11. The Routing Table Structure
- 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
Destination type is either "network" or "router". Only network
entries are actually used when forwarding IP data traffic.
Router routing table entries are used solely as intermediate
steps in the routing table build process.
A network is 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 into this category.
Router entries are kept for area border routers and AS boundary
routers. Routing table entries for area border routers are used
when calculating the inter-area routes (see Section 16.2), and
when maintaining configured virtual links (see Section 15).
Routing table entries for AS boundary routers 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 routers, 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 this field indicates the
optional OSPF capabilities supported by the destination router.
The only optional capability defined by this specification is
the ability to process AS-external-LSAs. 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 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 field.
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
"router", there may be separate sets of paths (and therefore
separate routing table entries) associated with each of several
areas. For example, this will happen when two area border
routers share multiple areas in common. For destinations of
type "network", only the set of paths associated with the best
area (the one providing the preferred 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 decreasing 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-LSAs. AS external paths are
paths to destinations external to the AS. These are detected
through the examination of received AS-external-LSAs.
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 LSA
(router-LSA or network-LSA) that directly references the
destination. For example, if the destination is a transit
network, this is the transit network's network-LSA. If the
destination is a stub network, this is the router-LSA for the
attached router. The LSA is discovered during the shortest-path
tree calculation (see Section 16.1). Multiple LSAs may
reference the destination, however a tie-breaking scheme always
reduces the choice to a single LSA. 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 broadcast, Point-to-MultiPoint and NBMA
networks, the next hop also includes the IP address of the next
router (if any) in the path towards the destination.
Advertising router
Valid only for inter-area and AS external paths. This field
indicates the Router ID of the router advertising the summary-
LSA or AS-external-LSA that led to this path.
11.1. Routing table lookup
When an IP data packet is received, an OSPF router finds the
routing table entry that best matches the packet's destination.
This routing table entry then provides the outgoing interface
and next hop router to use in forwarding the packet. This
section describes the process of finding the best matching
routing table entry.
Before the lookup begins, "discard" routing table entries should
be inserted into the routing table for each of the router's
active area address ranges (see Section 3.5). (An area range is
considered "active" if the range contains one or more networks
reachable by intra-area paths.) The destination of a "discard"
entry is the set of addresses described by its associated active
area address range, and the path type of each "discard" entry is
set to "inter-area".[10]
Several routing table entries may match the destination address.
In this case, the "best match" is the routing table entry that
provides the most specific (longest) match. Another way of
saying this is to choose the entry that specifies the narrowest
range of IP addresses.[11] For example, the entry for the
address/mask pair of (128.185.1.0, 0xffffff00) is more specific
than an entry for the pair (128.185.0.0, 0xffff0000). The
default route is the least specific match, since it matches all
destinations. (Note that for any single routing table entry,
multiple paths may be possible. In these cases, the calculations
in Sections 16.1, 16.2, and 16.4 always yield the paths having
the most preferential path-type, as described in Section 11).
If there is no matching routing table entry, or the best match
routing table entry is one of the above "discard" routing table
entries, then the packet's IP destination is considered
unreachable. Instead of being forwarded, the packet should then
be discarded and an ICMP destination unreachable message should
be returned to the packet's source.
11.2. Sample routing table, without areas
Consider the Autonomous System pictured in Figure 2. No OSPF
areas have been configured. A single metric is shown per
outbound interface. The calculation of Router RT6's routing
table proceeds as described in Section 2.2. The resulting
routing table is shown in Table 12. Destination types are
abbreviated: Network as "N", Router as "R".
There are no instances of multiple equal-cost shortest paths in
this example. Also, since there are no areas, there are no
inter-area paths.
Routers RT5 and RT7 are AS boundary routers. Intra-area routes
have been calculated to Routers RT5 and RT7. This allows
external routes to be calculated to the destinations advertised
by RT5 and RT7 (i.e., Networks N12, N13, N14 and N15). It is
assumed all AS-external-LSAs originated by RT5 and RT7 are
advertising type 1 external metrics. This results in type 1
external paths being calculated to destinations N12-N15.
11.3. Sample routing table, with areas
Consider the previous example, this time split into OSPF areas.
An OSPF area configuration is pictured in Figure 6. Router
RT4's routing table will be described for this area
configuration. Router RT4 has a connection to Area 1 and a
backbone connection. This causes Router RT4 to view the AS as
the concatenation of the two graphs shown in Figures 7 and 8.
The resulting routing table is displayed in Table 13.
Again, Routers RT5 and RT7 are AS boundary routers. Routers
RT3, RT4, RT7, RT10 and RT11 are area border routers. Note that
there are two routing entries for the area border router RT3,
since it has two areas in common with RT4 (Area 1 and the
backbone).
Backbone paths have been calculated to all area border routers.
These are used when determining the inter-area routes. Note
that all of the inter-area routes are associated with the
backbone; this is always the case when the calculating router is
itself an area border router. Routing information is condensed
at area boundaries. In this example, we assume that Area 3 has
been defined so that networks N9-N11 and the host route to H1
Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
____________________________________________________________
N N1 0 intra-area 10 RT3 *
N N2 0 intra-area 10 RT3 *
N N3 0 intra-area 7 RT3 *
N N4 0 intra-area 8 RT3 *
N Ib 0 intra-area 7 * *
N Ia 0 intra-area 12 RT10 *
N N6 0 intra-area 8 RT10 *
N N7 0 intra-area 12 RT10 *
N N8 0 intra-area 10 RT10 *
N N9 0 intra-area 11 RT10 *
N N10 0 intra-area 13 RT10 *
N N11 0 intra-area 14 RT10 *
N H1 0 intra-area 21 RT10 *
R RT5 0 intra-area 6 RT5 *
R RT7 0 intra-area 8 RT10 *
____________________________________________________________
N N12 * type 1 ext. 10 RT10 RT7
N N13 * type 1 ext. 14 RT5 RT5
N N14 * type 1 ext. 14 RT5 RT5
N N15 * type 1 ext. 17 RT10 RT7
Table 12: The routing table for Router RT6
(no configured areas).
are all condensed to a single route when advertised into the
backbone (by Router RT11). Note that the cost of this route is
the maximum of the set of costs to its individual components.
There is a virtual link configured between Routers RT10 and
RT11. Without this configured virtual link, RT11 would be
unable to advertise a route for networks N9-N11 and Host H1 into
the backbone, and there would not be an entry for these networks
in Router RT4's routing table.
In this example there are two equal-cost paths to Network N12.
However, they both use the same next hop (Router RT5).
Router RT4's routing table would improve (i.e., some of the
paths in the routing table would become shorter) if an
additional virtual link were configured between Router RT4 and
Router RT3. The new virtual link would itself be associated
with the first entry for area border router RT3 in Table 13 (an
intra-area path through Area 1). This would yield a cost of 1
for the virtual link. The routing table entries changes that
would be caused by the addition of this virtual link are shown
Type Dest Area Path Type Cost Next Adv. Hops(s) Router(s)
N N1 1 intra-area 4 RT1 * N N2 1 intra-area 4 RT2 * N N3 1 intra-area 1 * * N N4 1 intra-area 3 RT3 * R RT3 1 intra-area 1 * *
N Ib 0 intra-area 22 RT5 * N Ia 0 intra-area 27 RT5 * R RT3 0 intra-area 21 RT5 * R RT5 0 intra-area 8 * * R RT7 0 intra-area 14 RT5 * R RT10 0 intra-area 22 RT5 * R RT11 0 intra-area 25 RT5 *
N N6 0 inter-area 15 RT5 RT7 N N7 0 inter-area 19 RT5 RT7 N N8 0 inter-area 18 RT5 RT7 N N9-N11,H1 0 inter-area 36 RT5 RT11
N N12 * type 1 ext. 16 RT5 RT5,RT7 N N13 * type 1 ext. 16 RT5 RT5 N N14 * type 1 ext. 16 RT5 RT5 N N15 * type 1 ext. 23 RT5 RT7
Table 13: Router RT4's routing table
in the presence of areas.
in Table 14.