From: Chris Lewis \(chrlewis\) (chrlewis@cisco.com)
Date: Thu Aug 25 2005 - 13:12:33 GMT-3
Hi,
I think the first step is to understand what the forwarding address is,
what it is used for and then look at why suppressing it may be
beneficial.
Starting from the position that NSSA is there to allow the injection of
external routes from a stub area to the backbone, we know that the NSSA
area will have type 7 LSAs in it that represent the external prefixes
being injected. These will be translated at the NSSA ABR to type 5s. The
NSSA ABR has to tell the backbone routers how to reach these external
prefixes, so there are two choices, either the NSSA ABR propagates the
actual address of the router injecting these external routes, or the
NSSA router sends a default in to the backbone and says contact me to
get to these external routes and I will take it from there. The address
of the injecting router or the default, is advertized as the forwarding
address in the external LSA.
So you end up with two choices. If you propagate all the NSSA routes in
to the backbone, there is no need to suppress the forwarding address.
However if you do not advertize all the NSSA internal routes in to the
backbone, you will need to ensure the forwarding address is suppressed
as the backbone routers will not know how to get to the injecting
router.
In either case, the suppress fa command has nothing to do with
suppressing the routes from the NSSA in to the backbone, you have to
figure that out separately J but what it does is alter the value of the
forwarding address in the external LSA.
It is also worth noting the conditions necessary for a router to install
an external LSA in to the routing table. Basically a router needs to see
the ASBR (the one injecting the external route of interest) via an intra
or inter area route AND the forwarding address must be known via an
intra or inter area route.
In many cases you will see 0.0.0.0 as the forwarding address already
with the sho ip ospf database external A.B.C.D command. I recommend you
try out some configs with your setup to see when you get 0.0.0.0 or the
injecting router IP as the forwarding address and that should clear
things up.
Chris
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Montiean
Sent: Saturday, August 20, 2005 10:29 PM
To: ccielab@groupstudy.com
Subject: nssa translate type7 suppress-fa
Hi,
Someone please explains me about this command "nssa translate type7
suppress-fa". From cisco
http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/hi
rp_c/
ch15/hoadsup.htm
Per my understanding, forwarding addresses in type7 will be set to
0.0.0.0 into Type-5 LSAs. I lab it up but couldn't see any thing change
rather than under nssa ABR saying below
Perform type-7/type-5 LSA translation, suppress forwarding
address
I also found CSCec72160 in mainline 12.3 advise to take off area nssa
command and put it back in with "nssa translate type7 suppress" But it
didn't work for me.
My lab setup is r6 ----area0 ----- r4 ---- area147(nssa)
-------r1-----rip----- r2.
So I redistributed rip into r1 and use suppress-fa at r4. r6 still
seeing all the addresses advertised from rip.
I just want to understand why we use this command, cisco page does not
explains clearly for me or I miss understand some thing. I appreciate
any feedback on this question.
--Montiean
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