70 metro areas. In a given metro area there might be one major traffic exchange point and de-facto most important peering location (example: SeattleIX in Seattle, and the Westin), or in a larger metro area, multiple exchange points.
edit: for a list of the geographical (OSI layer 1/2) locations where AS15169/google peers, see the following: https://www.peeringdb.com/asn/15169
70 metros translate to many more POPs (also for redundancy reasons).
The metros can be seen in e.g. the 1e100.net hosts in a traceroute. They're usually the closest airport code, so e.g. lhr for London.
Somebody in China reverse engineered the metro/POP naming and addressing for latency reasons. You can see that, for example, there are at least three POPs in Sydney:
yes, the practice of using IATA airport codes or similar for reverse DNS is a practice that long pre-dates google. They just adopted industry best practices when they started building consistent rDNS.
this is why some ISPs use a format that maps to two letter country code, then state or subdivision, then POP. where 01 is the first pop in the city, 02 is second, etc. The ISP's own internal documentation system will tell their NOC and neteng staff which pop number is which street address/datacenter (info that does not need to be available in rDNS). in a totally random example, nyc01 POP for an ISP might be 25 broadway, nyc02 would be 60 hudson, nyc03 would be 111 8th, and so on.
example agg router:
agg1.nyc01.ny.us.ASNUMBER.net
and then individual interfaces and subinterfaces would be defined under hierarchically under agg1.
I wonder if they are referring to a peering Internet exchange point (IXP) when they say metro. Basically a building where networks converge and ISPs connect to each other.
yes, though "metro" is a better way to define it since many IXes are geographically distributed throughout their city. For example DE-CIX in frankfurt is in many different datacenters, with their core switches connected by DE-CIX controlled dark fiber. AMS-IX in amsterdam is in many facilities in the same metro area, all the same L2 peering fabric. The SIX in Seattle is in three facilities in the same metro and several local ISPs have built their own extensions of it to Vancouver BC.
A metro in Google-speak generally refers to all of the peering locations in a given city or metro area, not specifically a single IXP.
For example, if you look at the PeeringDB entry for AS15169 (https://www.peeringdb.com/asn/15169), for the London "metro" there's public peering available on LINX at 3 different POPs, and private peering available at Digital Realty, 3 different Equinix POPs, and 2 Telehouse POPs.
