Summary

  • RFC 10075 assigns bit 6 of the Router Advertisement Flags Extension Option to mark an advertisement as sent by a SNAC router.
  • The flag can classify a sender. It cannot authenticate that sender or authorize its advertisement.
  • A transit network protected by RA-Guard may filter the very advertisements that carry the new distinction.

A packet carries a modest claim: “I am a Stub Network Auto-Configuring router.” RFC 10075 gives that claim one bit in an IPv6 Router Advertisement. When set to one, the bit tells a receiving SNAC router that the advertisement came from another SNAC router, rather than from a transit router. The standard assigns bit 6; IANA’s current registry now records it as S - SNAC Router Flag.

That is useful classification. It is not a certificate. The RFC leaves the exact conditions that cause a SNAC router to set the bit outside its scope. It also says that devices which do not understand the flag silently ignore it, and that non-SNAC devices must not set it. A receiver can learn what the sender declares itself to be; the bit says nothing about who installed the device, whether the declaration is true, or whether the receiver should accept its route information.

IPv6 Neighbor Discovery already draws that boundary in plain language: it has no mechanism for deciding which neighbors are authorized to send particular messages such as Router Advertisements. A label can help a router process a message differently. Authorization still depends on the receiving network’s link, port, configuration and security policy.

Then comes the switch port. RFC 10075 states that if RA-Guard filters Router Advertisements from SNAC routers, devices on the transit network never receive the flag. RA-Guard is a Layer 2 filter, commonly used to limit which ports can send Router Advertisements. A deployment that treats every such advertisement as forbidden may erase the new signal before a transit-side device can read it. One that permits it too broadly may widen the path for messages it did not intend to trust. The setting is local; the consequence crosses the link.

The bit therefore creates a small but real coordination job. A network has to decide which interfaces may emit SNAC advertisements, whether downstream transit-facing equipment needs to see them, and how its filtering rules treat the new option. “The protocol has a flag” is not a rollout plan. The operative question is which device sees it, on which port, and what that device is allowed to do next.

The evidence here is the standard and its registry record. They do not provide a device support inventory, interoperability results or an observed failure. A multi-vendor packet trace or published support matrix would change that evidence picture; neither appears in these sources.

Sources