Subject: ospf limits...

From: Ravi Malhotra <[email-masked]>
Date: Thu, 6 Mar 2003 09:19:23 -0500

I am in the midst of redesigning an OSPF network to accommodate
significant growth, especially in the Area 0 backbone. There has been
some concern about the size of the OSPF topology that would result and
whether the routing protocol would remain robust, able to handle the odd
flapping link or memory leak. The network in question is using high-end
routers with high speed links (at least DS-3).

Are there any heuristics that someone has which may give us an idea of
the comfort zone in terms of the database size, number of neighbors,
routers, etc?

Or, could someone share information on the size of large OSPF
implementations that they are aware of?

I know that this issue is rather vague (clouded in too many unknowns)
and may only result in rather ambiguous
answers, but, one never knows.

Thanks,

Ravi

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From: Sanjai Narain <[email-masked]>
Date: Thu, 6 Mar 2003 09:34:43 -0500

Ravi: As far as I know, OSPF has been used for a network of eighteen
thousand routers in a financial company's network. -- Sanjai

--
   Sanjai Narain
   Senior Research Scientist
   Telcordia Technologies
   445 South Street
   Morristown, NJ 07960
   USA
   Tel: +1 973 829 4515 Fax: +1 973 829 5888
   Email: [email-masked]

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From: Acee Lindem <[email-masked]>
Date: Thu, 6 Mar 2003 10:45:37 -0500

Ravi,

I agree that modern routers and OSPF implementations have
blown away the previous limits for the number of routers in
an area. As an example, take a look at the results of Lightreading's
Edge Router test (apologies in the advance for vendor specific
information):

http://www.lightreading.com/document.asp?doc_id=25454&page_number=14

Also, it has been my experience that the limiting
factor is the number of neighbor adjacencies that must be
maintained. This number will vary by routing platform - ask your
vendor.

--
Acee

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From: Quaizar Vohra <[email-masked]>
Date: Thu, 6 Mar 2003 09:31:40 -0800

I think the main factor is the amount of flooding a router has to do
which depends on how many adjacencies you have and how many LSAs you
have to flood to each adjacency. So it is useful to divide your
network into multiple areas so as to reduce flooding. Also large
no. of adjacencies in a single area can result in rather large router
LSAs which causes a lots of fragmentation and adds to the flooding
load.

Quaizar

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From: klara moser <[email-masked]>
Date: Thu, 6 Mar 2003 13:11:52 -0500

Hi Ravi,

As far as I know the OSPF protocol has only one limitation on the topologies
size - the Network and Router LSA size. The max is 64KB. This limits the number
of links the router can have in a single area, because it has to advertise them
in a single Router LSA. You can do the math. It is something like 5 000 links.

Klara

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From: Erblichs <[email-masked]>
Date: Mon, 10 Mar 2003 10:43:56 -0800

Ravi,

        One major concern about with respect to the number of
        neighbors, is their consistency/frequency in sending hellos
        to prevent adjs from being torn down uncessarily vs the
        amount of time a link may be down and then tearing down
        the adj and running another SPF computation.

        In my opinion a core router, must be able to lookup / insert
        maybe 500 elements per second in its LSDB, run single digit
        ms duration SPF calculation times, and pass 98%+ of its pkts
        to the proper destination.

        I would think that the above core router would need to support
        up to 500 nbrs per area and a 1 million LSDB table size.

        Mitchell Erblich
        Sr Software Engineer
        =============================

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From: Quaizar Vohra <[email-masked]>
Date: Mon, 10 Mar 2003 11:59:52 -0800

Mitchell,

        Unless you do DoNotAge LSAs, you will end up flooding close to
400 LSA per second just due to periodic refreshes (asuuming around
2500 seconds as refresh interval for each LSA). If you flood this to
500 nbrs, you end up generating 400 * 500 LSAs per seconds, which is
close to 200,000 LSAs per second. Which amounts to approx 200,000 * 32
bytes per seconds (assuming most LSAs are external). This come to 6.4
mbytes of LSA traffic per second. And this alone in a single area, let
alone multiple areas.  Also we are not counting any processing
overheads or other ospf traffic.  I would be quite surprised if any
vendor comes even close to this.

Also 500 nbrs in a single area would mean very large router LSAs,
resulting in lots of fragmentation. Fragmentation coupled with
flooding to 500 nbrs will be quite CPU intensive.

I would be more comfortable with 50 to 100 nbrs per area, 500 to 1000
nbrs across multiple areas, with upto 10K LSAs per area.

Quaizar

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From: Erblichs <[email-masked]>
Date: Mon, 10 Mar 2003 13:37:25 -0800

Yes,

        He did mention area 0 backbone.

        So, I am assuming "signifciant growth" and multiple
        Gb or 10Gb links bandwidth links.

        The "maybe 500 elements per sec" to
        "maybe 500 elements per nbr per sec" suggestion is based
        on whether DR and BDRs exist within the OSPF backbone
        OR a full-mesh exists between nbrs in an area. And of course
        500LSAs per sec * 60 = 30K. And 30k * 30 minutes = 900k.

        Flooding is only over DR/BDR links when they exist, so
        that would minimize the LSA flooding overhead to what
        would minimally be about 545 elements per sec.

        Mitchell Erblich
        Sr Software Engineer
        =========================