Re: Getting pretty close to default IPv4 route maximum for 6500/7600 routers.

[Blake Hudson <blake () ispn net>]

Matthew Petach wrote the following on 6/10/2014 7:03 PM:
> On the couple Cisco platforms I have available with full tables, Cisco 
> summarizes BGP by default. Since this thread is talking about Cisco 
> gear, I think it's more topical than results from BIRD.
>
>
>     One example from a non-transit AS:
>     ASR#sh ip route sum
>     IP routing table name is default (0x0)
>
>     IP routing table maximum-paths is 32
>     Route Source    Networks    Subnets     Replicates Overhead  
>      Memory (bytes)
>     connected       0           10          0 600         1800
>     static          1           2           0 180         540
>     application     0           0           0 0           0
>     bgp xxxxx       164817      330796      0 29736780  89210340
>       External: 495613 Internal: 0 Local: 0
>     internal 5799  20123680
>     Total           170617      330808      0 29737560  109336360
>
>
>
> I'm not sure you're reading that correctly.
> 164817+330796 = 495613
>
> That is, the BGP routing table size is the
> union of the "Networks" and the "Subnets";
> it's not magically doing any summarization
> for  you.
>
> Matt

Thank you Matt for directly addressing my question. My interpretation, 
which seems likely incorrect, was that smaller announcements could be 
discarded if there was a covering prefix (that otherwise matched the 
same AS path and other BGP metrics) and that many smaller prefix 
announcements could be bundled (again, assuming that all BGP metrics 
were the same between the prefixes). The numbers I was seeing in my 
routers for subnets coincided closely with the cidr-report's 
summzarization numbers http://www.cidr-report.org/as2.0/aggr.html, and I 
assumed the two used the same logic (not magic) to calculate how to 
reduce routes without losing any routing functionality. Your explanation 
that I was simply interpreting the numbers incorrectly seems the most 
logical now that I look again.

Thanks,
--Blake