Jeff,
Thanks for the clarification.
Yes I am from cisco background, well who isnt? :-)
Not that my IQ does not allow me to understand this, but I am not seeing
anything wrong with cisco redistribution command. To juniper, the import and
export policy both used for filter routes within the protocol and during
redistribution. The former applyes to routes already exist, you just apply
the filters, but in the redistribustion, you do not have any routes from one
protocol to another. The nature logical thoughout is:
1) there is an export policy configured under this protocol, for example,
ISIS, this should apply to routes being advertised from routing table to
ISIS, as by default, all routes are being propogated from routing table to
protocol, so I just apply the policy to the routes;
2) looking at policy, it says match from bgp to ISIS, hem? What routes can I
match by this policy? nothing. Because, the routing table by default does
not advertise routes from bgp to ISIS.
I will have to call this confusion.
As Juniper is standing on the shoulder of cisco, (how many former cisco
employees they have?), I can see JUNOS definitely is way better than IOS
from the bottom of the structure and many other aspects(I believe cisco had
this embeded style in early age), but the way they define redisditribution
this isnot clever at all.
Thanks.
Kent
----- Original Message ----- hg
From: "Jeff Aitken" <jaitken@aitken.com>hg
To: "Kent Yu" <yux@lucent.com>
Cc: <juniper-nsp@puck.nether.net>
Sent: Monday, May 14, 2001 8:59 AM
Subject: Re: [j-nsp] Route redistribution
> 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