Using the OSPF Protocol in Your Network

The Open Shortest Path First (OSPF) Link-state routing protocol is considered a TCP/IP internet routing Interior Gateway Protocol (IGP). OSPF distributes routing information between routers belonging to a single Autonomous System (AS). The OSPF protocol is based on link-state or SPF technology. The advantages associated with a link-state routing protocol are:

• Rapid convergence
• Reduced routing updates traffic over traditional distance-vector protocols

This OSPF implementation supports RFC 2328, OSPF Version 2.

The OSPF protocol is designed expressly for the TCP/IP Internet environment. It provides for the authentication of routing updates, and utilizes IP multicast when sending and receiving the updates.

OSPF routes IP packets based solely on the destination IP address found in the IP packet header. IP packets are not encapsulated in any further protocol headers as they transit the Autonomous System. OSPF is a dynamic routing protocol in that it quickly detects topological changes in the AS, such as router interface failures, and calculates new loop-free routes after a period of convergence. This period of convergence is short and involves a minimum of routing traffic. In a link-state routing protocol, each router maintains a database describing the AS‘s topology. This database is referred to as the link-state database. Each participating router has an identical database. Each individual piece of this database is a particular router‘s local state made up of such information as the router‘s usable interfaces and reachable neighbors. The router distributes its local state throughout the AS by flooding.

Each network that has at least two attached routers has a designated router. The designated router generates an LSA for the network and has other special responsibilities in the running of the protocol, enabling a reduction in the number of adjacencies required on a network. This in turn reduces the amount of routing protocol traffic and the size of the link-state database.

All routers run the exact same algorithm, in parallel. From the link-state database, each router constructs a tree of shortest paths with itself as root. This shortest-path tree provides the route to each destination in the AS. Externally derived routing information appears on the tree as leaves. When several equal-cost routes to a destination exist, traffic is distributed equally among them. The cost of a route is described by a single dimensionless metric.

OSPF allows sets of networks to be grouped together. Such a grouping is called an area. The topology of an area is hidden from the rest of the AS. This information hiding enables a significant reduction in routing traffic. Also, routing within the area is determined only by the area‘s own topology, lending the area protection against bad routing data. An area is a generalization of an IP subnetted network. OSPF enables the flexible configuration of IP subnets. Each route distributed by OSPF has a destination and mask. Two different subnets of the same IP network number may have different masks providing a different range of addresses for that subnet. This is commonly referred to as Variable Length Subnet Masking (VLSM). A packet is routed to the longest or most specific match. Host routes are considered to be subnets whose masks are “all ones” (0xffffffff).

All OSPF protocol exchanges are authenticated. This means that only trusted routers can participate in the AS‘s routing. The S- K- and 7100-Series platform supports either simple or MD5 authentication schemes. Separate authentication schemes can be configured for each IP subnet.

Route redistribution is supported for RIP, connected, and static routes. Route redistribution of BGP is supported on S- and 7100-Series platforms.

The Bidirectional Forwarding Detection (BFD) protocol providing sub-second failure detection on OSPF forwarding interfaces is enabled by default on all OSPF interfaces (S-, K-Series).

An OSPF Customer Edge (CE) router can be configured as a peer to a Provider Edge (PE) router by enabling the PE-CE protocol on the PE-CE associated routers.