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    Creating Multiple Forwarding Solutions with IP Policy Lists

    By default, the router uses a single route table lookup to determine the forwarding solution for packets. For IP policy lists only, the forward command enables you to configure one or more unique forwarding solutions (interfaces or next-hop addresses) that override the route table lookup. By creating a group of forwarding solutions, you can ensure that there is a reachable solution for the packets.

    You can use the order keyword to specify the order of the group of forwarding solutions within a single forward rule. If no order value is specified, then the default order of 100 is assigned to a solution. The router evaluates the forwarding solutions in the group, starting at the solution with the lowest order value, and then uses the first reachable solution. To be considered a reachable solution, a solution must be a reachable interface or a next-hop address that has a route in the routing table. If no solutions are reachable, the traffic is dropped.

    The following guidelines apply when you create a group of forwarding solutions in an IP policy list:

    • You can specify a maximum of 20 forwarding solutions for a classifier.
    • The interface and next-hop elements of a forwarding solution must exist within a single virtual router:
      • Next-interface elements are associated with the virtual router where that interface exists.
      • You can include an optional parameter to specify the virtual router when you define next-hop elements.
      • If only next-hop elements exist and you do not use the virtual router option, then the policy assumes the virtual router context of the command-line interface (CLI), making the policy specific to that VR. The policy can be attached only to interfaces that belong to that VR. However, the policy can still be displayed and modified from any VR. The output of the show configuration command displays the policy in the section of output related to that VR rather than in the section for the default VR. This behavior ensures that when you use that output for a configuration script, the policy is specific to the correct VR, and the original configuration is re-created.
    • If you specify both an interface element and a next-hop address element, then they both must be reachable to be used. Also, the interface must be the correct interface for the next-hop address.
    • If you specify a next-hop address, then you can optionally specify that the default route be ignored.
    • If you delete the target (interface or next-hop address) referenced in a rule, that solution is replaced by the null interface but retains the same order number in the policy list. The null interface is always considered unreachable.
    • When a forwarding solution with a lower order value than the currently active solution becomes reachable, the router switches to the lower-ordered solution.
    • If two rules that have the same order value are reachable, then the rule that was created first is used.

      Note: The forward interface and forward next-hop commands are replacing the next-interface and next-hop commands, which do not support multiple forwarding solutions in a single forward rule.

    In the following sample classifier group of a policy list, the forwarding solution of ATM interface 0/0.1 has the lowest order value in the group, and would therefore be selected as the solution for the policy list. However, if this interface is not reachable, the router then attempts to use the solution with the next higher order; which would be ATM interface 12/0.1. If none of the solutions in the group is reachable, the traffic is dropped.

    host1(config-policy-list)#classifier-group westfordClacl precedence 200host1(config-policy-list-classifier-group)#forward interface atm 0/0.1 order 10host1(config-policy-list-classifier-group)#forward interface atm 12/0.1 order 50host1(config-policy-list-classifier-group)#forward interface atm 3/0.25 order 300

    Note: You can use the suspend version of the command to suspend an individual entry in a group of forwarding solutions. The forward rule remains active as long as there is a reachable or active entry in the group of forwarding solutions. If you suspend all entries in the group, the status of the forward rule is changed to suspended.

    Published: 2014-08-14