It seems the situation with finding routers in IPv4 with DHCP was okay and now IPv6 has RA's and we have to worry about rogue RA, different vendors provide protection against it, etc. What is IPv6 trying to fix/improve with RAs that makes the downsides worth it?
Fix? I don't know. But it's abundantly clear the committee members were blind to history: IPv4 had the same horrible, nasty, and insecure feature that was almost instantly abandoned -- ICMP Router Advertisements.
The purpose of the RA is to announce a router, the prefix for the segment (served by that router, there can be more than one router on a segment), and flags for "managed" (i.e. use DHCPv6 for addresses) and "other" (i.e. use DHCPv6 for other, non-address elements -- dns servers, domain name, etc.) IPv6 compliant hosts are required to accept and process all RAs they see, however many systems limit the number to mitigate one of the GREAT MANY security issues with RAs. (crashing systems with millions of unique RAs) As there is no support for signing or authentication, any node on a network can originate an RA and pretend to be a router, spoof the origin making some other node (possibly non-existent) eat traffic, or as previously mentioned, spew RAs until everything crashes.
I guess you never had to worry about rogue DHCP which is much easier to get on your network than rogue RAs.
Router Advertaisement are part of the Neighbor Discovery Protocol (NDP) not just a replacement for DHCP.
There are also methods to use RAs to migrate netwok (v6 to v6... from one prefix to another) without ever having to touch or even reboot the hosts.
DHCP has a few characteristics that could be regarded as problems.
- There is no way for a router to say "I have routes to these networks but not the internet in general".
- There are no provisions made for redundancy and load balancing of multiple gateways (this can be hacked arround at lower levels).
- DHCP servers are stateful
- There is no provision for running multiple adresses in paralell. IPv6 proponents saw running multiple addresses in parallel as a soloution to support multihoming without resorting to NAT or bloating the gloabl routing table with PI space.
- There is no provision for telling a client that due to unexpected network structure changes previous promises made in the DHCP lease can no longer be kept.