IPv6, multiple routers and possible bad prefix advertisement in ICMPv6 RA

icmpv6ipv6

I have two 'router' type devices setup on my network.

One is my actual IPv6 router – a Cisco 2621 (fe80::xxxx:3e60). It's the correct default router.

Another is a DLINK DIR-615 (fe80::xxxx:5760). It's IPv6 aware but at the moment is only configured for link-local addressing as it's only being used as a WAP.

Both devices are sending out ICMPv6 RA messages and as a result both are being added to my clients' routing table:

$ ip -f inet6 route | grep default
default via fe80::xxxx:3e60 dev wlan0  proto kernel  metric 1024  expires 1741sec mtu 1500 advmss 1440 hoplimit 64
default via fe80::xxxx:5760 dev wlan0  proto kernel  metric 1024  expires 1791sec mtu 1500 advmss 1440 hoplimit 64

The trouble arises from the fact is that my clients now have a choice of default routers. One sends packets merrily on their way, the other doesn't. I can remove the incorrect route but it just gets re-added when the next RA comes along.

fe80::xxxx:3e60 (the correct router) is sending out RAs advertising a prefix of 2001:xxxx:xxxx::/64 (my network)

fe80::xxxx:5760 (the incorrect router) is sending out RAs advertising a prefix of fe80::/64 (link-local)

So… who's at fault here?

  • fe80::xxxx:5760 for sending out RAs when it can't provide a valid prefix?
  • fe80::xxxx:5760 for sending out RAs with the link-local prefix?
  • my clients for adding fe80::xxxx:5760 to the route table when it's advertising the link-local prefix?
  • Something else?

Best Answer

radvd (router advertising daemon) should be disabled on the DLink, at least on the interfaces facing the Cisco router.

Alternatively, you could setup a static default route to the Cisco router on the DLink. This will make the route one hop longer, but still valid assuming the DLink will route back out the interface to the Cisco router.

If the WAN port is the one connected to the Cisco router, the DLink router should not be advertising routing on that link.

EDIT: You may be able to influence routing by setting AdvRoutePreference high; and/or AdvDefaultPreference high; in the Cisco router's radvd configuration.

EDIT2: The problem appears to be that the DLink router does not have a global IP address or route. Default route for routers is usually not discovered, so it may need to be set manually.

Step for setting ipv6 router preference from CISCO documentation are:

  1. enable
  2. configure terminal
  3. interface type number
  4. ipv6 nd router-preference {high | medium | low}
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