• Link state ID is the IP address of the physical interface.

Example: R1 in Area 0 is the DR.

Type 3 LSA - Summary LSA

• Generated by the ABR.
• Represent the Type 1 LSA from another area, received by the ABR.
• Scope - Inter-area.

Example: R3 and R4 are connected to area 0 and are 1 and 2 respectively.

Type 4 LSA - Summary ASB Link States

• Inter-area LSA
• Generated by an ABR that is attached to an area that includes and ASBR.
• Advertised out of the area that includes the ASBR
• They Represent reachability information to the ASBR and the routes from ASBR.

Example: R3 is advertising Type 4 LSA into Area 2, because it has connection to an area with ASBR.

Type 5 LSA - Type-5 AS External Link States

• Generated by ASBR in normal area.
• Represent external routes that are being redistributed into OSPF.
• Type 1 external router E bit = 0, Type 2 E-bit = 1.
• Are not allowed to enter any special area type (Stub, Totally Stub, NSSA, totally not so stubby).

Type 7 LSA - Type-7 AS External Link States 

• Identical to type 5
• Generated by an ASBR attached to a special area type (NSSA or TNSSA).
• Not allowed to enter other special areas.

Virtual Link

• Virtual link cannot be created across Backbone area.
• Virtual link cannot traverse more than one area.
• OSPF cost limit is 65535. If the virtual link is established across a link with high overall cost, the virtual link will not form.

Example.

Below is a topology which as a connection between two non-backbone areas. A virtual link will be configured to connect area 2 to area 0. Below is the configuration from R3 and R4.

R3

R3#sho run | sec router o

router ospf 1

 area 1 virtual-link 150.1.4.4

R3#sho ip osp in bri

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

VL1          1     0               155.1.23.3/24      30    P2P   1/1

Et0/0        1     1               155.1.23.3/24      10    DR    1/1

Lo0          1     2               150.1.3.3/32       1     LOOP  0/0

Et0/1        1     2               155.1.36.3/24      10    BDR   1/

R3#sho ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

150.1.4.4         0   FULL/  -           -        155.1.14.4      OSPF_VL1

150.1.2.2         1   FULL/BDR        00:00:30    155.1.23.2      Ethernet0/0

150.1.6.6         1   FULL/DR         00:00:37    155.1.36.6      Ethernet0/1

R4

R4#sho run | sec r o

router ospf 1

 area 1 virtual-link 150.1.3.3

R4#sho ip ospf int bri

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

VL2          1     0               155.1.14.4/24      30    P2P   1/1

Lo0          1     0               150.1.4.4/24       1     LOOP  0/0

Et0/1        1     0               155.1.45.4/24      10    BDR   1/1

Et0/0        1     1               155.1.14.4/24      10    DR    1/1

R4#sho ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface

150.1.3.3         0   FULL/  -           -        155.1.23.3      OSPF_VL2

150.1.5.5         1   FULL/DR         00:00:35    155.1.45.5      Ethernet0/1

150.1.1.1         1   FULL/BDR        00:00:38    155.1.14.1      Ethernet0/0

Another verification commands.

R3#sho ip ospf virtual-links

Virtual Link OSPF_VL1 to router 150.1.4.4 is up

  Run as demand circuit

  DoNotAge LSA allowed.

  Transit area 1, via interface Ethernet0/0

 Topology-MTID    Cost    Disabled     Shutdown      Topology Name

        0           30        no          no            Base

  Transmit Delay is 1 sec, State POINT_TO_POINT,

  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

    Hello due in 00:00:02

    Adjacency State FULL (Hello suppressed)

    Index 1/1/3, retransmission queue length 0, number of retransmission 0

    First 0x0(0)/0x0(0)/0x0(0) Next 0x0(0)/0x0(0)/0x0(0)

    Last retransmission scan length is 0, maximum is 0

    Last retransmission scan time is 0 msec, maximum is 0 msec