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    Example: Overriding the Default BGP Routing Policy on PTX Series Packet Transport Routers

    Understanding the Default BGP Routing Policy on Packet Transport Routers

    On PTX Series Packet Transport Routers, the default BGP routing policy differs from that of other Junos OS routing devices.

    The PTX Series routers are MPLS transit platforms that do IP forwarding, typically using interior gateway protocol (IGP) routes. The PTX Series Packet Forwarding Engine can accommodate a relatively small number of variable-length prefixes.

    Note: A PTX Series router can support full BGP routes in the control plane so that it can be used as a route reflector (RR). It can do exact-length lookup multicast forwarding and can build the multicast forwarding plane for use by the unicast control plane (for example. to perform a reverse-path forwarding lookup for multicast).

    Given the PFE limitation, the default routing policy for PTX Series routers is for BGP routes not to be installed in the forwarding table. You can override the default routing policy and select certain BGP routes to install in the forwarding table.

    The default behavior for load balancing and BGP routes on PTX Series routers is as follows. It has the following desirable characteristics:

    • Allows you to override the default behavior without needing to alter the default policy directly

    • Reduces the chance of accidental changes that nullify the defaults
    • Sets no flow-control actions, such as accept and reject

    The default routing policy on the PTX Series routers is as follows:

    user@host# show policy-options | display inheritance defaults no-comments
    policy-options {
    policy-statement junos-ptx-series-default {
    term t1 {
    from {
    protocol bgp;
    rib inet.0;
    }
    then no-install-to-fib;
    }
    term t2 {
    from {
    protocol bgp;
    rib inet6.0;
    }
    then no-install-to-fib;
    }
    term t3 {
    then load-balance per-packet;
    }
    }
    }
    routing-options {
    forwarding-table {
    default-export junos-ptx-series-default;
    }
    }
    user@host# show routing-options forwarding-table default-export | display inheritance defaults no-comments
    default-export junos-ptx-series-default;

    As shown here, the junos-ptx-series-default policy is defined in [edit policy-options]. The policy is applied in [edit routing-options forwarding-table], using the default-export statement. You can view these default configurations by using the | display inheritance flag.

    Also, you can use the show policy command to view the default policy.

    user@host> show policy junos-ptx-series-default
    Policy junos-ptx-series-default:
        Term t1:
            from proto BGP
             inet.0
            then install-to-fib no
        Term t2:
            from proto BGP
             inet6.0
            then install-to-fib no
        Term t3:
            then load-balance per-packet

    Caution: We strongly recommend that you do not alter the junos-ptx-series-default routing policy directly.

    Junos OS chains the junos-ptx-series-default policy and any user-configured export policy. Because the junos-ptx-series-default policy does not use flow-control actions, any export policy that you configure is executed (by way of the implicit next-policy action) for every route. Thus you can override any actions set by the junos-ptx-series-default policy. If you do not configure an export policy, the actions set by junos-ptx-series-default policy are the only actions.

    You can use the policy action install-to-fib to override the no-install-to-fib action.

    Similarly, you can set the load-balance per-prefix action to override the load-balance per-packet action.

    Example: Overriding the Default BGP Routing Policy on PTX Series Packet Transport Routers

    This example shows how to override the default routing policy on packet transport routers, such as the PTX Series Packet Transport Routers.

    Requirements

    This example requires Junos OS Release 12.1 or later.

    Overview

    By default, the PTX Series routers do not install BGP routes in the forwarding table.

    For PTX Series routers, the configuration of the from protocols bgp condition with the then accept action does not have the usual result that it has on other Junos OS routing devices. With the following routing policy on PTX Series routers, BGP routes do not get installed in the forwarding table.

    user@host# show policy-options
    policy-statement accept-no-install {
    term 1 {
    from protocol bgp;
    then accept;
    }
    }
    user@host# show routing-options
    forwarding-table {
    export accept-no-install;
    }
    user@host> show route forwarding-table
    Routing table: default.inet
    Internet:
    Destination        Type RtRef Next hop           Type Index NhRef Netif
    default            perm     0                    rjct    36     2

    No BGP routes are installed in the forwarding table. This is the expected behavior.

    This example shows how to use the then install-to-fib action to effectively override the default BGP routing policy.

    Configuration

    CLI Quick Configuration

    To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

    set policy-options prefix-list install-bgp 66.0.0.1/32
    set policy-options policy-statement override-ptx-series-default term 1 from prefix-list install-bgp
    set policy-options policy-statement override-ptx-series-default term 1 then load-balance per-prefix
    set policy-options policy-statement override-ptx-series-default term 1 then install-to-fib
    set routing-options forwarding-table export override-ptx-series-default

    Installing Selected BGP Routes in the Forwarding Table

    Step-by-Step Procedure

    The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

    To install selected BGP routes in the forwarding table:

    1. Configure a list of prefixes to install in the forwarding table.
      [edit policy-options prefix-list install-bgp]
      user@host# set 66.0.0.1/32
    2. Configure the routing policy, applying the prefix list as a condition.
      [edit policy-options policy-statement override-ptx-series-default term 1]
      user@host# set from prefix-list install-bgp
      user@host# set then install-to-fib
      user@host# set then load-balance per-prefix
    3. Apply the routing policy to the forwarding table.
      [edit routing-options forwarding-table]
      user@host# set export override-ptx-series-default

    Results

    From configuration mode, confirm your configuration by entering the show policy-options and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

    user@host# show policy-options
    prefix-list install-bgp {
    66.0.0.1/32;
    }
    policy-statement override-ptx-series-default {
    term 1 {
    from {
    prefix-list install-bgp;
    }
    then {
    load-balance per-prefix;
    install-to-fib;
    }
    }
    }
    user@host# show routing-options
    forwarding-table {
    export override-ptx-series-default;
    }

    If you are done configuring the device, enter commit from configuration mode.

    Verification

    Confirm that the configuration is working properly.

    Verifying That the Selected Route Is Installed in the Forwarding Table

    Purpose

    Make sure that the configured policy overrides the default policy.

    Action

    From operational mode, enter the show route forwarding-table command.

    user@host> show route forwarding-table destination 66.0.0.1
    Internet:
    Destination        Type RtRef Next hop           Type Index NhRef Netif
    66.0.0.1/32        user     0                    indr 2097159     3
                                                     ulst 2097156     2
                                  5.1.0.2            ucst   574     1 et-6/0/0.1
                                  5.2.0.2            ucst   575     1 et-6/0/0.2
    

    Meaning

    This output shows that the route to 66.0.0.1/32 is installed in the forwarding table.

    Modified: 2017-09-13