From: Volkov, Dmitry (Toronto - BCE) (dmitry_volkov@ca.ml.com)
Date: Wed Sep 04 2002 - 23:20:19 GMT-3
> distribute-list X in - filters routes learned via all incoming
> > interfaces from going into the routing table
>
> Distribute lists are of 2 types, not to mention their
> different workings
> when used with DV or LS protocols. There are global
> distribute lists and
> interface distribute lists. When the router receives an
> update its first
> checked against the 'interface specific' distribute list,
> then against the
> 'global distribute list' , and whatever is filtered (read
> allowed) thru gets
> installed into the routing table.
>
> This is the generic function when applying distribute lists
> inbound with DV
> protocols.
Nick what do You mean "global" ?
I assume it is when nothing followed after "in" ou "out"
I think when we use 'global' in - that means - filtering incoming routing
updates via all interfaces. In - it's always about interfaces
http://127.0.0.1:8080/cc/td/doc/product/software/ios121/121cgcr/ip_r/iprprt2
/1rdindep.htm#1017647
in - Applies the access list to INCOMING routing updates.
type (Optional) Interface type.
number - (Optional) Interface number on which the access list should be
applied to incoming updates. If no interface is specified, the access list
will be applied to all incoming updates.
Sorry, maybe You meant the same.
>
> With LS protocols, Dist list in can be used on 'any' OSPF
> router, however it
> just 'filters' the information/routes that can get into the
> routing table.
> The LSA's still remain in the databse, and every downstream
> router will
> still have the routes in the routing table.
With exception of case when You are filtering LSA 7 to 5 conversion on NSSA
(see P.v.Oene answer to D.Mateo (09/01 10:38). In this case downstream
routers will not get
LSA 5.
>
> > distribute-list X out - filters any external redistributed
> routes from
> > other protocols.
>
> Distribute lists applied outbound with DV protocols are of 3 types,
> interface specific, global & redistribution specific.
> Interface specific
> filters information going 'out' via the interface, global are applied
> 'after' the interface specific (if both are applied), if
> 'only' global is
> applied then all info going out to all neighbors is filtered thru the
> distribute list. These 2 can only be used with DV protocols.
So, again about 'global' (just 'out') -
I just want to clarify: It's related to sending routes out ALL interfaces
AND
from ALL other routing protocols running on router and redistributed into
this one.
Correct ?
>
> In case of 'redistribution specific' dist lists, you are essentially
> filtering what info gets from 'routing protocol A' into
> 'routing protocol
> B'. This can be used with DV & LS protocols.
Agree.
>
> > distribute-list X in - will still filters routes from all incoming
> > interfaces from entering into the routing table. I have read on
> > Parkhurst (OSPF Command & Reference) saying that this command can
> > filters "O" & "O IA" route, but not "O E2" routes, yet I
> have also read
> > on other books saying that only "O IA" routes can be
> filtered by this,
> > so which is correct?
>
> I think you can only filter O/IA routes.
Just tested it using "distr-list X in". Filters "O E2" and "O IA" (but the
stay in DB),
also filters "O" (the are not even in DB !!), all three types are
still propagated to downstream routers. Even "O" which is not in DB.
I even reload router for sure :)
>
> > Also, since OSPF uses LSAs, where OSPF neighbors determines
> routes from
> > the info in the Link State database, this command won't
> stop the routes
> > from going into OSPF Topology database.
See above "O" routes intra-area - are removed from DB.
Dmitry
>
> Correct
>
> > distribute-list X in <Serial 0> - same as above, except the
> filtering
> > only applied for routes learned via a specific interface.
>
> Well, I would think that a global distribute list or
> interface specific
> distribute list would work equally for OSPF, because
> basically we are just
> 'filtering' info from getting into the Routing table.
>
> > distribute-list X out - I have read from different books (as well as
> > CCO regarding this). Dr Parkhurst (OSPF Command &
> Reference) says that
> > this command is useless / has no effect on OSPF... yet I
> have read on
> > other sources saying this will still work, where ALL
> different external
> > protocols will be filtered with the list (e.g. if EIGRP &
> RIP are both
> > redistributed into OSPF, then they would both be filtered).
>
> distribute list x OUT by itself is useless for OSPF. But you
> can use the
> special distribute-list x OUT <routing protocol> in OSPF (and
> any other
> protocol). This will filter routes being redistributed from a
> particular
> protocol into another protocol (and it can be OSPF as well).
>
> > distribute-list X out <rip> - filters only a specific external
> > redistributed protocol (O E2) from entering into the ASBR's own OSPF
> > database, hence these external routes will be prevented to be
> > advertised to the rest of OSPF domain.
>
> Correct.
>
> > ... Apart from those, is it true that I should only use the above 4
> > commands from OSPF ABR or OSPF ASBR??
> >
>
> Nope, distribute list in can be used on any router,
> distribute list x out
> <routing protocol> can be used only on ASBR (because a
> redistribution router
> becomes ASBR)
>
> Nick
> > To unsubscribe from the CCIELAB list, send a message to
> > majordomo@groupstudy.com with the body containing:
> > unsubscribe ccielab
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