[c-nsp] OSPF NSSA Question

Jay Hennigan jay at west.net
Sun Oct 3 23:24:40 EDT 2004

On Sun, 3 Oct 2004, Dan Armstrong wrote:

> Numbers?  Ok.  Each "access" router is a 3550, 24 ports, 24 customers.
> Each dist router is a 6509 with MSFC2.
> The economics of the design should scale up to about 2000 to 3000
> customers per 6509, so that is about 100 or so 3550 switches hanging on
> it, which with an area for each would be 100 or so OSPF areas, each with
> 1 router each.  The churn of routes would be fairly high.... larger
> customers tend not to flap, but smaller customers unplug, turn stuff
> off, etc. quite often.
> I guess I am so concerned about large routing tables on the access
> routers, since 3550s are not really designed for large routing tables.
> Each 3550 is attached to each 6509 with a gig link, setup as a trunk,
> and vlan interfaces in the 6509 and a vlan interface in the 3550.  (I am
> oversimplifying the design a bit for clarity... each 3550 is actually
> hung off of 2 6509s)
> We "try" and keep the cusotmer's subnets as contiguous as possible, but
> as people move around the network, summarization becomes difficult.
> I have no problem with making an OSPF area for each access router... I
> have been told that I might blow up the 6509 pretty quickly, but who
> knows.

I would think the opposite.  The 6509 has to keep track of all of the
customer routes regardless of if one area or several.  With several
areas, the SPF updates when a customer route changes go between the
attached access router and the 6509 only, and the route is added or
withdrwawn from the 6509 alone.  With a single area for all of the
access routers, the change must ripple through 100 or so routers that
must by the design of OSPF have identical LSA databases.

I'd rather have 100 areas with one router than one area with 100 routers
unless there's something that is very CPU-intensive with regard to the
number of areas.  The very reason for multiple areas within OSPF is to
solve the exact problem you're experiencing.  It sure looks like IOS
doesn't have an issue with lots of areas...

r1(config-router)#network area ?
  <0-4294967295>  OSPF area ID as a decimal value

Probably more than you'll need.   :-)

> I am also considering if EIGRP would be a better way to implement this....

If you have Smartnet you can always open a priority 4 TAC case and ask
about the implications of a large number of ospf areas, specifically on
your hardware.

Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
WestNet:  Connecting you to the planet.  805 884-6323      WB6RDV
NetLojix Communications, Inc.  -  http://www.netlojix.com/

More information about the cisco-nsp mailing list