6RD: how to tie prefix lifetimes to (expiring) DHCPv4 lease times?

[Marc Luethi]

Hi all

One of our local ISPs (Swisscom) is in a beta program for a 6RD deployment. I'm using their 6RD BR with a 881-SEC/K9 with 15.1(4)M, and it works pretty well.

I wonder:

Should a router adapt the prefix lifetimes in it's router advertisments, once the address lease (here: DHCPv4) on the WAN interface starts to expire? How should a router behave if the address assignment isn't lease-based, but "on/off" as with PPPoE?

And if yes, how would I configure an 881 to do exactly that?

RFC5969, section 4 states that

The prefix lifetimes advertised in Router Advertisements or
used by DHCP on the CE LAN side MUST be equal to or shorter than the
IPv4 address lease time.

It however neglects to state if that lifetime was to be static (for example lease duration as given in the DHCP exchange) or if it was to follow the remaining lifetime dynamically ("counting down") once the lease is issued.

The DHCPv4 lease on my 881 looks like this:

router6rd#show dhcp lease

Temp IP addr: xx.bbb.ccc.42  for peer on Interface: FastEthernet4

Temp  sub net mask: 255.255.252.0

   DHCP Lease server: ddd.hhh.c.pp, state: 5 Bound

   DHCP transaction id: 1F49

   Lease: 28800 secs,  Renewal: 3600 secs,  Rebind: 5400 secs

Temp default-gateway addr: xx.bbb.ccc.1

   Next timer fires after: 00:01:13

   Retry count: 0   Client-ID: cisco-c89c.1dea.8eee-Fa4

As shown in the many config examples, I am using a "general prefix" and the proverbial "Tunnel 6" interface...

ipv6 general-prefix 6RD-SWISSCOM 6rd Tunnel6

interface Tunnel6

description * Swisscom 6RD Tunnel *

no ip address

load-interval 30

ipv6 enable

ipv6 mtu 1480

tunnel source FastEthernet4

tunnel mode ipv6ip 6rd

tunnel 6rd prefix 2A02:1200::/28

tunnel 6rd br 164.128.155.23

interface FastEthernet4

description * DSL-LAN *

ip address dhcp

ip nat outside

ip nat enable

... I can provide the parameters for Clients to use SLAAC on the LAN side. Trying to mimic the DHCPv4 values, I tried adapting the default timers

interface Vlan42

ip address 172.20.42.1 255.255.255.0

     ip nat inside

load-interval 30

ipv6 address 6RD-SWISSCOM ::2:0:0:0:1/64

ipv6 enable

ipv6 nd prefix default 28800 3600

end

But I doubt that is the correct approach. What I do observe is that my Linux clients keep resetting their valid/preferred prefix lifetimes to these values after an RA arrives at their interfaces (seems reasonable enough). But what if the DHCPv4 lease can't be renewed and the address becomes invalid, and is possibly going to be replaced by a different address next time the DHCPv4 service becomes reachable again? Shouldn't the address' expiring lifespan be propagated to the systems on the network?

Thanks for sharing your comments...

Marc

[Ole Troan]

Marc,

a 6rd CE should indeed adapt its advertised prefix lifetimes to the DHCP IPv4 address lease time.

this should be counting down in real time. 6rd Prefix Delegation should work just like Prefix Delegation specificed in RFC3633 in that regard.

in cases like PPP, there is no choice but to follow the defaults in RFC4861. if the prefix changes, you will get an "instant renumbering" event. that has some issues in IPv6, but see RFC6204 for recommendations on how a CPE should handle that.

the IOS general prefix has lifetimes associated with it; I'm not able to verify if this is done correctly in IOS right now, but I fear it is an omission. please let me know and I'll open a bug.

as is also stated in RFC5969, it is recommended that the IPv4 address is long lived (i.e. change rarely) because of the problems caused by instant renumbering. even if the lifetimes in the RA is advertised correctly, there is no way to do a "proper" IPv6 renumbering event with 6rd (an old prefix being phased out for a new one).

cheers,

Ole

[Marc Luethi]

Ole,

Thanks a lot for your reply.

As of 15.1(4)M1, my 881 shows no means of setting preferred or valid lifetimes for the ipv6 general-prefix:

router6rd(config)#ipv6 general-prefix 6RD-SWISSCOM 6rd Tunnel6 ?

.. and neither does the Cisco IOS IPv6 Command Reference from the IOS 15.1M&T command references give hints this direction.

Still, the general prefix does seem to have lifetime properties associated with it:

router6rd#show ipv6 general-prefix

IPv6 Prefix 6RD-SWISSCOM, acquired via 6rd

  2A02:1205:C6AA:82A0::/60 Valid lifetime infinite, preferred lifetime infinite

This might indeed be an omission, and is possibly worth filing a bug.

best regards & thanxalot

Marc

[Ole Troan]

Marc,

apologies for the delay. I've opened CSCtt34625.

cheers,

Ole