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Policy routing is
a means of controlling routes that relies on the source, or source
and destination, of traffic rather than destination alone. Policy
routing can be used to control traffic
inside an AS as well as between ASs. Policy routing is a glorified
form of static routing. It is used when you want to force a routing
behavior different from what the dynamic
routing protocols dictate.
Static routing
enables you to direct traffic based on the traffic destination.
Traffic toward destination 1 can go via point A, whereas traffic
toward destination 2 can go via point B.
Policy routing, on
the other hand, enables you to direct traffic based on traffic
source or a combination of source and destination. Traffic coming
from network 1 can go via point A, or traffic coming from network 1
and going toward network 2 can go via point B.
Consider the example
illustrated in Figure . Assume that AS1 was assigned network
numbers from two different providers. The 10.10.10.0/24 range was
taken from AS3, and the 11.11.11.0/24 range was taken from AS4. AS1
wants to have any traffic originated from its 10.10.10.0/24 networks
to be directed toward AS3 and traffic from its 11.11.11.0/24
networks to be directed to AS4, irrespective of the destination of
the traffic. AS1 could use policy routing to achieve this
requirement by forcing all traffic with a source IP address
belonging to 10.10.10.0/24 to have a next hop of 1.1.1.1, and
traffic with source IP belonging to 11.11.11.0/24 to have a next hop
of 2.2.2.2.
Policy routing can
also be based on a source/destination combination. This is
illustrated in Figure . Assume that RTA wants to use the San
Francisco link for any traffic originating from network
10.10.10.0/24 and reaching network 12.12.12.0/24 in New York. Also,
RTA wants to use the San Jose link for any traffic originating from
network 10.10.10.0/24 and reaching network 13.13.13.0/24 in New
York. Policy routing can be used to set the next hop for the traffic
combination (Source =10.10.10.0/24, Destination = 12.12.12.0/24) to
be 1.1.1.1. The traffic combination (Source = 10.10.10.0/24,
Destination = 13.13.13.0/24) will be set with next hop 2.2.2.2.
Whenever static behavior is enforced, backup
becomes an issue. It is important to ensure that if policy routed
traffic cannot be delivered because the next hop is down, some other
alternative is available. Cisco offers a creative way of doing
policy routing by offering multiple next hops for policy routed
traffic. If the first next hop is down or not available, the second
next hop will be tried, and so on. If none of the statically defined
next hops are available, the router can be configured to send the
traffic according to the normal dynamic routing (that is, based on
destination). (See Figure .)
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Lab
Activity |
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In
this lab, you will learn how to configure
a simple route-map to control traffic flow. |
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