IPv6 Address Types

Lessons Discussion
82

Hello, everyone.

I found something today that doesn’t make much sense to me. IANA defines FF01 as a interface-local multicast scope, therefore it shouldn’t leave the local device.
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However, when studying about CAPWAP today, I found the following:

CAPWAP Multicast Discovery—Broadcast does not exist in IPv6 address. Access point sends CAPWAP discovery message to all the controllers multicast address (FF01::18C). The controller receives the IPv6 discovery request from the AP only if it is in the same L2 segment and sends back the IPv6 discovery response.
https://www.cisco.com/c/en/us/td/docs/wireless/controller/8-10/config-guide/b_cg810/ap_connectivity_to_cisco_wlc.html#discover-and-join-controllers

What’s going on here? IPv6 CAPWAP multicast messages use FF01::18C? But that uses the interface-local scope, so what does this mean? :smiley:

Thank you.
David

83

Hello David

Indeed the FF01:: is a multicast IPv6 address that is reserved for interface-local address scope according to the IANA. However, there’s nothing that will stop a vendor from using that scope for other purposes. There’s no inherent characteristic in these addresses that will prevent them from being used in any way.

CAPWAP is an open standard, and it should adhere to IANA rules, but according to its RFC 5415, the all ACs multicast address is: (FF0X:0:0:0:0:0:0:18C). Which means X can be anything?

It’s not quite clear what is happening here. IANA has indeed specified that this range should be used for this purpose, however, the RFC is not clear in its usage. And ultimately, Cisco can implement these mechanisms however it chooses. It can “violate” the IANA scopes to serve their purposes.

I hope this has been helpful!

Laz