[c-nsp] Rate limiting questions

Justin Shore justin at justinshore.com
Fri Oct 26 09:45:29 EDT 2007

Does anyone have any tricks for rate-limiting a pair of customer facing 
L3 interfaces w/ HSRP across 2 chassis?  I'm working on a co-lo setup 
and I was thinking about how I'm going to rate-limit the customer and 
still implement HSRP for access layer redundancy.  Another engineer's 
question got me wondering.

I was planning on applying the same rate-limits to the customer's L3 
HSRP interfaces on both routers.  However the customer could easily 
point half their servers at the interface IP on the standby L3 interface 
and it will gladly accept and route their traffic.  Of course all return 
traffic would come through the active HSRP interface but since most 
co-lo traffic is upstream that rate-limit wouldn't prevent the 
simultaneous use of both HSRP routers.

Is there a HSRP option to tell the standby router to only route traffic 
when it's active?  VRRP and GLBP would have the same problem I imagine. 
  Or is the solution to not pull the interface IPs out of the same block 
as the floating standby IP, ie address the interfaces with RFC1918 
addresses?  This way the interface IPs wouldn't be routable from the 
customer's server unless the customer bound the appropriate private 
subnet to one of their interfaces and at that point their traffic 
wouldn't be routable across the 'Net.  I know this has been discussed 
here dozens of times but I can't remember what the consensus was.

My other rate-limit question was about calculating the figures for CAR. 
  I once found a nice page that gave suggestions for what formula to use 
in certain circumstances to calculate burst sizes.  I can't find that 
bookmark now and my Google-fu isn't turning up anything helpful.  Does 
anyone know the page or a similar one?  Or does anyone have a suggestion 
for burst size calculations?


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