Discussion

Let’s take a look at the three routing policies to see how they are evaluated when
chained together:
[edit]
aviva@router1> show configuration policy-options
policy-statement block-private {
term 1 {
from {
route-filter 0.0.0.0/0 upto /7;
route-filter 0.0.0.0/0 prefix-length-range /25-/32;
route-filter 10.0.0.0/8 orlonger;
route-filter 127.0.0.0/8 orlonger;
route-filter 172.16.0.0/12 orlonger;
route-filter 192.168.0.0/16 orlonger;
route-filter 224.0.0.0/4 orlonger;
}
then reject;
}
}
}
policy-statement remove-communities {
term 1 {
then {
community delete all-communities;
}
}
}
policy-statement send-statics {
term 1 {
from protocol [ static direct ];
then accept;
}
}
community all-communities members *:*;
For routes being exported by the BGP group external-group, the policy evaluation
first checks against the prefix ranges listed in the block-private policy, which keeps
private addresses from being advertised to external ASs. Routes matching any one of
the prefixes are rejected, and policy evaluation stops. Otherwise, routes are checked
against the remove-communities policy. This policy has no match conditions, so all
routes match and all community strings are removed from the route. Because this
policy has no action that terminates policy evaluation (accept or reject), the evaluation
continues with the next policy in the chain, send-statics, which redistributes
static and direct routes to the BGP peers. At this point, if a route does not match any
of the three routing policies, the default BGP export action is taken, which is to
export only those routes learned from BGP and reject everything else.
You can also design routing policies that are implicitly chained together. If a policy
has no flow control action (then accept, then reject, then next term, orthen next
policy) and has no more terms, policy evaluation automatically continues with the
next policy if one is configured. The second two policies, remove-communities and
send-statics, illustrate how this works. After the remove-communities policy removes
298 | Chapter 9: Routing Policy and Firewall Filters
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