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Unformatted text preview: ement from node F in a network. The arrows show the same advertisement
being re-broadcast (at different points in time) as part of the ﬂooding process once per node, along all of
the links connected to the node. The link state is shown in this example for one node; in practice, there is
one of these originating from each node in the network, and re-broadcast by the other nodes. consists of the nodes and currently working links (as evidenced by the HELLO protocol at
the nodes). Armed with the complete map of the network, each node can independently
run a centralized computation to ﬁnd the shortest routes to each destination in the network.
As long as all the nodes optimize the same metric for each destination, the resulting routes
at the different nodes will correspond to a valid path to use. In contrast, in a distancevector protocol, the actual computation of the routes is distributed, with no node having
any signiﬁcant knowledge about the topology of the network. A link-state protocol distributes information about the state of each link (hence the name) and node in the topology
to all the nodes, and as long as the nodes have a consistent view of the topology and optimize the
same metric, routing will work as desired.
18.5.1 Flooding link-state advertisements Each node uses the HELLO protocol (mentioned earlier, and which we will discuss in the
next lecture in more detail) to maintain a list of current neighbors. Periodically, every
ADVERT INTERVAL, the node constructs a link-state advertisement (LSA) and sends it along
all its links. The LSA has the following format:
[origin addr, seq, (nbhr1, linkcost1), (nbhr2, linkcost2), (nbhr3, linkcost3), ...]
Here, “origin addr” is the address of the node constructing the LSA, each “nbhr” refers
to a currently active neighbor (the next lecture will describe more precisely what “currently
active” means), and the “linkcost” refers to the cost of the corresponding link. An example
is shown in Figure 18-5.
In addition, the LSA has a sequence number, “seq”,...
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This document was uploaded on 02/26/2014 for the course CS 6.02 at MIT.
- Fall '13