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17 - Day 28 - OSPF Part 3.md

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Day 28 - OSPF Part 3

Reviewed: No

1. Loopback Interfaces:

  • Overview:
    • A loopback interface is a virtual, software-based interface on a router.
    • It is always in the up/up state unless manually shut down.
    • Loopback interfaces provide a stable IP address that can be used to identify and reach the router, even if physical interfaces fail.

2. OSPF Network Types:

  • Broadcast Network Type:
    • Enabled by default on Ethernet and FDDI interfaces.
    • Routers dynamically discover neighbors using the multicast address 224.0.0.5.
    • DR (Designated Router) and BDR (Backup Designated Router) elections are required on each subnet.
    • Routers that are neither DR nor BDR become DROther.
  • DR/BDR Election Process:
    • Priority: The router with the highest OSPF interface priority becomes the DR. The default priority is 1.
    • Router ID: If priorities are equal, the router with the highest Router ID becomes the DR.
    • The election is non-preemptive: Once a DR/BDR is elected, it remains in that role until OSPF is reset or the interface goes down.
  • Point-to-Point Network Type:
    • Enabled by default on serial interfaces using PPP (Point-to-Point Protocol) or HDLC (High-Level Data Link Control) encapsulations.
    • No DR/BDR elections are held because the connection is between just two routers.
    • Routers form a full adjacency with each other.
  • Serial Connections Overview:
    • DCE (Data Communications Equipment): The side that sets the clock rate for the connection.
    • DTE (Data Terminal Equipment): The receiving side of the clock rate.
    • Default encapsulation is HDLC (Cisco HDLC in Cisco devices).
    • Command for clock rate: clock rate <value> (in bits per second) on the DCE side.
  • Manually Configuring Network Types:
    • Command: ip ospf network <network-type> (in interface configuration mode).
    • Not all network types are compatible with all link types (e.g., serial links cannot use the broadcast network type).

3. OSPF Neighbor and Adjacency Requirements:

  • Area Number Must Match: Both routers must be in the same area to become neighbors.
  • Subnet Must Match: Routers’ interfaces must be in the same subnet.
  • OSPF Process Must Be Active: The OSPF process must not be in the shutdown state.
  • Unique Router IDs: Router IDs must be unique across the OSPF domain.
  • Hello and Dead Timers Must Match: These timers must be consistent between neighbors.
  • Authentication Settings: OSPF passwords and authentication settings must match.
  • IP MTU Must Match: Maximum Transmission Unit (MTU) settings on interfaces must match for proper OSPF operation.
  • Network Type Must Match: The OSPF network type configuration must be consistent between neighbors.

4. OSPF LSA Types:

  • Type 1 - Router LSA:
    • Generated by every OSPF router.
    • Identifies the router and lists the networks connected to the router's OSPF-activated interfaces.
  • Type 2 - Network LSA:
    • Generated by the DR of each multi-access network (e.g., Ethernet networks using the broadcast network type).
    • Lists all routers attached to the multi-access network.
  • Type 5 - AS-External LSA:
    • Generated by Autonomous System Boundary Routers (ASBRs).
    • Describes routes to destinations outside the OSPF domain (e.g., routes to the internet).

5. Quiz Review:

  • Point-to-Point Network Type Characteristic:
    • Answer: DR and BDR elections are not held.
  • Full Adjacencies in OSPF Broadcast Network:
    • Answer: The DR forms a full adjacency with all neighbors in the broadcast network segment.
  • OSPF Neighbor Requirements:
    • Answers: Hello and Dead timers must match, and interfaces must be in the same area.
  • Type 2 LSA:
    • Answer: Generated only by the DR of a multi-access network, such as the broadcast network type.
  • Priority and DR Election:
    • Answers: The DR and BDR of the network are unchanged; if the current DR goes down, R1 will become the BDR

6. Commands used:

Configure OSPF on an Interface:

ip ospf <process-id> area <area-id>

Change OSPF Interface Priority:

ip ospf priority <value>

Reset the OSPF Process:

clear ip ospf process

Configure Serial Interface Clock Rate:

clock rate <bits-per-second>

Change Serial Interface Encapsulation to PPP:

encapsulation ppp

Check Serial Interface Controller (to see if it's DCE or DTE):

show controllers <interface-id>

Configure OSPF Network Type:

ip ospf network <network-type>

Configure OSPF Hello Interval:

ip ospf hello-interval <seconds>

Configure OSPF Dead Interval:

ip ospf dead-interval <seconds>

Configure OSPF Authentication Key:

ip ospf authentication-key <password>

Enable OSPF Authentication:

ip ospf authentication

Configure IP MTU:

ip mtu <bytes>

Disable OSPF Process:

shutdown (in OSPF configuration mode)

Re-enable OSPF Process:

no shutdown (in OSPF configuration mode)