Link State Routing
Protocols
Introduction
Link State protocols,
unlike Distance Vector
broadcasts, use multicast.
Multicast
is a "broadcast" to a group of hosts, in this case
routers (Please see the
multicast page for more information). So if I had 10
router of which 4 where part of a "mutilcast group"
then, when I send out a multicast packet to this group,
only these 4 routers will receive the updates, while the
rest of them will simply ignore the data. The multicast
address is usually 224.0.0.5 & 224.0.0.6, this address
is defined by the IGRP
(Interior Gateway Routing
Protocol).
Link State routing
protocols do not view networks in terms of adjacent
routers and hop counts, but they build a comprehensive
view of the overall network which fully describes the
all possible routes along with their costs. Using the
SPF (Shortest
Path First) algorithm, the router creates a
"topological database" which is a hierarchy reflecting
the network routers it knows about. It then puts it's
self on the top of this hierarchy, and has a complete
picture from it's own perspective.
When a
router using a Link State
protocol, such a OSPF (Open
Shortest Path First) knows about a change on the
network, it will multicast this change instantly, there
for flooding the network with this information. The
information routers require to build their databases is
provided in the form of Link State
advertisement packets (LSAP). Routers do not advertise
their entire routing tables, instead each router
advertises only its information regarding immediately
adjacent routers.
Link State protocols in
comparison to Distance Vector
protocols have:
-
Shortest path computations
require many CPU circles
-
If network is stable little
bandwidth is used; react quickly to topology changes
-
Announcements cannot be
“filtered”. All items in the database must be sent
to neighbors
-
All neighbors must be trusted
-
Authentication mechanisms can
be used to avoid undesired adjacencies
-
No split horizon techniques
are possible
Even though
Link State protocols work more efficiently,
problem can arise. Usually problems occur cause of
changes in the network topology (links go up-down), and
all routers don't get updated immediately cause they
might be on different line speeds, there for, routers
connected via a fast link will receive these changes
faster than the others on a slower link.
Different
techniques have been developed to deal with these
problem and these are :
1)
Dampen update frequency
2)
Target link-state updates to
multicast
3)
Use link-state area hierarchy for
topology
4)
Exchange route summaries at area
borders
5)
Use Time-stamps Update numbering &
counters
6)
Manage partitions using a area
hierarchy
Please select one of the following
Link State routing protocols:
Open Shortest
Path First - OSPF
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