Intra-AS Routing in the Internet: OSPF

Intra-AS Routing in the Internet: OSPF

Like RIP, OSPF routing is extensively used for intra-AS routing in the Internet. OSPF and its closely related cousin, IS-IS, are normally deployed in upper-tier lSPs whereas RIP is deployed in lower-tier ISPs and enterprise networks. The Open in OSPF indicates that the routing protocol specification is publicly available (for instance, as opposed to Cisco's EIGRP protocol).

OSPF was conceived as the successor to RIP and as such has a number of advanced features. At its heart, on the other hand, OSPF is a link-state protocol that uses flooding of link-state information and a Dijkstra least-cost path algorithm. With OSPF, a router constructs a complete topological map (that is, a graph) of the entire autonomous system. The router then locally runs Dijkstra's shortest-path algorithm to determine a shortest-path tree to all subnets, with itself as the root node. Individual link costs are configured by the network administrator (see Principles in Practice: Setting OSPF Link Weights). The administrator might choose to set all link costs to 1, thus achieving minimum-hop routing, or might choose to set the link weights to be inversely proportional to link capacity in order to discourage traffic from using low-bandwidth links. OSPF does not mandate a policy for how link weights are set (that is the job of the network administrator), but instead provides the mechanisms (protocol) for determining least-cost path routing for the given set of link weights.


With OSPF, a router broadcasts routing information to all other routers in the autonomous system, not just to its neighboring routers. A router broadcasts link-state information whenever there is a change in a link's state (for instance, a change in cost or a change in up/down status). It also broadcasts a link's state periodically (at least once every 30 minutes), even if the link's state has not changed. OSPF advertisements are contained in OSPF messages that are carried directly by IP, with an upper-layer protocol of 89 for OSPF.  In this way, the OSPF protocol must itself implement functionality such as reliable message transfer and link-state broadcast. The OSPF protocol also checks that links are operational (via a HELLO message that is sent to an attached neighbor) and allows an OSPF router to get a neighboring router's database of network-wide link state.

Some of the advances embodied in OSPF contain the following:

●  Security. Exchanges between OSPF routers (for instance, link-state updates) can be authenticated. With authentication, only trusted routers can participate in the OSPF protocol within an AS, thus preventing malicious intruders (or networking students taking their newfound knowledge out for a joyride) from injecting incorrect information into router tables. By default, OSPF packets between routers are not authenticated and could be forged. Two types of authentication can be configured - simple and MD5. With simple authentication, the same password is configured on each router. When a router sends an OSPF packet, it contains the password in plaintext. Clearly, simple authentication is not very secure. MD5 authentication is based on shared secret keys that are configured in all the routers. For each OSPF packet that it sends, the router computes the MD5 hash of the content of the OSPF packet appended with the secret key. Then the router contains the resulting hash value in the OSPF packet. The receiving router, using the preconfigured secret key, will compute an MD5 hash of the packet and compare it with the hash value that the packet carries, thus verifying the packet's authenticity. Sequence numbers are also used with MD5 authentication to protect against replay attacks.

●  Multiple same-cost paths. When multiple paths to a destination have the same cost, OSPF allows multiple paths to be used (that is, a single path need not be chosen for carrying all traffic when multiple equal-cost paths exist).

●  Integrated support for unicast and multicast routing. Multicast OSPF (MOSPF) provides simple extensions to OSPF to provide for multicast routing. MOSPF uses the existing OSPF link database and adds a new type of link-state advertisement to the existing OSPF link-state broadcast mechanism.

●  Support for hierarchy within a single routing domain. Perhaps the most important advance in OSPF is the ability to structure an autonomous system hierarchically. In "Hierarchical Routing" we have already examined the many advantages of hierarchical routing structures. We cover the implementation of OSPF hierarchical routing in the remainder of this section.

An OSPF autonomous system can be configured hierarchically into areas. Each area runs its own OSPF link-state routing algorithm, with each router in an area broadcasting its link state to all other routers in that area. Within each area, one or more area border routers are responsible for routing packets outside the area. Finally, just one OSPF area in the AS is configured to be the backbone area. The main role of the backbone area is to route traffic between the other areas in the AS. The backbone always includes all area border routers in the AS and may include nonborder routers as well. Inter-area routing within the AS requires that the packet be first routed to an area border router (intra-area routing), then routed through the backbone to the area border router that is in the destination area, and then routed to the final destination. OSPF is a comparatively complicated protocol, and our coverage here has been necessarily brief; [Huitema 1998; Moy 1998;] provide further details.


autonomous system, ospf packet, area border routers, link weights

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