Transit LAN vs. Individual LANs

Mark Smith random at
Sat Feb 25 22:11:45 UTC 2006

On Sat, 25 Feb 2006 13:56:37 -0600
"Stephen Sprunk" <stephen at> wrote:

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


> 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.



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

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