What is Open Shortest Path First – OSPF?
OSPF, which stands for Open Shortest Path First, is a routing protocol that was created in 1991. It is a link-state protocol, which means that it builds a map of the network topology and calculates the shortest path between any two nodes.
OSPFv2 is the newer version of OSPF for IPv4 and was introduced in RFC 1247 by John Moy. It has been updated several times since then, with the most recent update being in RFC 2328. Additionally, OSPFv3 was published in RFC 2740 for IPv6 networks, providing similar functionality to OSPFv2 but with support for the newer protocol.
OSPF was designed to replace the distance vector protocol RIP. Unlike RIP, OSPF is a classless protocol that uses the concept of areas to enhance network scalability and reduce the size of routing tables.
In OSPF, the metric that determines the best path is called cost, which is a random value. In Cisco IOS, the OSPF cost is determined based on the bandwidth of the network.
One of the most significant benefits of using Open Shortest Path First (OSPF) over Routing Information Protocol (RIP) is its ability to converge quickly and scale effectively to much larger network implementations. OSPF uses a more intelligent and efficient algorithm for calculating shortest path routing, which results in faster network convergence times.
Furthermore, OSPF is designed to handle larger network implementations with multiple routers and subnets, making it a more reliable and scalable option for complex network infrastructures. These features make OSPF a preferred choice for enterprise-level networks that require high availability and performance.
Administrative Distance
Administrative distance is a value that determines the trustworthiness of routing information from different sources. In OSPF, the default administrative distance is 110, which means that OSPF routes are preferred over routes from other protocols with higher administrative distances.
OSPF Hello Packets
The OSPF Hello packet is used to establish neighbor adjacencies. By default, OSPF Hello packets are sent :
- Every 10 seconds on multi-access and point-to-point segments
- Every 30 seconds on non-broadcast multi-access (NBMA) segments (Frame Relay, X.25, ATM).
OSPF Dead Intervals
In OSPF, the dead interval is a crucial parameter that specifies the duration for which an OSPF router waits to receive a hello packet from its neighbor, when that duration elapse; it will terminate the adjacency.
By default, the OSPF dead interval is set to four times the hello interval, which is the duration between the hello packets exchanged between the routers. This parameter plays a critical role in ensuring the stability and efficiency of the OSPF network by preventing a router from continuing to send traffic to a dead neighbor. These dead intervals time lapse are :
+ For multi-access and point-to-point segments, this period is 40 seconds.
+ For NBMA networks, the Dead interval is 120 seconds.
+ For routers to become adjacent, their Hello interval, Dead interval, Network types and subnet masks must match.
OSPF Router ID
In the OSPF routing domain, every router needs to be uniquely identified. This is where the OSPF router ID comes into play. The router ID is used to identify each router in the OSPF network. Cisco routers use three criteria to derive the router ID, and these are prioritized in the following order of preference.:
+ the Use of the IP address configured with the OSPF router ID command.
+ If the router ID is not configured, the router chooses highest IP address of any of its loopback interfaces.
+ If no loopback interfaces are configured, the router chooses highest active IP address of any of its physical interfaces.
OSPF Area IDs
The OSPF network domain area is commonly referred to as the area ID. It is a group of routers that share link-state information, which is received from their neighbors and stored in their link-state databases. This information is vital for OSPF routers to form a complete view of the network topology.
In OSPF design, routers are connected to a backbone area, also known as area 0. This area is considered the heart of the network and serves as the reference point for all other areas.
All other routers must connect to this area, which is why it is also known as OSPF Multi-Area. This design ensures that all routers in the network have the same link-state information, allowing them to communicate effectively and maintain network stability.
How to Configure OSPF Multiarea on a network.
How to Configure OSPF on Cisco Routers
How to Configure OSPF with Authentication
Troubleshooting and Verifying OSPF Configurations
How to Configure OSPF using Wildcards
In Summary,
Open Shortest Path First (OSPF) is a popular routing protocol for IP networks for several key reasons: –
* It is classless,
* Offers full CIDR and VLSM support,
* It scales well, converges quickly, and guarantees loop-free routing.
It also supports address summarization and the tagging of external routes, similar to EIGRP.
OSPF uses a large, dimensionless metric on every link – which can also be referred to as “cost –, with a maximum value of 65,535. It is important to remember that OSPF doesn’t add these metrics the same way that RIP and EIGRP do.
In those protocols, each router updates the total metric as it passes the route onto the next router. However, in OSPF, the routers distribute the individual link costs to one another.
The maximum cost for an individual link, then, is 65,535. Any given path through an OSPF network can include many high-cost links, but still be usable. This is quite different from RIP, for example, where a few high-cost links along a path can make the entire path unusable.
BGP EIGRPv6 RIPv6 or RIPng OSPFv3 DHCPv6
