PI space and colocation

Stephen Sprunk stephen at sprunk.org
Wed Jan 18 22:49:18 UTC 2006


Thus spake "Chris Adams" <cmadams at hiwaay.net>
> Once upon a time, Patrick W. Gilmore <patrick at ianai.net> said:
>> It adds zero useful data to the global table, but increases RAM, CPU,
>> etc. on every router looking at the global table.
>
> How much difference is there between one AS (the colo provider)
> announcing a prefix and another AS (the customer) announcing it through
> the first AS (the colo provider)?  If the space is ARIN assigned PI, it
> isn't going to aggregate with the colo provider's space, so the prefix
> will still be a separate announcement.  The only difference is the AS
> path is one entry longer.

Routing slots aren't the only resource you're consuming.  In general, many 
of the prefixes coming out of a given AS have common attributes, e.g. path, 
MEDs, etc.  Those attributes are stored only once (at least in the BGP 
implementation I know) even if they're used by hundreds of prefixes.  If you 
attach a new leaf AS, it must, by necessity, consume another one of those 
slots.  If the customer prefix were announced by the upstream, however, it 
would not require an additional attribute slot; it'd reuse one of the 
existing ones.

Now, I'm not aware that there's any serious shortage of attribute slots like 
there is routing slots, but there's no sense wasting them if there's no gain 
to be had doing it.  Save your (and everyone else's) RAM for more prefixes.

S

Stephen Sprunk        "Stupid people surround themselves with smart
CCIE #3723           people.  Smart people surround themselves with
K5SSS         smart people who disagree with them."  --Aaron Sorkin 




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