Information Technology Reference
In-Depth Information
Small Hello packets are periodically exchanged between two adjacent neighbors, which serve as a
“keepalive”
function to monitor the state of the neighbor. If a router stops receiving Hello packets
from a neighbor, that neighbor is considered
unreachable
and the adjacency is broken.
Building the Link-State Packet
The third step in the link-state routing process is
each router builds a link-state packet (LSP) contain-
ing the state of each directly connected link.
After a router has established its adjacencies, it can build its link-state packets (LSP) that contain the
link-state information about its links. The table you filled out previously contains all the information
for R5's LSP.
Flooding Link-State Packets to Neighbors
The fourth step in the link-state routing process is
each router floods the LSP to all neighbors, who
then store all LSPs received in a database.
What happens when a router receives an LSP from a neighbor?
Whenever a router receives an LSP from a neighboring router, it immediately sends that LSP out all
other interfaces except the interface that received the LSP. This process creates a flooding effect of
LSPs from all routers throughout the routing area.
With triggered updates, distance vector routing protocols also send out updates when there is a
change. What is one reason why link-state routing protocols converge faster?
Unlike distance vector routing protocols, which must first run the Bellman-Ford algorithm to process
routing updates before sending them to other routers, link-state routing protocols calculate the SPF
algorithm after the flooding is complete. As a result, link-state routing protocols reach convergence
much faster than distance vector routing protocols.
Constructing a Link-State Database
The final step in the link-state routing process is
each router uses the database to construct a com-
plete map of the topology and computes the best path to each destination network.
After each router has propagated its own LSPs using the link-state flooding process, each router will
then have an LSP from every link-state router in the routing area. These LSPs are stored in the link-
state database. Each router in the routing area can now use the SPF algorithm to construct the SPF
trees that you saw earlier.
Figure 10-3 shows the entire topology, with network addresses.
