IP subnet routing
Chapter 3. Routing Protocols
3.3 Interior Gateway Protocols (IGP)
3.3.4 Open Shortest Path First (OSPF)
3.3.4.1 OSPF Terminology
OSPF uses specific terminology which must be understood before the protocol can be described.
Areas: OSPF internetworks are organized into areas. An OSPF area consists of a number of networks and routers that are logically grouped together. Areas can be defined on a per location or a per region basis, or they can be based on administrative boundaries. All OSPF networks consist of at least one area, the backbone, plus as many additional areas as are demanded by network topology and other design criteria. Within an OSPF area all routers maintain the same topology database, exchanging link state information to maintain their
synchronization. This ensures that all routers calculate the same network map for the area.
Information about networks outside an area is summarized by an area border or AS boundary routers (see “Intra-Area, Area Border and AS Boundary Routers”) and flooded into the area. Routers within an area have no knowledge of the topology of networks outside the area, only of routes to destinations provided by area borders and AS boundary routers.
The importance of the area concept is that it limits the size of the topology
database that must be held by routers. This has direct impact on the processing to be carried out by each router, and on the amount of link state information that must be flooded into individual networks.
The OSPF Backbone: All OSPF networks must contain at least one area, the backbone, which is assigned an area identifier of 0.0.0.0. (This is different
definition from IP address 0.0.0.0.) The backbone has all the properties of an area, but has the additional responsibility of distributing routing information between areas attached to it. Normally an OSPF backbone should be contiguous, that is with all backbone routers attached one to another. This may not be possible because of network topology, in which case backbone continuity must be maintained by the use of virtual links (see below). Virtual links are backbone router-to-backbone router connections that traverse a non-backbone area.
Routers within the backbone operate identically to other intra-area routers and maintain full topology databases for the backbone area.
Intra-Area, Area Border and AS Boundary Routers: There are three possible types of routers in an OSPF network. Figure 81 on page 114 shows the location of intra-area, area border and AS boundary routers within an OSPF internetwork.
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OSPF Backbone
Area 2 AB
As External Links
Area 1 AB ASB
As External Links AB
ASB
Area 4 AB
Area 3
IA IA
Area 4
Key ASB AB IA
= AS Boundary Router
= Area Border Router
= Intra-Area Router Figure 81. OSPF Network
Intra-Area Routers
Routers that are situated entirely within an OSPF area are called intra-area routers. All intra-area routers flood router links advertisements into the area to define the links they are attached to. If they are elected designated or backup-designated routers (see “Designated and Backup Designated Router”
on page 116), they also flood network links advertisements to define the identity of all routers attached to the network. Intra-area routers maintain a topology database for the area in which they are situated.
Area Border Routers
Routers that connect two or more areas are referred to as area border routers. Area border routers maintain topology databases for each area to which they are attached, and exchange link state information with other routers in those areas. Area border routers also flood summary link state advertisements into each area to inform them of inter-area routes.
AS Boundary Routers
Routers that are situated at the periphery of an OSPF internetwork and exchange reachability information with routers in other ASs using exterior gateway protocols are called AS boundary routers. Routers that import static routes or routes from other IGPs, such as RIP, into an OSPF network are also AS boundary routers. AS boundary routers are responsible for flooding AS external link state advertisements into all areas within the AS to inform them of external routes.
Virtual Link: A virtual link is part of the backbone. Its endpoints are two area border routers that share a common non-backbone area. The link is treated as a point-to-point link with metrics cost equal to the intra-area metrics between the endpoints of the links. The routing through the virtual link is done using normal
intra-area routing (see Figure 82 on page 115). Virtual endpoints are area border routers (ABRs) that share Area 2 as a transit area.
Transit Area: An area through which a virtual route is physically connected. In Figure 82, Area 2 is transit area. In Figure 82, virtual endpoints are ABRs that share Area 2 as a transit area.
ABR Area 0
Backbone
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Area 1
Area 2 ABR
Figure 82. OSPF Virtual Link, Transit Area
Stub Area: An area configured to use default routing for inter-AS routing. A stub area can be configured where there is only a single exit from the area, or where any exit can be used without preference for routing to destinations outside the autonomous system. By default inter-AS routes are copied to all areas, so the use of stub areas can reduce the storage requirements of routers within those areas for autonomous systems where a lot of inter-AS routes are defined.
Neighbor Routers: Two routers that have interfaces to a common network. On multiaccess networks, neighbors are dynamically discovered by the Hello protocol.
Each neighbor is described by a state machine, which describes the conversation between this router and its neighbor. A brief outline of the meaning of the states follows. See the section immediately following for a definition of the terms adjacency and designated router.
Down
Initial state of a neighbor conversation. It indicates that there has been no recent information received from the neighbor.
Attempt
A neighbor on a non-broadcast network appears down and an attempt should be made to contact it by sending regular Hello packets.
Init A Hello packet has recently been received from the neighbor. However, bidirectional communication has not yet been established with the neighbor.
(That is, the router itself did not appear in the neighbor's Hello packet.) 2-way
In this state, communication between the two routers is bidirectional.
Adjacencies can be established, and neighbors in this state or higher are eligible to be elected as (backup) designated routers.
ExStart
The two neighbors are about to create an adjacency.
Exchange
The two neighbors are telling each other what they have in their topological databases.
Loading
The two neighbors are synchronizing their topological databases.
Full The two neighbors are now fully adjacent; their databases are synchronized.
Various events cause a change of state. For example, if a router receives a Hello packet from a neighbor that is down, the neighbor's state changes to init, and an inactivity timer is started. If the timer fires (that is, no further OSPF packets are received before it expires), the neighbor will return to the down state. Refer to RFC 2173 for a complete description of the states and information on the events which cause state changes.
Adjacent Router: Neighbor routers can become adjacent. They are said to be adjacent when they have synchronized their topology databases through the exchange of link state information.
Link state information is exchanged only between adjacent routers, not between neighbor routers.
Not all neighbor routers become adjacent. Neighbors on point-to-point links do so, but on multi-access networks adjacencies are only formed between individual routers and the designated and backup designated routers.
The exchange of link state information between neighbors can create significant amounts of network traffic. Limiting the number of adjacencies on multi-access networks in this way achieves considerable reductions in network traffic.
Designated and Backup Designated Router: All multi-access networks have a designated and a backup designated router. These routers are elected
automatically for each network once neighbor routers have been discovered by the Hello protocol.
The designated router performs two key roles for a network:
It generates network links advertisements that list the routers attached to a multi-access network.
It forms adjacencies with all routers on a multi-access network and therefore becomes the focal point for forwarding of all link state advertisements.
The backup designated router forms the same adjacencies as the designated router. It therefore has the same topology database and is able to assume designated router functions should it detect that the designated router has failed.
Physical Network Types: All OSPF areas consist of aggregates of networks linked by routers. OSPF categorizes networks into the following different types.
Point-to-point Network
Point-to-point networks directly link two routers.
Multi-Access Network
Multi-access networks are those that support the attachment of more than two routers. They are further subdivided into two types:
Broadcast
Non-broadcast Point-to-Multipoint Network
Point-to-multipoint networks describe a special case of multiaccess
non-broadcast where not every router has a direct connection to any other router (also referred to as partial mesh).
Broadcast networks have the capability of directing OSPF packets to all attached routers, using an address that is recognized by all of them. An Ethernet LAN and token-ring LAN are examples of a broadcast multi-access network.
Non-broadcast networks do not have this capability and all packets must be specifically addressed to routers on the network. This requires that routers on a non-broadcast network be configured with the addresses of neighbors. Examples of a non-broadcast multi-access network are the X.25 public data network or a frame relay network
Interface: The connection between a router and one of its attached networks.
Each interface has state information associated with it that is obtained from the underlying lower-level protocols and the OSPF protocol itself. A brief description of each state is given here. Please refer to RFC 2173 for more details, and for information on the events that will cause an interface to change its state.
Down
The interface is unavailable. This is the initial state of an interface.
Loopback
The interface is looped back to the router. It cannot be used for regular data traffic.
Waiting
The router is trying to determine the identity of the designated router or its backup.
Point-to-Point
The interface is to a point-to-point network or is a virtual link. The router forms an adjacency with the router at the other end.
Note: The interfaces do not need IP addresses. Since the remainder of the internetwork has no practical need to see the routers' interfaces to the point-to-point link, just the interfaces to other networks, any IP
addresses for the link would be needed only for communication between the two routers. To conserve the IP address space, the routers can dispense with IP addresses on the link. This has the effect of making the two routers appear to be one to IP but this has no ill effects. Such a link is called an unnumbered link.
DR Other
The interface is on a multiaccess network but this router is neither the designated router nor its backup. The router forms adjacencies with the designated router and its backup.
Backup
The router is the backup designated router. It will be promoted to designated router if the present designated router fails. The router forms adjacencies with every other router on the network.
DR The router itself is the designated router. The router forms adjacencies with every other router on the network. The router must also originate a network links advertisement for the network node.
Type of Service (TOS) Metrics: In each type of link state advertisement, different metrics can be advertised for each IP Type of Service. A metric for TOS 0 (used for OSPF routing protocol packets) must always be specified. Metrics for other TOS values can be specified; if they are not, these metrics are assumed equal to the metric specified for TOS 0.5
Link State Database: Also called the directed graph or the topological database.
It is created from the link state advertisements generated by the routers in the area.
Note: RFC 2328 uses the term link state database in preference to topological database. The former term has the advantage that it describes the contents of the database, the latter is more descriptive of the purpose of the
database, to describe the topology of the area. We have previously used the term topological database for this reason, but for the remainder of this section where we discuss the operation of OSPF in more detail, we refer to it as the link state database.
Shortest-Path Tree: Each router runs the SPF (see SPF in 3.2.3.1,
“Shortest-Path First (SPF)” on page 105) algorithm on the link state database to obtain its shortest-path tree. The tree gives the route to any destination network or host as far as the area boundary. It is used to build the routing table.
Note: Because each router occupies a different place in the area's topology, application of the SPF algorithm gives a different tree for each router, even though the database is identical.
Area border routers run multiple copies of the algorithm but build a single routing table.
Routing table: The routing table contains entries for each destination: network, subnet or host. For each destination, there is information for one or more types of service (TOS). For each combination of destination and type of service, there are entries for one or more optimum paths to be used.
Area ID: A 32-bit number identifying a particular area. The backbone has an area ID of zero.
5 The use of TOS has been dropped in recent OSPF implementations.
Router ID: A 32-bit number identifying a particular router. Each router within the AS has a single router ID. One possible implementation is to use the lowest numbered IP address belonging to a router as its router ID.
Router Priority: An 8-bit unsigned integer, configurable on a per-interface basis indicating this router's priority in the selection of the (backup) designated router. A router priority of zero indicates that this router is ineligible to be the designated router.
Link State Advertisements: Link state information is exchanged by adjacent OSPF routers to allow area topology databases to be maintained, and inter-area and inter-AS routes to be advertised.
Link state information consists of five types of link state advertisement. Together these provide all the information needed to describe an OSPF network and its external environment:
1. Router links 2. Network links
3. Summary links (type 3 and 4) 4. AS External links
Router link advertisements
Router link advertisements are generated by all OSPF routers and describe the state of the router's interfaces (links) within the area. They are flooded throughout a single area only.
Network link advertisements
Network link advertisements are generated by the designated router on a multi-access network and list the routers connected to the network. They are flooded throughout a single area only.
Summary link advertisements
Summary link advertisements are generated by area border routers. There are two types: one describes routes to destinations in other areas; the other descirbes routes to AS boundary routers. They are flooded throughout a single area only.
AS external link advertisements
AS external link advertisements are generated by AS boundary routers and describe routes to destinations external to the OSPF network. They are flooded throughout all areas in the OSPF network.
Router Router Links
Advertised by router Describes state/cost of routers' links
DR Network Links
Advertised by designated router Describes all routers attached to network
ABR Summary Links
Advertised by ABR Describes inter-area and ASBR reachability
Area X Area O ASBR
External Links
Advertised by ASBR Describes networks outside of OSPF AS
Area X Area O
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Figure 83. OSPF Link State Advertisements