From: Yves Fauser (Yves@xxxxxxxxx)
Date: Wed Aug 15 2001 - 19:47:38 GMT-3
Hi all,
Paduh's last mail exactly explains the Bug that you experience with the 0.0.0.0
Wildcard ASBR's.
Hans and I ran into this problem some time ago :
http://www.groupstudy.com/archives/ccielab/200106/msg02298.html
The following link speaks of the forwarding database, but does not exactly ment
ion
the 0.0.0.0 Wildcard..
http://www.cisco.com/warp/public/104/10.html
------------------- From above link --------------------------
The value of the forwarding address specified by the autonomous system boundary
router(ASBR) (Router 1 in the figure above) can be either 0.0.0.0 or non-zero.T
he
0.0.0.0 address indicates that the originating router (the ASBR) is the next ho
p.
The forwarding address is determined by the following conditions:
The forwarding address is set to 0.0.0.0 if the ASBR is redistributing routes a
nd
OSPF is not enabled on the next hop interface for those routes. This would be t
rue
in the figure above if Router 1 didn't have OSPF enabled on the Ethernet
interface.
The following condition sets the forwarding address field to a non-zero
address:
OSPF is enabled on the ASBR's next hop interface AND
ASBR's next hop interface is non-passive under OSPF AND
ASBR's next hop interface is not point-to-point AND
ASBR's next hop interface is not point-to-multipoint AND
!!!!! ASBR's next hop interface address falls under the network range !!!!!!!
!!!!! specified in the router ospf
command. !!!!!!!
Any other conditions besides these set the forwarding address to 0.0.0.0.
-------------------------------------------------------------------
A.
1. You have 3 routers on a broadcast or non-broadcast network with one ASBR, on
e
non-ASBR and one EIGRP router
2. On the ASBR the network is in OSPF and EIGRP
3. On the non-ASBR the network is only in OSPF
4. Routes are exchanged between the ASBR and the EIGRP router and redistributed
into OSPF
Now if the network (lets say 192.168.10.0/24) is put into OSPF on the ASBR with
192.168.10.0 0.0.0.255 area 0, the next hop of the route is in the range of the
network statement, this means that the forwarding address is the address of the
EIGRP router. On the non-ASBR the LSA is examined and the route is dropped beca
use
the next hop of the route is set to an address that is not a valid interface
address of an OSPF neighbor router in the network LSA of the DR (for that
network). If you would have used 192.168.10.1 0.0.0.0 on the ASBR the next hop
would have been "use me", and the Problem would not have happened.
B.
The opposite can happen if you have situations like 3 routers in 2 areas (from
which one is an ABR) on the same broadcast or non-brodcast network. If you use
the
0.0.0.0 wildcard on the ABR you'll get an extra hop because of the "use me"
forwarding address. If you would have taken the 0.0.0.255 wildcard the forwardi
ng
address would be the next hop.
Hope this helps a bit, it's hard to explain.
Good luck to all, Yves
Yves Fauser wrote:
> Hi Jeffrey,
>
> If you want to take an Interface into ospf it doesn't matter which of the
> statement you use.
> But be aware of the consequences that the 0.0.0.0 wildcard mask has, it
> changes the forwarding address in the database.
> To be short if you use the 0.0.0.0 Mask the forwarding address will be 0.0.0.
0
> in the LSA. If you take 0.0.0.255 the forwarding address will be the address
> of the next hop. You can have various Problems with 0.0.0.0 or next hop
> depending on the topology. Another thing is that there is a Bug in all
> versions when redistributing ospf into RIP.
>
> I'll send some links later.
>
> Yves
>
> Jeffrey Levine wrote:
>
> > I have noticed in some lab scenarios that when the instructions say that a
> > router interface should be advertised in a routing protocol, the labs
> > sometimes advertise only the interface, othertimes an entire subnet.
> >
> > For example, let's say it's an ethernet interface with address
> > 192.168.1.1/24 being advertised under OSPF. I've seen the following:
> >
> > router ospf 64
> > net 192.168.1.1 0.0.0.0 area 0
> >
> > or
> >
> > router ospf 64
> > net 192.168.1.1 0.0.0.255 area 0
> >
> > I wouldn't be concerned if there were some consistency. I've seen the lab
> > instructions state "interface" and then seen the entire network advertised
> > and vice-versa.
> >
> > Any thought?
> >
> > Jeffrey S. Levine
> >
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