>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. I do recommend summarizing as much as possible, without being compulsive about it, even in fairly small networks. But forwarding performance isn't the major motivation when you have, say, 500 routes or less. There are a number of good reasons for doing it. One is to enforce hierarchical design and efficient address space use. This will definitely be important if you ever need to justify assignments of public address space, and it tends to make life generally simpler. Hierarchy tends to localize the effects of mergers and divestitures. It can localize the effects of problems and simplify troubleshooting. It can ease your capacity planning. Summarization also tends to contain the effect of route flapping and similar instabilities, which can have an appreciable load on router processors, especially small ones. As far as your point about suboptimal routing, I find that this tends to be an issue only in the largest networks. The reality is that small networks -- and even large networks -- don't have huge numbers of alternate paths that could be found for optimality. In one 2500 router network I redesigned, only 400 routers routinely had alternate paths (i.e., not dial backup) they used. If anything, the discipline of a hierarchical address plan helps you catch configuration errors early in the process. > >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? > To paraphrase Thomas Jefferson (I think), the tree of enterprise network topology must periodically be watered by the blood of renumbering. Hierarchical addressing vastly reduces the amount of blood that must be spilled, along with other good practices. See RFC 2072. I'm off to the IETF and pre- and post-IETF meetings, so may not be posting much for the next 10-14 days. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=14615&t=14615 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
