What Is OSPFv2 Commands and How to Use It in Cisco Routing

An Autonomous System (AS) can use the link-state Interior Gateway Protocol (IGP) known as Open Shortest Path First version 2 (OSPFv2) to route IPv4 traffic. With its speedier convergence and increased scalability for big, complicated networks, this open-standard protocol described in RFC 2328 replaced distance-vector protocols like RIP.

Core Operational Principles

As opposed to “routing by rumor,” OSPFv2 works by creating a comprehensive map of the network topology.

• SPF Algorithm: Every router determines a loop-free route to each destination using the Dijkstra Shortest Path First (SPF) algorithm.
• Link-State Database (LSDB): Each router in a given region keeps an identical Link-State Database (LSDB), which is synchronized by exchanging Link-State Advertisements (LSAs).
• Metric (Cost): Cost is the statistic used by OSPF, and it is inversely proportional to interface bandwidth. Higher-speed networks have a lower, more desirable cost since Cisco’s default reference bandwidth is 100 Mbps. The formula is often computed as Cost = Reference Bandwidth/Interface Bandwidth.
• Administrative Distance: OSPFv2’s default Administrative Distance (AD) is 110, which makes it less reliable than EIGRP (AD 90) but more reliable than RIP (AD 120).

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Hierarchical Area Structure

The two-layer hierarchical design used by OSPFv2 helps to manage big networks and lower CPU/memory overhead:

1. Backbone Area (Area 0): This is an OSPF network’s required core area. For all other locations to share routing information, they must be physically or logically connected to it.
2. Non-Backbone Areas: Traffic must travel via the backbone between non-backbone sectors, which are used to divide the network.
3. Area Border Router (ABR): An area border router (ABR) is a router that summarizes routing data between non-backbone and backbone regions and has interfaces in several locations.
4. Autonomous System Boundary Router (ASBR): A router that links the OSPF domain to external networks or other routing protocols, like BGP, is known as an autonomous system boundary router (ASBR).

Neighbor Adjacencies and Packet Types

Before sharing information, routers need to build an adjacency, or relationship.

• The Hello Protocol: Every ten seconds (on broadcast links), routers transmit Hello packets to the multicast address 224.0.0.5 to find neighbors. Routers must agree on the Area ID, subnet mask, authentication, and Hello/Dead intervals for an adjacency to develop.
• Adjacency States: Routers go through a number of stages known as adjacency states, including Down, Init, Two-Way (bidirectional communication), ExStart, Exchange, Loading, and Full (completely synchronized databases).
• Packet Types: OSPFv2 has five distinct packet types: Link-State Request (LSR) (requesting particular records), Link-State Update (LSU) (sending LSAs), Database Description (DBD) (LSDB summary), and Link-State Acknowledgment (LSAck).

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Designated Routers (DR and BDR)

To stop excessive traffic on multi-access networks like Ethernet, OSPF chooses a Backup Designated Router (BDR) and a Designated Router (DR). They only establish complete adjacencies with the DR and BDR, rather than with every other router. having the Router ID (RID) acting as a tiebreaker, the election is determined by the interface having the highest priority.

Basic Configuration and Verification

The following command format is commonly used by administrators to enable OSPFv2:

1. router ospf [process-id] – Starts the OSPF process (the ID is locally significant).
2. network [network-address] [wildcard-mask] area [area-id] – Identifies which interfaces will participate in OSPF. Alternatively, OSPF can be enabled directly on an interface using the ip ospf [process-id] area [area-id] command.

Typical instructions for verification are:

• show ip ospf neighbor: To view current adjacencies and their states.
• show ip ospf database: To examine the LSAs and topology database.
• show ip protocols: To verify the configured OSPF process ID and RID.
• show ip route ospf: To see all IPv4 routes learned specifically via OSPF

Security

To safeguard routing updates, OSPFv2 offers both plain-text and MD5/HMAC-SHA cryptographic authentication. Although the routing table’s integrity is guaranteed, the actual data flow is not encrypted; IPsec can be used to fully encrypt OSPF.

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