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If I write

interface Vlan1
 ... {ipv4 config} ...
 ipv6 address 2001:DB8:5432:101::/64 eui-64
 ipv6 address FDAA:DB8:5432:101::/64 eui-64
 ipv6 enable
 ipv6 nd prefix 2001:DB8:5432:101::/64
 ipv6 nd prefix FDAA:DB8:5432:101::/64
 ipv6 nd other-config-flag
 ipv6 nd ra interval 4
 ipv6 dhcp server DHCP6_POOL
!

then the NDP RAs I see look like (tcpdump, machine connected to Vlan1)

IP6 (class 0xe0, hlim 255, next-header ICMPv6 (58) payload length: 96) fe80::{router's eui-64} > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 96
hop limit 64, Flags [other stateful], pref medium, router lifetime 1800s, reachable time 0ms, retrans timer 0ms
  source link-address option (1), length 8 (1): {router's eui-48}
  mtu option (5), length 8 (1):  1500
  prefix info option (3), length 32 (4): 2001:db8:5432:101::/64, Flags [onlink, auto], valid time 2592000s, pref. time 604800s
  prefix info option (3), length 32 (4): fdaa:db8:5432:101::/64, Flags [onlink, auto], valid time 2592000s, pref. time 604800s

exactly as expected. The prefixes being advertised are the ones configured with ipv6 nd prefix *::/64, no more, no fewer.

However, if I configure additional addresses on the interface with ipv6 address but don't add those prefixes as ipv6 nd prefix as follows

interface Vlan1
 ... {ipv4 config} ...
 ipv6 address 2001:DB8:5432:101::/64 eui-64
 ipv6 address 2001:DB8:5432:102::/64 eui-64
 ipv6 address FDAA:DB8:5432:101::/64 eui-64
 ipv6 address FDAA:DB8:5432:102::/64 eui-64
 ipv6 enable
 ipv6 nd prefix 2001:DB8:5432:101::/64
 ipv6 nd prefix FDAA:DB8:5432:101::/64
 ipv6 nd other-config-flag
 ipv6 nd ra interval 4
 ipv6 dhcp server DHCP6_POOL
!

then I would expect RAs for only the prefixes specified with ipv6 nd prefix, however I see RAs containing all four prefixes:

IP6 (class 0xe0, hlim 255, next-header ICMPv6 (58) payload length: 160) fe80::{router's eui-64} > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 160
hop limit 64, Flags [other stateful], pref medium, router lifetime 1800s, reachable time 0ms, retrans timer 0ms
  source link-address option (1), length 8 (1): {router's eui-48}
  mtu option (5), length 8 (1):  1500
  prefix info option (3), length 32 (4): 2001:db8:5432:101::/64, Flags [onlink, auto], valid time 2592000s, pref. time 604800s
  prefix info option (3), length 32 (4): 2001:db8:5432:102::/64, Flags [onlink, auto], valid time 2592000s, pref. time 604800s
  prefix info option (3), length 32 (4): fdaa:db8:5432:101::/64, Flags [onlink, auto], valid time 2592000s, pref. time 604800s
  prefix info option (3), length 32 (4): fdaa:db8:5432:101::/64, Flags [onlink, auto], valid time 2592000s, pref. time 604800s

What configuration can I use to give the router all four addresses on Vlan1, but only advertise the original two - so that dynamic devices will only autoconfigure in 101 subnets, but the router can still also talk to servers statically assigned addresses within 102 subnets?

3
ipv6 nd prefix default no-advertise

or list the interface addresses not to announce

ipv6 nd prefix 2001:DB8:5432:102::/64 no-advertise
ipv6 nd prefix FDAA:DB8:5432:102::/64 no-advertise
2
  • The former suppresses all prefixes in RAs and I don't see an option converse to no-advertise, so while I think default no-advertise + prefix 2001:101 advertise + prefix 2001:102 would be neater, I'm happy with the working solution prefix 2001:101 + prefix 2001:102 no-advertise. Thanks. – Scruffy Mar 3 at 3:00
  • The former is how it's supposed to work. Looks like Cisco has yet another bug in their IPv6 thinking. – Ricky Mar 3 at 16:15

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