OSPF

What is OSPF?

The Open Shortest Path First Protocol (OSPF) is an Interior Gateway Protocol (IGP) used to distribute information among routers belonging to an autonomous system (AS). Utilitizing the link state protocol, each OSPF router maintains identical databases describing the AS topology. Routers use this database to calculate and create shortest path routing tables. Routers supporting OSPF share and update information using Link State Advertisements (LSA). OSPF routers quickly learns and distributes routing information, which includes the set of all routers and links between them and the cost of each link.

RFC 1583 defines OSPF Version 2.

OSPF Features and Benefits


Benefits and features of OSPF routing include:
• Least cost routing — Lets you configure path costs based on any combination
of network parameters, for example, bandwidth, delay, and dollar cost.
• Scaliability - OSPF works with large networks and has no limitation to the
routing metric or hop count.
• Area routing — Decreases the resources, such as memory and network
bandwidth, consumed by the protocol and provides an additional level of
routing protection.
• TOS routing - Packet routing based on Type of Service (TOS).
• Variable Length Subnet Masks — Lets you break an IP address into variable
size subnets, conserving IP address space.
• Routing authentication — Provides additional routing security.
• CIDR — Classless Interdomain Routing.
• OSPF supports IP subnetting and the tagging of externally derived routing
information. It uses IP multicast when sending or receiving packets.

OSPF Routing Environment
Introduction

This section describes the AS (Autonomous System) or the OSPF domain including
concepts such as:
• Division of the AS into OSPF areas, OSPF backbone area, and stub areas.
• Variable-length subnetting.
• Functions of internal routers, neighboring routers, Area Border Routers
(ABRs), and AS Boundary Routers (ASBRs).
• Use of Hello protocol.
• Definition of Virtual Links when the OSPF backbone area is not contiguous.

Area
An area is a collection of contiguous networks and hosts together with one or more IP network address ranges. Each address range is an [address, mask] pair. OSPF lets you summarize all the networks in an area into one or a few IP address ranges. Then you assign the area a 32-bit area number.
The topology of any one area is hidden from that of the other areas, which reduces routing traffic and protects routing within an area from outside influence.

Backbone Area
The backbone area is the set of contiguous networks that interconnect all areas. All OSPF domains must have at least one backbone network. The backbone area distributes inter-area routing information. The backbone has an area ID of 0.0.0.0 and consists of any of the following:
• Networks belonging to Area 0.0.0.0.
• Routers attached to those networks.
• Routers belonging to multiple areas.
• Configured virtual links (see the “Virtual Links” section).
Many small- to medium-sized OSPF networks consist of all networks connected to the backbone.

Stub Area
A stub area is an area that does not allow advertisement of external routes. Frequently, branch offices are configured as stub areas and are connected in a star configuration to a hub router. We recommend that when there are more than 40 routers running OSPF, you should define the OSPF areas to limit the OSPF algorithm completely. Typically, you configure the set of branch routers connected to a central hub router as a separate OSPF area.

Internal Routers
Internal routers have all their directly connected networks belonging to the same area. Routers with only backbone interfaces belong to this category. These routers run a single copy of the basic routing algorithm.

Area Border Router (ABR)
An Area Border Router (ABR) connects an area to the backbone. This connection to the backbone can be direct or through a virtual link (see “Virtual Links”, below) and summarizes the area contents by advertising a single route for each address range. If an ABR is attached to multiple areas, it can run multiple copies of the basic algorithm, one copy for each attached area and an additional copy for the backbone. ABRs condense the topology information of attached areas for distribution to the backbone. The backbone distributes the information to other ABRs.

AS Boundary Router (ASBR)
An AS Boundary Router (ASBR) exchanges information with routers that belong to other autonomous systems. Such a router has AS external routes that are advertised throughout the AS. External routes are any routes learned by means of static routing and Routing Information Protocol (RIP).

Hello Protocol
The Hello protocol is the part of OSPF used to establish and maintain neighbors. This protocol is used to form neighbors, establish bidirectional communication, and elect a DR and a backup DR on multi-access networks.

Designated Router(DR)
Each multi-access network having at least two attached routers has a Designated Router (DR). The DR generates a link state advertisement (LSA) for the multi-access network and has other special responsibilities in the running of the protocol. The DR is elected by the Hello protocol. Having a DR reduces the number of adjacencies required on a multi-access network. This in turn reduces the amount of routing protocol traffic.

Backup DR
The backup DR is elected on a multi-access network to ease the transition of DRs when the current DR becomes inactive.


Virtual Links
What are Virtual Links?
The backbone must be contiguous so that all areas are reachable. Virtual links are OSPF neighbor adjacencies established between two ABRs to maintain backbone connectivity. The virtual link forms a logical point-to-point serial connection between the endpoints and is a part of the backbone. When you need to configure a virtual link, configure the following in both ABRs:
• The virtual link.
• The virtual endpoint, which is the other ABR.
• The non-backbone area that the routers have in common, which is called the transit area.
Note
A virtual link cannot transit though a stub area.

Types of Routing

Introduction The three types of OSPF routing are:
• Intra-area
• Inter-area
• External
Intra-Area Routing
Intra-area routing occurs when a packet’s source and destination addresses are in the same area within an AS. The router sends Hello packets to its neighbors and in turn receives their Hello packets.
The router attempts to form adjacencies with some of its newly acquired neighbors. The topological databases are synchronized between pairs of adjacent routers.
On multi-access networks, the DR determines which routers should become adjacent. Adjacencies control the distribution of routing protocol packets. Routing protocol packets are sent and received only on adjacencies. In particular, distribution of topological database updates proceeds along adjacencies.
Inter-Area Routing
Inter-area routing occurs when the packet’s source and destination addresses are in different areas within an AS. The ABRs form the adjacencies and synchronize the topological database.

External Routing
Routers that have information regarding other autonomous systems can flood this information throughout an AS. This external routing information is distributed to every router except for those connecting to stub areas.

OSPF Link State Advertisements
Description
A Link State Advertisement (LSA) describes a link from a router to its interface,
including the metric. An LSA is flooded to all routers in an area to form the area database describing the topology of the area. It is originated on a delta basis, whenever information changes, or at least every 30 minutes otherwise. From this database, each router generates a Shortest Path First (SPF) route with itself as the route of the tree. Then the router forms the routing table.
LSA Format
An LSA is sent in LS update packets that are queued on LS Retransmit queues. Each LSA must be explicitly acknowledged in an LS Acknowledgment packet or it is retransmitted at the retransmission interval (5 seconds).
Each LSA has a sequence number established by its originator. Sequence numbers
are signed 32-bit integers starting at -2**32 -1 (0x8000001) and incrementing to
2**32-1 (0x7FFFFFFF).
LSA Components
An LSA has:
• Link State (LS) type (8 bits) — values 1 to 5.
• Link State ID (32 bits) — the destination of the link.
• Advertising Router (32 bits) — the source of the link.

LSA Types

Router Link - Originated by all routers and flooded throughout a single area only. It describes a router’s interfaces to a network or a router’s links to another router.
Network Link - Generated for multi-access advertisement networks by the DR. It contains the list of the attached routers to the network.
Net Summary - Sent by the ABR summarizing net and mask of all networks in the area.
ASBR - Sent by the ABR and contains the routes to the ASBRs.
External - Sent by ASBRs and contains AS external routes. It is flooded into all but stub areas.

When Are LSAs Generated?
LSAs are generated in these cases:
• The LS refresh time expires.
• An interface’s state changes.
• An attached network’s DR changes.
• One of the neighboring routers changes to or from full state.

LSAs are generated by ABRs when:
• An intra-area route (see the “Types of Routing” section) has been added,
deleted, or modified in the routing table.
• An inter-area route has been added, deleted, or modified in the routing table.
• The router becomes newly attached to an area.

Variable-Length Subnetting
Introduction OSPF attaches an IP address mask to each advertised route. The mask indicates the range of addresses being described by the particular route. Including the mask with each advertised destination enables variable-length subnetting. This means that you can break a single IP class A, B, or C number into many subnets of various sizes. A key advantage of defining OSPF areas is that you can summarize the networks at the area with a single variable-length subnet advertisement.

Example of VLSM

An area that contains networks with IP net numbers 128.185.1.0 to 128.185.31.0 may be summarized in a single advertisement of the network 128.185.0.0 with a mask of 255.255.224.0. The Area Summary Statistics defines how an Area Border Router (see “Area Border Router (ABR)” section on page 1-3) summarizes the networks in its area when advertising on the OSPF backbone. In this example, the ABR sends one network summary advertisement rather than 32 individual network advertisements.

CIDR Support for OSPF
Classless Interdomain Routing (CIDR) is supported for OSPF.
CIDR Feature CIDR support for OSPF includes the following:
• Enhancements of the area range configuration to allow aggregation of routes
on a classless boundary.
• Configuration of a mask on each interface to allow a classless boundary. Configuration of the classless mask on the IP interface allows aggregation of directly connected multiple networks on that interface. The router LSA will contain classless routes only if the interface is not a point-to-point interface. The router LSA will be built as specified by the RFC and would use the configured classless mask.
• Support for external route aggregation on classbased or classless boundary.