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Some interesting stuff I found on IX LANs

These days the internet as a whole is mostly constructed out of point to point ethernet circuits, meaning an ethernet interface (mostly optical) attached directly from one routing device to another routing device. However that is not always the case, as the humble “internet exchange” (IX) still exists, and while the relevancy of IXs are progressively being diminished by the internet increasingly being concentrated into a small handful of content networks and IXs not keeping up with the lowering price of transit or private fiber connections to the largest networks, there are still a large number of networks that’s attached to at least one IX fabric.

This isn’t that magical, at its core it works by running tcpdump on each IX port, and picking up the BUM traffic, parsing what it is looking at (and throwing away the unknown unicast, since that is a separate common problem that I don’t want to get involved with), and reporting that data back up the chain to bgp.tools’s website. DEC-MOP, RoMON, STP, CDP, IS-IS, ES-IS, LLDP, VRRP, OSPF, IPv6 RA are remarkably common, and yet they use specific MAC address destinations that could just be filtered out on all ports, preventing them from being seen by other IX participants. While [LLMNR, NetBIOS, PIM, LDP, MDNS, DHCPv4 /DHCPv6, SSDP, DNS-Broadcast, Broadcast NTP, MikroTik Discovery] do require the IX device to be able to inspect Layer 3 headers like UDP port numbers in ACLs, this feature is very common among deployed hardware in the industry.

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