27. OSPF Part 2

OSPF Metric (Cost)

• Metric Name: In OSPF, the metric used to determine the best path is called Cost.
• Calculation: OSPF automatically calculates the cost based on the bandwidth (speed) of the interface. The formula is:

$\text{Cost}=\frac{\text{Reference Bandwidth}}{\text{Interface Bandwidth}}$

• Default Reference Bandwidth: 100 Mbps here some Example Calculations :

$\frac{100\text{Mbps}}{10\text{Mbps}}=\text{Cost of }10$

$\frac{100\text{Mbps}}{100\text{Mbps}}=\text{Cost of }1$

$\frac{100\text{Mbps}}{1000\text{Mbps}}=\text{Cost of }1$

$\frac{100\text{Mbps}}{10000\text{Mbps}}=\text{Cost of }1$

• Minimum Cost: If the calculated cost is less than 1, it is rounded up to 1.
• Fast Ethernet, Gigabit Ethernet, and Faster Links: Interfaces with 100 Mbps, 1 Gbps, 10 Gbps, etc., will all have the same cost of 1.

Changing the Reference Bandwidth

• Command:

  R1(config-router)# auto-cost reference-bandwidth <megabits-per-second>

✏️ The reference bandwidth is specified in megabits per second (default is 100 Mbps).

Example: $\frac{100000\text{Mbps}}{100\text{Mbps}}=\text{Cost of }1000:forFastEthernet$

$\frac{100000\text{Mbps}}{1000\text{Mbps}}=\text{Cost of }100:forGigEthernet$

• Recommendation: Set the reference bandwidth higher than the fastest links in your network to accommodate future upgrades.
• Important: Changing the reference bandwidth should be done on all OSPF routers in the network.

OSPF Cost to a Destination

• Total Cost: The OSPF cost to a destination is the sum of the costs of the outgoing (exit) interfaces.
• Loopback Interfaces: Default cost is 1.

Modifying OSPF Cost

• Manual Cost Configuration:

  R1(config-if)# ip ospf cost <cost>

• Precedence: Manually configured costs take precedence over automatically calculated costs.

• Changing Interface Bandwidth:

  • The OSPF cost can also be altered by changing the interface bandwidth using the bandwidth command.
  • Formula:

$\text{Cost}=\frac{\text{Reference Bandwidth}}{\text{Interface Bandwidth}}$

• Note: Changing the bandwidth value does not actually change the physical speed of the interface; it only alters the OSPF cost calculation.

• To change the actual operating speed of the interface, use the speed command.

• Best Practice:

  • It is recommended to adjust the reference bandwidth rather than the interface bandwidth.
  • Then, use the ip ospf cost command for more precise cost adjustments.

Summary of OSPF Cost Modifications

1. Change the Reference Bandwidth:

   R1(config-router)# auto-cost reference-bandwidth <megabits-per-second>

2. Manual Configuration:

   R1(config-router)# ip ospf cost <cost>

3. Change the Interface Bandwidth:

   R1(config-if)# bandwidth <kilobits-per-second>

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Becoming OSPF Neighbors

• Key Task: Ensuring routers successfully become OSPF neighbors is crucial for configuring and troubleshooting OSPF.
• Process:
  • Once routers become neighbors, they automatically share network information and calculate routes.

Hello Messages

• Activation: When OSPF is activated on an interface, the router sends out Hello messages at regular intervals.
• Default Interval: 10 seconds on an Ethernet connection.
• Destination: Multicast to 224.0.0.5 (all OSPF routers).
• Protocol: OSPF messages are encapsulated in an IP header with a protocol value of 89.

OSPF Neighbor States

1. Down State:

   • Initial state when OSPF is activated on an interface.
   • Router sends Hello messages but doesn’t know about any neighbors yet.

   

2. Init State:

   • When a router receives a Hello packet, it adds the sender to its OSPF neighbor table.
   • The neighbor state is Init until the router sees its own Router ID (RID) in a Hello packet from the neighbor.

   

3. 2-Way State:

   • Achieved when a router sees its own RID in the Hello packet from the neighbor.
   • At this point, the routers are ready to share LSAs and build a common LSDB.
   • In some networks, this is where DR (Designated Router) and BDR (Backup Designated Router) elections occur.

   

4. ExStart State:

   • Routers prepare to exchange information about their LSDB.
   • They must first decide which router will start the exchange (Master/Slave role).
   • The router with the higher RID becomes the Master.

   

5. Exchange State:

   • Routers exchange DBD (Database Description) packets, which contain a list of the LSAs in their LSDBs.
   • Only basic information about the LSAs is included.

   

6. Loading State:

   • Routers send Link State Requests (LSR) to request missing LSAs.
   • LSAs are sent in Link State Update (LSU) messages.
   • Routers acknowledge receipt of LSAs with LSAck messages.

   

7. Full State:

   • The routers have a full OSPF adjacency and identical LSDBs.
   • They continue to send and listen for Hello packets (every 10 seconds by default) to maintain the neighbor adjacency.
   • If the Dead timer (40 seconds by default) reaches zero without receiving a Hello message, the neighbor is removed.
   • Routers will continue to share LSAs as the network changes.

   

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OSPF Neighbors Summary

1. Become Neighbors:

   • Down State
   • Init State
   • 2-Way State
   • (DR/BDR Election)

2. Exchange LSAs:

   • ExStart State
   • Exchange State
   • Loading State

Summary of OSPF Message Types

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Additional OSPF Configurations

Activate OSPF on an Interface

You can activate OSPF directly on an interface with:

R1(config-if)# ip ospf <process-id> area <area>

Configure All Interfaces as Passive

To make all interfaces passive by default:

R1(config-router)# passive-interface default

Exclude Specific Interfaces:

R1(config-router)# no passive-interface <interface-id>

Different Output in show ip protocols

Activating OSPF directly on interfaces will display a different output in show ip protocols:

Routing on Interfaces Configured Explicitly (Area #): Displays the specific interfaces where OSPF is activated.

Viewing the OSPF LSDB

To view the LSDB of a device:

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