Ipv6 link-local

[getaway51]

Hi,

May I know What is the function of link-local? What is the difference for ipv6 address without link-local like below? Do we need both ipv6 address line like below? Many many thanks!!

int Gi0/4

ipv6 address 2403:8300:500:3::/64

ipv6 address FE80::8300:500:1:1 link-local

[Francesco Molino]

Hi

Link local addresses are local to broadcast domain. The router won't forward traffic out using this address.
They're mostly used for auto-address configuration and neighbour discovery.

The 2nd ip is what we call global address and it's routable across the Internet.

Then yes you need both.

[getaway51]

Hi,

Many thanks for your explanation.

1) Can I say tht local-link address is just local significance. Therefore every site/router can configure the same link-local address

like router is FE80::8300:500:1:1 and core switch is FE80::8300:500:1:2. It will not conflict. 

2)Can I say every interfaces in a router can also configure the same link-local address?

3) Is link-local used just to verify p2p link connectivity and interface (i.e. router to core switch connectivity & its connected interfaces). Is it use for other purposes?

5) Also I noticed link-local address doesnt have any subnet like /64, /127,etc. Does it means it is /128?

[Harold Ritter]

> 1) Can I say tht local-link address is just local significance. Therefore every site/router can configure the same link-local

> address like router is FE80::8300:500:1:1 and core switch is FE80::8300:500:1:2. It will not conflict. 

Correct.

> 2)Can I say every interfaces in a router can also configure the same link-local address?

Also correct.

> 3) Is link-local used just to verify p2p link connectivity and interface (i.e. router to core switch connectivity & its

> connected interfaces). Is it use for other purposes?

Link-Local address are used for many purposes. Among others, for the Neighbor Discovery (ND) process and for routing protocol communication between adjacent routers.

> 5) Also I noticed link-local address doesnt have any subnet like /64, /127,etc. Does it means it is /128?

The link-local addresses are defined by RFC4291 and have a fix format, hence we do not need to configure the prefix length.

2.5.6. Link-Local IPv6 Unicast Addresses

   Link-Local addresses are for use on a single link.  Link-Local
   addresses have the following format:

   |   10     |
   |  bits    |         54 bits         |          64 bits           |
   +----------+-------------------------+----------------------------+
   |1111111010|           0             |       interface ID         |
   +----------+-------------------------+----------------------------+

   Link-Local addresses are designed to be used for addressing on a
   single link for purposes such as automatic address configuration,
   neighbor discovery, or when no routers are present.

   Routers must not forward any packets with Link-Local source or
   destination addresses to other links.

Regards,

[Craddockc]

Harold,

I apologize for hijacking the thread but I also had some questions about IPv6. I am trying to study for and pass the CCNP ROUTE exam before the February deadline and I ashamedly admit that I dont really know much about IPv6. I am currently getting a grasp of it so that I can actually understand how to configure and troubleshoot IPv6 routing for the exam. I had a couple of questions if you would be so inclined to assist:

Global Unicast Address: I gather that this is the equivalent to a "public" IPv4 address but for IPv6 and that your internal addresses would be NATed to one of these addresses for routing on the internet? Im assuming the ISP doles these out to their customers the same as they do public IPv4 addresses?

Unique-Local Address: I gather that this is equivalent to a private IPv4 address that is only routable on the internal network? Is this an address you would give out internally via DHCP?

Link-Local Address: I gather that this is always autoassigned by the NIC to itself (similar to an APIPA address) and is used for local broadcast domain communications only?

With all these different address types, Im assuming that an IPv6 NIC could have one or more of these addresses assigned to it at any one time?

Thanks for any feedback you can provide. 

[Harold Ritter]

> Global Unicast Address: I gather that this is the equivalent to a "public" IPv4 address but for IPv6

Correct.

> and that your internal addresses would be NATed to one of these addresses for routing on the internet?

Your internal address will most likely be the GUA as well, as NAT is not recommended in the IPv6 context.

> Im assuming the ISP doles these out to their customers the same as they do public IPv4 addresses?

Correct.

> Unique-Local Address: I gather that this is equivalent to a private IPv4 address that is only routable on the internal network?

Correct.

> Is this an address you would give out internally via DHCP?

In general, the recommendation is to assign GUA in the internal network to provide global connectivity.

> Link-Local Address: I gather that this is always autoassigned by the NIC to itself (similar to an APIPA address)

The LLA is derived from the MAC address by default. It is auto assigned by the IPv6 stack, not by the NIC.

> and is used for local broadcast domain communications only?

Correct.

> With all these different address types, Im assuming that an IPv6 NIC could have one or more of these addresses assigned

> to it at any one time?

Correct. In the context of IPv6, an interface generally have one LLA and one or more GUA or ULA.

Regards,

[Craddockc]

Harold,

Thanks so much for the responses, they are very helpful! 

[Craddockc]

Harold,

I apologize for making you come back to this thread but did have a few other questions.

Under what circumstances would you choose to use a Unique Local Address (ULA) over a Global Unicast Address (GUA) for your hosts? If GUA's are globally unique all over the world, in what situations would using ULA's be appropriate? Since there is no NAT in IPv6, im assuming that the ULA will be used for all internal communications between internal subnets and GUA address is used for anything that needs to go external?

From a routing table perspective, if you are using GUA's internally, will the routing table in the router just be filled with GUA addresses? 

I've never worked with IPv6 so these address types and use cases are little confusing.

I appreciate your help! 

[Harold Ritter]

> Under what circumstances would you choose to use a Unique Local Address (ULA) over a Global Unicast Address (GUA) for

> your hosts? If GUA's are globally unique all over the world, in what situations would using ULA's be appropriate?

I would personally not use ULA. I guess one could plan to use ULA if there is a certain level of certainty that the application will never have to access the Internet, but would could be sure of that these days.

> Since there is no NAT in IPv6, im assuming that the ULA will be used for all internal communications between internal subnets > and GUA address is used for anything that needs to go external?

I would just use GUA for both.

> From a routing table perspective, if you are using GUA's internally, will the routing table in the router just be filled with GUA > addresses? 

The internal routing table would have GUA indeed, but only internal to your network. You would still use a default route to reach the Internet.

> I've never worked with IPv6 so these address types and use cases are little confusing.

Not a problem. This is exactly what this forum is all about. Let us know if you have more doubts/questions.

Regards,

[Craddockc]

Harold,

Thank so much for taking time to answer my questions. I did need some clarification on the Gloabl Routing Prefix/Site prefix and the Subnet ID. My understanding is as follows:

Global Routing Prefix/Site prefix: This is a prefix that is owned and allocated to a certain ISP, it identifies the a portion of the "address domain" allocated to the ISP itself?

Subnet ID: This is a prefix that identifies the internal IPv6 address space of the private site connected to that ISP? 

Thanks.

[Seb Rupik]

Hi there,

Your statement about the Global routing prefix is correct. Typically a large organisation would expect to receive a /48, eg:

2001:630:d0::/48

The global routing prefix above leaves 16 bits until the interface/ host ID portion of the address is reached. This subnet ID portion is used to allocate addresses to fit your network topology. eg: 2001:630:d0:ed00::/64

https://tools.ietf.org/html/rfc4291#section-2.5.4

cheers,

Seb.