On Mon, May 14, 2001 at 11:33:44AM -0700, Kent Yu wrote:
> Junipers route redistribution concept really confuses me.
> By definition, routes from the protocols are imported into the routing
> table, then exported to their respective routing protocols.
Right.
> So in order to redistribute routes, I will have to use policy, right?
Yes.
> But to me, isnt policy only used to filter routes that already exist?
In the example you cited, the routes "already exist", they're just in
another protocol. By default, BGP routes are not exported into ISIS.
The example shows how you configure a policy to change the default
behavior. The application of that policy in the 'protocols isis'
section is what enables that policy (i.e., it is the equivalent of
the cisco 'redistribute' statement in the 'router isis' section).
> I mean, by default, there should no routes from bgp to ISIS, so to
> what routes the above policy applied?
The policy applies to whatever routes the policy says it applies to. :-)
In this case:
policy-options {
community Edu members 666:5;
policy-statement edu-to-isis {
from {
protocol bgp;
community Edu;
}
to protocol isis;
Note the compound structure of the clause; it says that routes must come
from bgp *and* match the 'Edu' community, and be destined for ISIS.
Only if all of these conditions are satisfied will the 'then' portion of
the policy apply.
> Or can I understand as that the policy configuration actually
> activates redistribution?
See above. What activates the redistribution is:
protocols {
isis {
export edu-to-isis;
}
> If it is, I must say it is really confusing, at least to me.
Confusing? Not really. Different from Cisco? Absolutely. If you
come from a Cisco background, JUNOS policy language and the application
of policies will take a little while to get used to.
> Also, the manual states: For IS-IS, you should not apply routing policies
> that affect how routes are imported into the routing table; doing so with a
> link-state protocol could easily lead to an inconsistent topology database.
Yes, you generally don't want to muck with your IGP in this fashion.
Personally I would have used a different example; redistributing ISIS L1
routes into L2 or redistributing static routes into BGP would both be
more "practical" as examples IMHO.
--Jeff
This archive was generated by hypermail 2b29 : Mon Aug 05 2002 - 10:42:42 EDT