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