[129323] in North American Network Operators' Group
Re: largest OSPF core
daemon@ATHENA.MIT.EDU (Christian Martin)
Thu Sep 2 21:40:55 2010
In-Reply-To: <m2occfsoqd.wl%randy@psg.com>
From: Christian Martin <christian.martin@teliris.com>
Date: Thu, 2 Sep 2010 21:40:39 -0400
To: Randy Bush <randy@psg.com>
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On Sep 2, 2010, at 7:35 PM, Randy Bush <randy@psg.com> wrote:
>> The stability of the topology plays a most prominent role, but it
>> wouldn't surprise me if a OSPF network largely comprised of router
>> LSAs (no redistribution), using today's hardware, could easily scale
>> to 1000 nodes in an area.
>=20
> i believe the original poster asked about actual operating deployment,
> not theory.
>=20
> and, i suspect one wants to know about full mesh under real load, i.e.
> topology change, which can be exciting when one gets to a network of
> significant size.
Randy,
Fair enough. 7 years ago, I was privy to an OSPF BB of 300 or so routers su=
pporting a BGP overlay. No NBMA, passive broadcast subnetworks, all running=
on systems without the capacity to offload adjacency maintenance into linec=
ards. I'd argue that this type of network is also uninteresting from a NANO=
G viewership POV.
I also operated a network that supported over 70 OSPF VRF instances on a sin=
gle PE. CPU loads were higher, but we didn't observe intractable workloads.=
And this was with a 500 route limit per VRF, with who knows what kinds of m=
essiness running in those VRFs. (and yea there were sham links and router L=
SAs flying around!!) :)
There are many variables, and several studies have tried to capture, algorit=
hmically and in terms of computational complexity, a formulaic approach to d=
etermining the boundaries of OSPF network scalability. Admittedly, these ap=
proaches can be very approximate in nature. But the point stands.
Stable topologies absent large, frequent, compulsively updated data can scal=
e extremely well. Unstable topologies with lots of leaf data (20,000 type 5=
LSAs, for example), don't.
The most interesting point to make, however, is how much legacy thinking in t=
his area continues to be stranded in a rut that emerged 15 years ago. It is=
not uncommon to hear network folks cringe at the thought of an OSPF area e=
xceeding 100 routers. Really? When simulations using testing tools show th=
at properly tuned OSPF implementations (with ISPF, PRC, etc) comprised of 10=
00 can run full SPFs in 500 ms?
That said, my experience, as stated above, is that 300 routers is completely=
workable.
Cheers
Chris
>=20
> randy
