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11. The Routing Table Structure Connected: An Internet Encyclopedia
11. The Routing Table Structure

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11. The Routing Table Structure

11. The Routing Table Structure

The routing table data structure contains all the information necessary to forward an IP data packet toward its destination. Each routing table entry describes the collection of best paths to a particular destination. When forwarding an IP data packet, the routing table entry providing the best match for the packet's IP destination is located. The matching routing table entry then provides the next hop towards the packet's destination. OSPF also provides for the existence of a default route (Destination ID = DefaultDestination, Address Mask = 0x00000000). When the default route exists, it matches all IP destinations (although any other matching entry is a better match). Finding the routing table entry that best matches an IP destination is further described in Section 11.1.

There is a single routing table in each router. Two sample routing tables are described in Sections 11.2 and 11.3. The building of the routing table is discussed in Section 16.

The rest of this section defines the fields found in a routing table entry. The first set of fields describes the routing table entry's destination.

Destination Type

The destination can be one of three types. Only the first type, Network, is actually used when forwarding IP data traffic. The other destinations are used solely as intermediate steps in the routing table build process.

Network

A range of IP addresses, to which IP data traffic may be forwarded. This includes IP networks (class A, B, or C), IP subnets, IP supernets and single IP hosts. The default route also falls in this category.

Area border router

Routers that are connected to multiple OSPF areas. Such routers originate summary link advertisements. These routing table entries are used when calculating the inter- area routes (see Section 16.2). These routing table entries may also be associated with configured virtual links.

AS boundary router

Routers that originate AS external link advertisements. These routing table entries are used when calculating the AS external routes (see Section 16.4).

Destination ID

The destination's identifier or name. This depends on the Destination Type. For networks, the identifier is their associated IP address. For all other types, the identifier is the OSPF Router ID.[9]

Address Mask

Only defined for networks. The network's IP address together with its address mask defines a range of IP addresses. For IP subnets, the address mask is referred to as the subnet mask. For host routes, the mask is "all ones" (0xffffffff).

Optional Capabilities

When the destination is a router (either an area border router or an AS boundary router) this field indicates the optional OSPF capabilities supported by the destination router. The two optional capabilities currently defined by this specification are the ability to route based on IP TOS and the ability to process AS external link advertisements. For a further discussion of OSPF's optional capabilities, see Section 4.5.

The set of paths to use for a destination may vary based on IP Type of Service and the OSPF area to which the paths belong. This means that there may be multiple routing table entries for the same destination, depending on the values of the next two fields.

Type of Service

There can be a separate set of routes for each IP Type of Service. The encoding of TOS in OSPF link state advertisements is described in Section 12.3.

Area

This field indicates the area whose link state information has led to the routing table entry's collection of paths. This is called the entry's associated area. For sets of AS external paths, this field is not defined. For destinations of type "area border router", there may be separate sets of paths (and therefore separate routing table entries) associated with each of several areas. This will happen when two area border routers share multiple areas in common. For all other destination types, only the set of paths associated with the best area (the one providing the shortest route) is kept.

The rest of the routing table entry describes the set of paths to the destination. The following fields pertain to the set of paths as a whole. In other words, each one of the paths contained in a routing table entry is of the same path-type and cost (see below).

Path-type

There are four possible types of paths used to route traffic to the destination, listed here in order of preference: intra-area, inter-area, type 1 external or type 2 external. Intra-area paths indicate destinations belonging to one of the router's attached areas. Inter-area paths are paths to destinations in other OSPF areas. These are discovered through the examination of received summary link advertisements. AS external paths are paths to destinations external to the AS. These are detected through the examination of received AS external link advertisements.

Cost

The link state cost of the path to the destination. For all paths except type 2 external paths this describes the entire path's cost. For Type 2 external paths, this field describes the cost of the portion of the path internal to the AS. This cost is calculated as the sum of the costs of the path's constituent links.

Type 2 cost

Only valid for type 2 external paths. For these paths, this field indicates the cost of the path's external portion. This cost has been advertised by an AS boundary router, and is the most significant part of the total path cost. For example, a type 2 external path with type 2 cost of 5 is always preferred over a path with type 2 cost of 10, regardless of the cost of the two paths' internal components.

Link State Origin

Valid only for intra-area paths, this field indicates the link state advertisement (router links or network links) that directly references the destination. For example, if the destination is a transit network, this is the transit network's network links advertisement. If the destination is a stub network, this is the router links advertisement for the attached router. The advertisement is discovered during the shortest- path tree calculation (see Section 16.1). Multiple advertisements may reference the destination, however a tie- breaking scheme always reduces the choice to a single advertisement. The Link State Origin field is not used by the OSPF protocol, but it is used by the routing table calculation in OSPF's Multicast routing extensions (MOSPF).

When multiple paths of equal path-type and cost exist to a destination (called elsewhere "equal-cost" paths), they are stored in a single routing table entry. Each one of the "equal-cost" paths is distinguished by the following fields:

Next hop

The outgoing router interface to use when forwarding traffic to the destination. On multi-access networks, the next hop also includes the IP address of the next router (if any) in the path towards the destination. This next router will always be one of the adjacent neighbors.

Advertising router

Valid only for inter-area and AS external paths. This field indicates the Router ID of the router advertising the summary link or AS external link that led to this path.


Next: 11.1. Routing table lookup

Connected: An Internet Encyclopedia
11. The Routing Table Structure

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