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