This topic is to discuss the following lesson:
Nice info, thanx!!
A couple of questions, for stateful:
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why did was the āipv6 nd managed-config-flagā not added to interface f0/0, so the RA sends the āMā flag for clients to make a DHCPv6 Solicit message request?
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why was āipv6 nd prefix 2001:1111:1111:1111::/64 no-autoconfigā not added to interface f0/0 so the RA does not send an āAā flag to tell clients not to autoconfig an addr (SLAAC off)?
Thanxā¦Jeff
Hello RENE,
Thank you for your nice posts.
I have some questions about adressing a network in IPv6:
Is there any rules to address an IPv6 network?
Can we for example use some small subnets such as /120 or should we always work with /64?
And if we are using stateful, shoud we use /64?
Thank you for your help
Hello Ali,
this document is good:
It helped me a lot.
Marek
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Thank you Marek. Nice Doc
Hi Jeff,
I just did some debugging and updated the post.
By default, Cisco routers only have the A flag enabledā¦the M flag is disabled and as a result my Windows 7 and Ubuntu client both preferred SLAAC instead of DHCPv6.
I enabled the M flag and Windows 7 and Ubuntu both prefer DHCPv6 instead of SLAAC thenā¦having the A flag enabled or disabled didnāt matter but it sounds like a good idea to keep it disabled anyway.
Thanks 
Rene
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Thatās a nice document indeedā¦good share Marek.
Hi Rene,
Thanks for the elaborate explanation.
I have a questions:
The command āipv6 address dhcpā and āipv6 address dhcp rapid-commitā is weirdly not recognized as a valid command by my cisco router while the server supports the āipv6 dhcp server STATEFUL rapid-commitā is valid.
My IOS verion is (C880DATA-UNIVERSALK9-M), Version 12.4(20)T
I did a quick search about the support of rapid-commit and apparently it is supported.
Do you have any idea why Iām not able to use those two commands to get a a stateful address?
Hi Imin,
Itās probably the IOS version. I wrote this tutorial using my 2800 routers running IOS 15.x. I checked it on a 3725 router in GNS3 running 12.4T and it also doesnāt support this command.
Rene
Hi Rene,
Thnx again for uber-supper post againā¦
I would like to ask something that;
I make stateless / stateful config as exactly same you wrote. Client got Ipv6 addressing but is not able to get dns or domain informations both even I already added on dhcp pools separately.
You have any idea about that ?
Deniz
Thanx
Deniz
Hi Deniz,
Thatās a good questionā¦it should work out of the box. The question is, does the server not send the options or does the client refuse them. What client did you use?
You could try a quick wireshark capture, take a look to see if the options are included in the DHCP messages from the server.
Rene
Hi Rene again,
I have captured the packets from fa 0/0 statefull DHCP leg that contains ;
Solicit XID
Advertise XID
Request XID
Reply XID
Each of these messages contain Domain Search List and DNS Recursive Name ( as long as I checked the values , all of them have bulk datagram messages ), DNS and domain names are delivered properly.
I think the IOS images may cause the problem ?
Deniz
Thank You
Hi Deniz,
Hmm thatās strange. What IOS image / router did you use? Also what client did you use?
Rene
Rene a question, what does this command do?
āipv6 nd prefix 2001:1111:1111:1111::/64 14400 14400 no-autoconfigā
the ipv6 nd prefix 2001:1111:1111:1111::/64
and the 14400 14400 ?
Thank you
i just simulate this command and based on my understanding the ipv6 nd prefix is to advertise the prefix i want to advertise? which is 2001:1111:1111:1111::/64?
then the 14400 14400 are the valid time and preferred time in seconds.
i have config all the commands in the DHCPv6 server, but i have problem about the clients. Theres no āipv6 address dhcpā the next in ipv6 address is
WORD General prefix name
X:X:X:X::X IPv6 link-local address
X:X:X:X::X/ IPv6 prefix
autoconfig Obtain address using autoconfiguration
theres no DHCP.
im using 7200 15.2ios and also 3745 12.4ios.
is there any other command? thank you
Hi John,
Iāve seen this before and I think (for whatever reason) that itās not yet supported in these versions. Thereās no other command that I know of.
Perhaps use a Windows 7 or Linux client as the DHCP client instead?
Rene
Hi,
just one question
DHCPV6(config-if)#ipv6 nd prefix 2001:1111:1111:1111::/64 14400 14400 no-autoconfig
14400 , what is this for?
Hi,
The first value advertises how long the IPv6 prefix is valid. The second one advertises how long the prefix is the āpreferredā prefix. This is useful if you are advertising multiple prefixes.
Rene
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Hi. Couple questions/validations.
- Assume that in either case the default gateway is learned via the RA, not stateful or stateless DHCPv6?
2 . For the stateful DHCPv6 example, what is it required to define the āipv6 nd prefix 2001:1111:1111:1111::/64 14400 14400 no-autoconfigā under the interface - is this just to inform to not use stateless configuration - if so why need to define the prefix as isnt that provided as part underneath the DHCP pool? - The host portion when using stateful DHCPv6 in this example is completely random in this case? Of course assume real world one would want to use a formal DHCPv6 like QIP/Infoblox and and have some sort of strategy/pattern for host recognition?
- I noticed some other option/choices instead of āā¦ no-autoconfigā highlighted in question #2 above such as no-onlink, no-rtr-address, and off-link. Can you elaborate when might want to use those?
Many thanks in advance.
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Hi Thomas,
-
Sending a RA when using DHCPv6 might sound redundant. You have to keep in mind that the RA is not only used for autoconfiguration. IPv4 uses the subnet mask to check if a destination is inside or outside the subnet. When the destination is outside of the subnet, weāll use the default gateway. In IPv6 we can use the RA to advertise which prefixes are āon linkā or āoff linkā. The no-autoconfig flag informs the host to use DHCP instead of autoconfiguration for its IPv6 address.
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The hosts will use EUI-64 to configure the 64 bits of address. This should be fine, you can always use DNS registration so that you can reach your hosts with hostnames instead of the IPv6 addresses.
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Thereās a lot of stuff, letās see what the ND/RA options are:
R1(config-if)#ipv6 nd prefix 2001:1111:1111:1111::/64 14400 14400 ?
no-autoconfig Do not use prefix for autoconfiguration
no-onlink Do not use prefix for onlink determination
no-rtr-address Do not send full router address in prefix advert
off-link Prefix is offlink
R1(config-if)#ipv6 nd ?
advertisement-interval Send an advertisement interval option in RAās
autoconfig Automatic Configuration
cache Cache entry
dad Duplicate Address Detection
managed-config-flag Hosts should use DHCP for address config
ns-interval Set advertised NS retransmission interval
nud Neighbor Unreachability Detection
other-config-flag Hosts should use DHCP for non-address config
prefix Configure IPv6 Routing Prefix Advertisement
ra Router Advertisement control
reachable-time Set advertised reachability time
router-preference Set default router preference value
secured Configure SEND
no-onlink / off-link: this tells the host if a prefix is on-link or off-link. Iāll have to think of some examples when and why we want to use thisā¦something for another tutorial.
no-rtr-address: this removes the router address so the hosts donāt have a default gateway, useful if hosts have to remain in their own subnet.
managed-config-flag: weāve seen this one, itās for DHCPv6.
other-config-flag: this is for stateless DHCPv6.
nud: neighbor unreachability detectionā¦no idea how it works, need to lab it up
router-preference: useful when you have multiple routers, you can use it to tell hosts what default gateway to prefer.
secured: havenāt tried this before but itās a secure version of NDP (Neighbor Discovery).
Rene
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