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Another possible source for a routing loop
occurs when incorrect information that has been sent back to a router
contradicts the correct information that the router originally
distributed. Here is how this problem occurs:
- Router A passes an update to Router
B and Router D, indicating that Network 1 is down. Router C,
however, transmits an update to Router B, indicating that Network 1
is available at a distance of 4, by way of Router D. This does not
violate split-horizon rules.
- Router B concludes, incorrectly,
that Router C still has a valid path to Network 1, although at a
much less favorable metric. Router B sends an update to Router A
advising Router A of the new route to Network 1.
- Router A now determines that it can
send to Network 1 by way of Router B; Router B determines that it
can send to Network 1 by way of Router C; and Router C determines
that it can send to Network 1 by way of Router D. Any packet
introduced into this environment will loop between routers.
- Split-horizon attempts to avoid this
situation. If a routing update about Network 1 arrives from Router
A, Router B or Router D cannot send information about Network 1 back
to Router A.
 Split-horizon thus reduces incorrect routing
information and reduces routing overhead.
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