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Single-Area OSPFv2 configuration, 2.3.1 OSPF Network Types, imagen, 2.1.1…

- - - - To configure OSPF directly on the interface, use the ip ospf interface configuration mode command.
      - For R1, remove the network commands by using the no form of the network commands. And then go to each interface and configure the ip ospf command, as shown in the command window.
  - - - Sending out unneeded messages on a LAN affects the network in three ways
        
        Inefficient Use of Resources
        
        All devices on the LAN must process and eventually discard the message.
        
        Increased Security Risk
        
        Without additional OSPF security configurations, OSPF messages can be intercepted with packet sniffing software. Routing updates can be modified and sent back to the router, corrupting the routing table with false metrics that misdirect traffic.
        
        Inefficient Use of Bandwidth
        
        Available bandwidth is consumed transporting unnecessary messages.
  - - - Any active interface that is configured with an IPv4 address belonging to that network will participate in the OSPFv2 routing process.
        
        Notice that in all cases, the area argument specifies area 0.
  - - - The show ip protocols command is then used to verify that the Loopback 0 interface is listed as passive.
        
        The interface is still listed under the heading, “Routing on Interfaces Configured Explicitly (Area 0)”, which means that this network is still included as a route entry in OSPFv2 updates that are sent to R2 and R3.
  - - - Wildcard mask bit 0 - Matches the corresponding bit value in the address.
      - Wildcard mask bit 1 - Ignores the corresponding bit value in the address.
    - - For /24
      - For /26
  - - - R1 is the BDR and R2 is the DR. The DR/ BDR election process is unnecessary as there can only be two routers on the point-to-point network between R1 and R2.
        
        Notice in the output that the router has designated the network type as BROADCAST. To change this to a point-to-point network, use the interface configuration command ip ospf network point-to-point on all interfaces where you want to disable the DR/BDR election process.
        
        R1(config)# interface GigabitEthernet 0/0/0
        R1(config-if)# ip ospf network point-to-point
  - - - The area area-id syntax refers to the OSPF area. When configuring single-area OSPFv2, the network command must be configured with the same area-id value on all routers.
        
        Although any area ID can be used, it is good practice to use an area ID of 0 with single-area OSPFv2.
      - The network-address wildcard-mask syntax is used to enable OSPF on interfaces.
  - - - By default, loopback interfaces are advertised as /32 host routes. For example, R1 would advertise the 10.10.1.0/24 network as 10.10.1.1/32 to R2 and R3.
        
        R2# show ip route | include 10.10.1 :check:
        O 10.10.1.1/32 [110/2] via 10.1.1.5, 00:03:05, GigabitEthernet0/0/0
        
        To simulate a real LAN, the Loopback 0 interface is configured as a point-to-point network so that R1 will advertise the full 10.10.1.0/24 network to R2 and R3.
        
        Now R2 receives the more accurate, simulated LAN network address of 10.10.1.0/24.
- - - - R1# show ip protocols
        Routing Protocol is "ospf 10"
        Router ID 1.1.1.1
        Routing on Interfaces Configured Explicitly (Area 0):
        Loopback0
        GigabitEthernet0/0/1
        GigabitEthernet0/0/0
        Routing Information Sources:
        Gateway Distance Last Update
        3.3.3.3 110 00:09:30
        2.2.2.2 110 00:09:58
        Distance: (default is 110)
  - - - R1# show ip ospf
        Routing Process "ospf 10" with ID 1.1.1.1
        Area BACKBONE(0)
        SPF algorithm executed 4 times
  - - - Additional commands:
        -show ip ospf neighbor
        -show ip protocols
        -show ip ospf
        -show ip ospf interface
        
        Reference topology:
        
        Show ip ospf neighbor command shows and verify that the router has formed an adjacency with its neighboring routers. If the router ID of the neighboring router is not displayed, or if it does not show as being in a state of FULL, the two routers have not formed an OSPFv2 adjacency.
        
        If two routers do not establish adjacency, link-state information is not exchanged. Incomplete LSDBs can cause inaccurate SPF trees and routing tables. Routes to destination networks may not exist, or may not be the most optimum path.
        
        Note: A non-DR or BDR router that has a neighbor relationship with another non-DR or BDR router will display a two-way adjacency instead of full.
        
        The following command output displays the neighbor table of R1:
        R1# show ip ospf neighbor
        Neighbor ID Pri State Dead Time Address Interface
        3.3.3.3 0 FULL/ - 00:00:19 10.1.1.13 GigabitEthernet0/0/1
        2.2.2.2 0 FULL/ - 00:00:18 10.1.1.6 GigabitEthernet0/0/0
        
        1 more item...
  - - - R1# show ip ospf interface GigabitEthernet 0/0/0
        Process ID 10, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 10
        Neighbor Count is 1, Adjacent neighbor count is 1
        Adjacent with neighbor 2.2.2.2
        
        To get a quick summary of OSPFv2-enabled interfaces, use the show ip ospf interface brief command, as shown in the following command output. This command is useful for seeing important information including the following:
        Interfaces are participating in OSPF
        Networks that are being advertised (IP Address/Mask)
        Cost of each link
        Network state
        Number of neighbors on each lin
- - - - An OSPF multi-access network allows one router to control the distribution of LSAs (Link-State Advertisements)
  - - - R2# show ip ospf interface GigabitEthernet 0/0/0
    - - R3# show ip ospf interface GigabitEthernet 0/0/0
    - - R1# show ip ospf interface GigabitEthernet 0/0/0
  - - - This is a DR or BDR router that is fully adjacent with a non-DR or BDR router.
    - - The router is fully adjacent with the indicated DR neighbor.
    - - The router is fully adjacent with the indicated BDR neighbor.
    - - The non-DR or BDR router has a neighbor relationship with another non-DR or BDR router.
- - - - In the topology in the figure, R2 is connected to the internet and should propagate a default route to R1 and R3. The router connected to the internet is sometimes called the edge router or the gateway router. However, in OSPF terminology, the router located between an OSPF routing domain and a non-OSPF network is called the autonomous system boundary router (ASBR).
        
        All that is required for R2 to reach the internet is a default static route to the service provider.
        
        To propagate a default route, the edge router (R2) must be configured with the following:
        A default static route using the ip route 0.0.0.0 0.0.0.0 [next-hop-address | exit-intf] command.
        The default-information originate router configuration command. This instructs R2 to be the source of the default route information and propagate the default static route in OSPF updates.
        
        In the following example, R2 is configured with a loopback to simulate a connection to the internet. Then a default route is configured and propagated to all other OSPF routers in the routing domain.
        
        Example (Note: a loopback interface with IPv4 address 64.100.0.1 is used to simulate the connection to the service provider.)
        
        R2(config)# interface lo1
        R2(config-if)# ip address 64.100.0.1 255.255.255.252
        R2(config-if)# exit
        R2(config)# ip route 0.0.0.0 0.0.0.0 loopback 1
        %Default route without gateway, if not a point-to-point interface, may impact performance
        R2(config)# router ospf 10
        R2(config-router)# default-information originate
        R2(config-router)# end
  - - - R2 Routing Table
        
        R2# show ip route | begin Gateway
        Gateway of last resort is 0.0.0.0 to network 0.0.0.0
        S* 0.0.0.0/0 is directly connected, Loopback1 10.0.0.0/8 is variably subnetted, 9 subnets, 3 masks
      - R3 Routing Table
        
        R3# show ip route | begin Gateway
        Gateway of last resort is 10.1.1.9 to network 0.0.0.0
        O*E2 0.0.0.0/0 [110/1] via 10.1.1.9, 00:12:04, GigabitEthernet0/0/1
      - R1 Routing Table
        
        R1# show ip route | begin Gateway
        Gateway of last resort is 10.1.1.6 to network 0.0.0.0
        O*E2 0.0.0.0/0 [110/1] via 10.1.1.6, 00:11:08, GigabitEthernet0/0/0