I agree that route summarisation may not speed up route lookup much. But
there's other far more valid reasons for doing it.
The network I work with is not ISP size in terms of routes, but it's pretty
big, with hundreds of geographically dispersed sites - without
summarisation, we'd have thousands of routes. Here's some reasons why we
summarise... mostly they would apply to smaller networks as well.
If you summarise (sensibly), you can hide route flaps from a large part of
the network. If an ethernet segment in Bourke falls over, the router in
Broome really shouldn't have to care less. By summarising, you restrict
the number of routers that have to recalculate routes, so routers spend
less time thinking about how to route and more time forwarding packets
(hopefully).
If you summarise (sensibly), you can reduce the amount of route information
in your routing tables. Forget routing lookup time - depending on your
routing protocol, this can substantially reduce the amount of data that has
to be transferred between routers (less overhead traffic), and reduce the
amount of calculations the router has to do. Again - less time doing (and
sending) background stuff, more time to route "real data".
If you summarise (sensibly), it's much easier to read the ip routing table
- fewer pages of info to wade through :-)
I tweaked the summarisation of our network several months ago. Previously,
we had been having occasional problems that were usually being put down to
"OSPF recalculations" (mostly erroneously, IMO, but it was creaking
occasionally). Since summarisation was beefed up, there have been no
problems (or maybe people just decided they couldn't point the finger at
OSPF any more :-)
JMcL
---------------------- Forwarded by Jenny Mcleod/NSO/CSDA on 02/08/2001
04:48 pm ---------------------------
"nrf" @groupstudy.com on 02/08/2001 02:42:45 pm
Please respond to "nrf"
Sent by: [EMAIL PROTECTED]
To: [EMAIL PROTECTED]
cc:
Subject: Just how important is route summarization in typical enterprise
[7:14601]
Hey all. I'm going to risk starting a flame war by asking the following:
I've been struck by just how much importance Cisco courseware places on
route summarization. For example, every student who goes through
CCNP-level
courseware learns about all the various kinds of summarization - OSPF area
summarization, OSPF stubs, EIGRP summarization, etc. etc., and how it
reduces the size of the route table, thereby improving router performance
by
speeding route lookup. It's gotten to the point that Cisco-trained
personnel treat summarization like the holy grail, and they go around
trying
to use summarization techniques wherever they can.
Yet, I seem to recall somebody wrote a book (I believe it was Berkowitz)
that basically stated that the performance gains associated with reducing
the route table via summarization is virtually nil in typical corporate
networks, because the real delays were caused simply by the serialization
time of sending packets over slow WAN links (T-1 and slower). Plus, with
fast-switching and its cousins (optimum switching, MLS, etc.), route lookup
isn't done all that often , so there is little lookup delay anyway. And
besides, most corporate networks aren't very big - typically less than 100
route entries, so how much lookup delay could there be? So, when I weigh
the cons of suboptimal routing as well as the possibility of
misconfiguration, I find it difficult to see why the typical enterprise
would ever really want to do summarization, as the gains are miniscule at
best.
Note, I know full well that ISP's/NSP's and very large enterprises (those
having on the order of thousands of routes) do indeed benefit substantially
from summarization. Of this I have no doubt. What I cannot see is why the
typical enterprise would really want to use summarization techniques.
Anybody have any thoughts on this?
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=14628&t=14628
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