From: Nick Shah (nshah@connect.com.au)
Date: Wed Sep 04 2002 - 21:32:43 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.
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.
> 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.
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.
> 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.
> 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.
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
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