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452 C
HAPTER
19: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF) R
OUTING
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Support for virtual links to noncontiguous areas — As discussed
earlier, OSPF can partition large autonomous systems into smaller,
more manageable subdivisions, called areas. An OSPF backbone is
responsible for distributing routing information between the areas of
an autonomous system. This backbone itself has all the properties of
an area and consists of those networks that are not contained in any
area. Although the backbone must be contiguous, backbone routers
can also be connected by means of a virtual link. Virtual links can be
configured between any two backbone routers that have an interface
to a common nonbackbone area. OSPF treats two routers that are
joined by a virtual link as if they were connected by an unnumbered
point-to-point network. For more information, see “Virtual Links”
later in this chapter.
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Variable length subnet mask support — OSPF considers both the
IP address and subnet mask in determining the best route for a
packet. An IP address mask is distributed with each advertised route.
The mask indicates the range of addresses that are being described by
the particular route. Including this mask enables the implementation
of variable-length subnet masks (VLSMs), which means that a single IP
network number can be
subnetworked or broken up into many
subnetworks of various sizes. When networks are subnetworked,
OSPF forwards each IP packet to the network that is the best match
for the packet’s destination. It determines the best match by
examining both the network address and the mask for each
destination and finding the longest or most specific match. VLSM
support is a key advantage for OSPF, especially when you consider the
shortage of IP addresses. Another advantage is its flexibility.
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Ability to import non-OSPF routing information — Connectivity
from one autonomous system to another is achieved through OSPF
autonomous system boundary routers (ASBRs). ASBRs can import
external link advertisements that contain information about external
networks from other protocols like RIP and redistribute them as LSAs
to the OSPF network. In this way, ASBRs flood information about
external networks to routers within the OSPF network.
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Assurance of loop-free networks — The algorithm that
dynamically calculates routing paths within the autonomous system
does not generate a path with internal loops.