[quagga-users 9690] Re: OSPF "summary-address"

Steve Bertrand steve at ibctech.ca
Sat Jun 28 14:39:39 IST 2008

Jason Sigurdur wrote:
> Sorry for the confusion.
> I am using zebra. The router(linux) has trunk interface with 2 networks
> and (area 1)
> I would just like to summarize these in one.

Future growth is hard to determine here without knowing how much of each 
/22 has already been assigned away (aka in use).

That said, you can aggregate this into a /21, and you will be ok.

If this is planning phase, might I recommend that you neglect to use 0.0 
  at this time, beginning with 4.0 and 8.0 instead, and plan your next 
usable /21 at Even better, hold enough space to allow future 
growth for a /22 on each side of the ones in use. It's hard to judge how 
you intend to use your /16, but if you are handing large chunks of it to 
big pieces of your network (whether it be clients or PoPs), reserving 
the first /22 will allow for this:

- assigned to PoP A
- assigned to PoP B


- PoP B needs more space, so you assign and aggregate to the next /21 
with the next /22 up to (incrementing with /23s would be better)

- PoP A needs more space, you have reserve with, and it grows 
(albeit backwards) to the /21 that precedes 4.0 (0.0)

Otherwise, you will have to assign PoP A a non-aggregate-able block 
outside of the encompassing /21 (this is a nightmare if you only have 
more than one level of aggregation)

Perhaps you have already considered this, and perhaps its why you are 
using /22s instead of /24 or /23s. Forgive me if this has been 
considered. Its more paperwork and detail involved this way, but I have 
run into a situation where once I assigned a /29 to a client "That's all 
we'll EVER need".

They have grown three times. After the second request for more space, I 
made them renumber, gave them a /27, and have the encompassing /25 
surrounding them (I expect more growth).

All in all, /21 will summarize them fine. But if you expect future 
growth, try to at LEAST double) whatever the highest expectation for 
that growth is, and go with that. Document it well, and/or perhaps null 
route the reserve. Either way, the space that you have reserved for 
areas that have been sitting idle and you realize will never be needed 
are already routed there, you can use them for other things.


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