Understanding the Default BGP Routing Policy on Packet Transport Routers (PTX Series)
On PTX Series Packet Transport routers, the default BGP routing policy differs from that of other Junos OS routing devices. The default routing policy of the PTX Series 3000 and 5000 Series routers will not install BGP routes in the forwarding table, unless you configure another policy to override it. All other PTX Series routers will install BGP learned routes to the forwarding information base (FIB) and packet forwading engine (PFE) without the need for a policy.
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.
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-packetWe 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.