- OSPF Route Class
Routing protocols that have areas or make a distinction between
internal and external routes divide their routes into classes
by the type of information used to calculate the route. A
route is always chosen from the most preferred class unless
none is available, in which case one is chosen from the second
most preferred class, and so on. In OSPF, the classes (in
order from most preferred to least preferred) are intra-area,
inter-area, type 1 external (external routes with internal
metrics), and type 2 external. As an additional wrinkle, a
router is configured to know what addresses ought to be
accessible using intra-area routes, and will not use inter-
area or external routes to reach these destinations even when
no intra-area route is available.
More precisely, we assume that each route has a class
attribute, called route.class, which is assigned by the routing
protocol. The set of candidate routes is examined to determine
if it contains any for which route.class = intra-area. If so,
all routes except those for which route.class = intra-area are
discarded. Otherwise, router checks whether the packet's
destination falls within the address ranges configured for the
local area. If so, the entire set of candidate routes is
deleted. Otherwise, the set of candidate routes is examined to
determine if it contains any for which route.class = inter-
area. If so, all routes except those for which route.class =
inter-area are discarded. Otherwise, the set of candidate
routes is examined to determine if it contains any for which
route.class = type 1 external. If so, all routes except those
for which route.class = type 1 external are discarded.
- IS-IS Route Class
IS-IS route classes work identically to OSPF's. However, the
set of classes defined by Integrated IS-IS is different, such
that there isn't a one-to-one mapping between IS-IS route
classes and OSPF route classes. The route classes used by
Integrated IS-IS are (in order from most preferred to least
preferred) intra-area, inter-area, and external.
The Integrated IS-IS internal class is equivalent to the OSPF
internal class. Likewise, the Integrated IS-IS external class
is equivalent to OSPF's type 2 external class. However,
Integrated IS-IS does not make a distinction between inter-area
routes and external routes with internal metrics - both are
considered to be inter-area routes. Thus, OSPF prefers true
inter-area routes over external routes with internal metrics,
whereas Integrated IS-IS gives the two types of routes equal
preference.
- IDPR Policy
A specific case of Policy. The IETF's Inter-domain Policy
Routing Working Group is devising a routing protocol called
Inter-Domain Policy Routing (IDPR) to support true policy-based
routing in the Internet. Packets with certain combinations of
header attributes (such as specific combinations of source and
destination addresses or special IDPR source route options) are
required to use routes provided by the IDPR protocol. Thus,
unlike other Policy pruning rules, IDPR Policy would have to be
applied before any other pruning rules except Basic Match.
Specifically, IDPR Policy examines the packet being forwarded
to ascertain if its attributes require that it be forwarded
using policy-based routes. If so, IDPR Policy deletes all
routes not provided by the IDPR protocol.