Transit LAN vs. Individual LANs

On Sat, 25 Feb 2006 13:56:37 -0600
"Stephen Sprunk" <stephensprunk.org> wrote:

> 
> Thus spake "Patrick W. Gilmore"
<patrickianai.net>
> > On Feb 24, 2006, at 9:03 PM, Scott Weeks wrote:

<snip>

> 
> There are a few advantages to going with PTP VLANs,
such as eliminating 
> DR/BDR elections needed on shared ones, but you'd need
10 of them to get a 
> full mesh, and 15 if you add one more router.  That's
just too much 
> complexity for virtually no gain, and as Owen notes, it
is generally bad for 
> your logical topology to not match the physical one.
> 

Even if you have a small number of routers on a segment, you
can set the
ethernet interface type to point-to-multipoint, at least on
Ciscos.

Automatic nighbour discovery via multicast hellos still
happens, the
difference is that the routers establish direct adjacencies
between each
other, rather than with the DR. While this costs additional
RAM, and CPU
during the SPF calc, the benefit of avoiding DR/BDR
elections, and the
'DR/BDR' approximately 40 second listening phase when a
third and
subsequent routers come online may be well worth those
costs.

I've also found you can set the OSPF interface type on
ethernets to
point-to-point. From memory, it results in a slightly
smaller Router LSA
than point-to-multipoint. That probably doesn't matter
much. I haven't
tested it, however setting the type to point-to-point might
prevent a
third OSPF router being accidentally added to the segment
and then
establishing an unwanted adjacency, which might provide a
robustness
against human error advantage.

Regards,
Mark.

-- 

        "Sheep are slow and tasty, and therefore must
remain constantly
         alert."
                                   - Bruce Schneier,
"Beyond Fear"

On Sun, 26 Feb 2006 08:41:45 +1030
Mark Smith <random72616e646f6d20323030342d30342d31360a.nosense.org>
wrote:

To qualify this better, there are no DR/BDR on the segment
at all,
rather than there being ones that just aren't used :

> Automatic nighbour discovery via multicast hellos still
happens, the
> difference is that the routers establish direct
adjacencies between each
> other, rather than with the DR. While this costs
additional RAM, and CPU
> during the SPF calc, the benefit of avoiding DR/BDR
elections, and the
> 'DR/BDR' approximately 40 second listening phase when
a third and
> subsequent routers come online may be well worth those
costs.
> 

-- 

        "Sheep are slow and tasty, and therefore must
remain constantly
         alert."
                                   - Bruce Schneier,
"Beyond Fear"