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Module 2: Single-Area OSPFv2 Configuration - Coggle Diagram
Module 2: Single-Area OSPFv2 Configuration
2.1 OSPF Router ID
2.1.1 OSPF Reference Topology
To get you started, this topic discusses the foundation on which OSPF bases its entire process, the OSPF router ID.
2.1.2 Router Configuration Mode for OSPF
OSPFv2 is enabled using the router ospf process-id global configuration mode command, as shown in the command window for R1.
2.1.3 Router IDs
An OSPF router ID is a 32-bit value, represented as an IPv4 address. The router ID is used to uniquely identify an OSPF router.
Participate in the election of the designated router (DR)
Participate in the synchronization of OSPF databases
2.1.4 Router ID Order of Precedence
The router ID is explicitly configured using the OSPF router-id
If the router ID is not explicitly configured, the router chooses the highest IPv4 address of any of configured loopback interfaces.
f no loopback interfaces are configured, then the router chooses the highest active IPv4 address of any of its physical interfaces.
2.1.5 Configure a Loopback Interface as the Router ID
2.1.6 Explicitly Configure a Router ID
2.1.7 Modify a Router ID
2.1.8 Syntax Checker - Configure R2 and R3 Router IDs
2.3 Multiaccess OSPF Networks
2.3.1 OSPF Network Types
Another type of network that uses OSPF is the multiaccess OSPF network. Multiaccess OSPF networks are unique in that one router controls the distribution of LSAs. The router that is elected for this role should be determined by the network administrator through proper configuration.
2.3.2 OSPF Designated Router
Recall that, in multiaccess networks, OSPF elects a DR and BDR as a solution to manage the number of adjacencies and the flooding of link-state advertisements (LSAs). The DR is responsible for collecting and distributing LSAs sent and received. The DR uses the multicast IPv4 address 224.0.0.5 which is meant for all OSPF routers.
2.3.3 OSPF Multiaccess Reference Topology
In the multiaccess topology shown in the figure, there are three routers interconnected over a common Ethernet multiaccess network, 192.168.1.0/24. Each router is configured with the indicated IPv4 address on the Gigabit Ethernet 0/0/0 interface.
2.3.5 Verify DR/BDR Adjacencies
To verify the OSPFv2 adjacencies, use the show ip ospf neighbor command, as shown in the example for R1. The state of neighbors in multiaccess networks can be as follows:
FULL/DROTHER - This is a DR or BDR router that is fully adjacent with a non-DR or BDR router. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments.
FULL/DR - The router is fully adjacent with the indicated DR neighbor. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments.
FULL/BDR - The router is fully adjacent with the indicated BDR neighbor. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments.
2-WAY/DROTHER - The non-DR or BDR router has a neighbor relationship with another non-DR or BDR router. These two neighbors exchange Hello packets.
2.3.6 Default DR/BDR Election Process
How do the DR and BDR get elected? The OSPF DR and BDR election decision is based on the following criteria, in sequential order:
OSPFv2 Multiaccess Reference Topology
In the figure, all Ethernet router interfaces have a default priority of 1. As a result, based on the selection criteria listed above, the OSPF router ID is used to elect the DR and BDR. R3 with the highest router ID becomes the DR; and R2, with the second highest router ID, becomes the BDR.
2.3.7 DR Failure and Recovery
After the DR is elected, it remains the DR until one of the following events occurs:
The DR fails.
The OSPF process on the DR fails or is stopped.
The multiaccess interface on the DR fails or is shutdown.
If the DR fails, the BDR is automatically promoted to DR. This is the case even if another DROTHER with a higher priority or router ID is added to the network after the initial DR/BDR election. However, after a BDR is promoted to DR, a new BDR election occurs and the DROTHER with the highest priority or router ID is elected as the new BDR.
2.3.8 The ip ospf priority Command
If the interface priorities are equal on all routers, the router with the highest router ID is elected the DR. It is possible to configure the router ID to manipulate the DR/BDR election. However, this process only works if there is a stringent plan for setting the router ID on all routers. Configuring the router ID can help control this. However, in large networks this can be cumbersome.
2.5 Default Route Propagation
2.5.1 Propagate a Default Static Route in OSPFv2
Network users will need to send packets outside the network to non-OSPF networks and will need a static default route.
To propagate a default route, the edge router must be configured:
A default static route.
The default-information originate router configuration command.
This instructs the router to be the originator of the default route information and to propagate the default static route in OSPF updates.
2.5.2 Verify the Propagated Default Route
You can verify the default route configuration on the Router using the show ip route command.
The meaning of E1 and E2 is beyond the scope of this module.
The E2 designation identifies that it is an external route.
The asterisk identifies it as a good candidate for the default route.
2.2 Point-to-Point OSPF Networks
2.2.1 The network Command Syntax
You can specify the interfaces that belong to a point-to-point network by configuring the network command.
Router(config-router)# network network-address wildcard-mask area area-id
2.2.2 The Wildcard Mask
Wildcard mask bit 0 - Matches the corresponding bit value in the address.
Wildcard mask bit 1 - Ignores the corresponding bit value in the address.
2.2.4 Configure OSPF Using the network Command
2.2.6 Configure OSPF Using the ip ospf Command
Router(config-if)# ip ospf process-id area area-id
2.2.8 Passive Interface
Inefficient Use of Bandwidth - Available bandwidth is consumed transporting unnecessary messages.
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.
2.2.9 Configure Passive Interfaces
2.2.11 OSPF Point-to-Point Networks
2.2.12 Loopbacks and Point-to-Point Networks
2.4 Modify Single-Area OSPFv2
2.4.1 Cisco OSPF Cost Metric
Recall that a routing protocol uses a metric to determine the best path of a packet across a network. A metric gives indication of the overhead that is required to send packets across a certain interface. OSPF uses cost as a metric. A lower cost indicates a better path than a higher cost.
2.4.2 Adjust the Reference Bandwidth
The cost value must be an integer. If something less than an integer is calculated, OSPF rounds up to the nearest integer. Therefore, the OSPF cost assigned to a Gigabit Ethernet interface with the default reference bandwidth of 100,000,000 bps would equal 1, because the nearest integer for 0.1 is 0 instead of 1.
2.4.3 OSPF Accumulates Costs
The cost of an OSPF route is the accumulated value from one router to the destination network. Assuming the auto-cost reference-bandwidth 10000 command has been configured on all three routers, the cost of the links between each router is now 10. The loopback interfaces have a default cost of 1
2.4.4 Manually Set OSPF Cost Value
OSPF cost values can be manipulated to influence the route chosen by OSPF. For example, in the current configuration, R1 is load balancing to the 10.1.1.8/30 network. It will send some traffic to R2 and some traffic to R3. You can see this in the routing table.
2.4.7 Hello Packet Intervals
As shown in the figure, OSPFv2 Hello packets are transmitted to multicast address 224.0.0.5 (all OSPF routers) every 10 seconds. This is the default timer value on multiaccess and point-to-point networks.
Note: Hello packets are not sent on the simulated the LAN interfaces because those interfaces were set to passive by using the router configuration passive-interface command.
2.6 Verify Single-Area OSPFv2
2.6.1 Verify OSPF Neighbors
Commad show ip ospf neighbor
Neighbor ID
Pri
- This is the OSPFv2 priority of the interface.
State
Dead Time
Address
Interface
2.6.2 Verify OSPF Protocol Settings
Command show ip protocols
It is a quick way to verify vital OSPF configuration information.
2.6.4 Verify OSPF Interface Settings
Command show ip ospf interface
The show ip ospf interface command provides a detailed list for every OSPFv2-enabled interface.
2.6.3 Verify OSPF Process Information
Command show ip ospf
This command displays the OSPFv2 area information and the last time the SPF algorithm was executed.
