[c-nsp] Sizing peering router / FIB

[Jared Mauch]

On Wed, Mar 14, 2007 at 02:07:36PM +0200, Sami Joseph wrote:
> Hello,
> 
> We are planning to get a new router as internet gateway, it should have one
> OC12 PoS and GigE.
> 
> I am trying to compare between different vendors.
> 
> We will peer with one provider and carry the Full table at the first stage
> and maybe later we will add another link and add another copy of the full
> table, but that wont be in the near future.
> 
> One router i am looking at supports up to 200K FIB entries, so i would like
> to clear somethings first.
> 
> Please correct me if i am wrong on the below:
> 
> The BGP table of the internet is almost 211K prefixes, if these routes are
> installed in the RIB (global table/without changing Admin. distance) then i
> need the same number of prefixes as FIB entries?

	This depends on who you get your 'feed' from.  Different providers
send a different number of (global) routes.  This may seem counterintuitive
but they may leak more specifics to you but not to their peers.  Their
customers may have a policy to announce their own routes to some
transit providers but not others.  There will be some variance between
different providers, and it can be large if they are leaking you large
internal deaggregated blocks...

> Assuming another transit peer is established and another copy of the routing
> table will be downloaded into the RIB, there will be comparisons and after
> decision is made, some more will be installed and FIB will increase?
> 
> I will peer with some customers of mine over this router and maybe give them
> the routing table, does that affect FIB in any way?

	Only in them sending you prefixes that may not currently match the
prefix length of other routes you may see in the existing table.

> Am i missing other significant number of FIB entries i should add to my
> consideration (if hte above is correct)?
> 
> If the above is not clear, i want to calculate the amount of FIB
> entries/Memory i need?
> 
> Private messages are always welcomed.

	My current recommendation is to plan to start at around 256k fib
entries, and to expect it to grow to around 310k within a year or so.
If you can't quickly upgrade the hardware (linecard memory and/or RP
memory) you may want to determine what the maximum memories each piece of
hardware can handle.  In the case of the 76/65 keep track of the
tcam space as well as this is not field upgradable like a GSR linecard
memory may be.

	It is valuable to make sure that you have MRTG (or similar) to track
the memory utilization of your devices as well (and actually look at it, or
alarm based on errors or warning levels you see/define).  This will help you
plan a bit better as opposed to running around at the last minute when the
routing tables are just too much for you to handle :)

	- Jared

-- 
Jared Mauch  | pgp key available via finger from jared at puck.nether.net
clue++;      | http://puck.nether.net/~jared/  My statements are only mine.