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Re: hosts as networks - Suboptimal Paths [7:86086] posted 03/19/2004
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At 06:55 PM 3/18/2004, Bob Sinclair wrote:
>John,
>
>I also have been in the habit of using the quad zero mask in OSPF - seemed
>like nailing it down specifically made sense.  But it can result in
>suboptimal paths when you do this on an ASBR.  You will find that when you
>use a quad zero mask, the ASBR always sends quad zero as the forwarding
>address for external routes (itself).  This can result in suboptimal routing
>when the next hop for the external route is on an OSPF-enabled, broadcast
>medium.  For details, check out the following paper in the
>netmasterclass.net technical library:

While I understand the point you are making, I'd much rather have my type 
5's hit the wire with all 0 forwarding addresses.  This allows them to be 
resolved properly by inter-area routers and you can pretty much be 
comfortable that in an OSPF network the best route to your ASBRs will be 
OSPF derived.  If you start using non zero forwarding addresses, you need 
to make sure that other routers in the network can resolve that address via 
a type 3, otherwise, they'll ignore the 5's.  This can be nasty ;-)

You'll not see the routing bit set in the LSDB when this occurs for what 
it's worth.


>"Forwarding Behavior of IGP Routing Protocols on Broadcast Subnet Part One"
>
>In the lab, it may be easier to use a quad zero mask, but  you should be
>aware of the implications.
>
>HTH,
>
>Bob Sinclair
>CCIE #10427, CISSP, MCSE
>www.netmasterclass.net
>
>
>----- Original Message -----
>From: "John Matus"
>To:
>Sent: Thursday, March 18, 2004 4:08 PM
>Subject: hosts as networks [7:86077]
>
>
> > i'm curious to see if anyone else does this and what the justification
is.
> > when configuring a routing protocol, i've been told that it is better to
>use
> > an actual host address:
> >
> > router ospf 100
> > network 172.16.100.5 0.0.0.0 area 0
> >
> > rather than:
> >
> > router ospf 100
> > network 172.16.100.0 0.0.0.255 area 0
> >
> > any benefits? any downsides?  lengthy explanations welcome!
> >
> > thanks in advance!
> >
> >
> > --
> >
> > Regards,
> >
> > John D. Matus
> > MCSE, CCNP
> > 818.782.2061 office
> > 818.430.8372 mobile
> > jmatus@xxxxxxxxxxx
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