BGP peering strategies for smaller routers

Blake Hudson blake at ispn.net
Mon May 2 20:34:26 UTC 2016


Mike, the ASR1k series has several ESP options (ESP5, 10, 20, 40, 100, 
200). Each ESP comes with a fixed amount of forwarding tcam which holds 
the forwarding information base (FIB). The ESP5 has 5MB of tcam can hold 
~500k routes. The ESP10 has 10MB of tcam, so theoretically should hold 
roughly double (1 million routes). The ESP20 and ESP40 have 40MB of tam; 
Cisco quotes these as 4 million routes.
http://www.cisco.com/c/en/us/products/collateral/routers/asr-1000-series-aggregation-services-routers/datasheet-c78-731640.html

There are two route processor (RP) options for the ASR1k series. The RP1 
and RP2. If you have 4GB, then you have an upgraded RP1. Which Cisco 
quotes at 1,000,000 IP4 routes *OR* 500k IP6 routes. Not to get too 
nitty gritty, but I would simplify these to say that there are 1,000,000 
route slots; each IP4 route takes 1 slot and each IP6 route takes 2.
http://www.cisco.com/c/en/us/products/collateral/routers/asr-1000-series-aggregation-services-routers/data_sheet_c78-441072.html

As others have noted, the best routes from the BGP table and other 
routing tables are condensed into the overall routing information base 
(RIB) and stored in DRAM. The RIB is then condensed into the FIB and 
stored as TCAM. Adding additional BGP peers will take minimal amounts of 
FIB if you're receiving the same full feed from both. The effect on the 
RIB and overall router memory is usually not that great either (I think 
on the ASR1k platform it's maybe ~100MB per peer). If you have an 
ESP10+, you're fine for two or more full feeds. If you have an ESP5, you 
really don't have the hardware to hold a single full feed.

--Blake

Mike wrote on 5/2/2016 2:07 PM:
> Hello,
>
>     I have an ASR1000 router with 4gb of ram. The specs say I can get 
> '1 million routes' on it, but as far as I have been advised, a full 
> table of internet routes numbers more than 530k by itself, so taking 2 
> full tables seems to be out of the question (?).
>
>      I am looking to connect to a second ip transit provider and I'm 
> looking for any advice or strategies that would allow me to take 
> advantage and make good forwarding decisions while not breaking the 
> bank on bgp memory consumption. I simply don't understand how this 
> would likely play out and what memory consumption mitigation steps may 
> be necessary here. Im open to ideas... a pair of route reflectors? 
> selective bgp download? static route filter maps?
>
> Thank you.
>
> Mike-
>



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