It’s time to explore the finer points of IPv6. A great place to start is by showing you how to address a host and what gives it the ability to find other hosts and resources on a network.
I’ll also demonstrate a device’s ability to automatically address itself—something called stateless autoconfiguration, plus another type of autoconfiguration known as stateful. Keep in mind that stateful autoconfiguration uses a DHCP server in a very similar way to how it’s used in an IPv4 configuration. I’ll also show you how Internet Control Message Protocol (ICMP) and multicast works for us on an IPv6 network.
Autoconfiguration
Autoconfiguration is an incredibly useful solution because it allows devices on a network to address themselves with a link-local unicast address. This process happens through first learning the prefix information from the router and then appending the device’s own interface address as the interface ID. But where does it get that interface ID? Well, you know every device on an Ethernet network has a physical MAC address, and that’s exactly what’s used for the interface ID. But since the interface ID in an IPv6 address is 64 bits in length and a MAC address is only 48 bits, where do the extra 16 bits come from? The MAC address is padded in the middle with the extra bits—it’s padded with FFFE.
For example, let’s say I have a device with a MAC address that looks like this: 0060.d673.1987.
After it’s been padded, it would look like this: 0260.d6FF.FE73.1987.
So where did that 2 in the beginning of the address come from? Another good question. You see, part of the process of padding (called modified eui-64 format) changes a bit to specify if the address is locally unique or globally unique. And the bit that gets changed is the seventh bit in the address. A bit value of 1 means globally unique, and a bit value of 0 means locally unique, so looking at this example, would you say that this address is globally or locally unique? If you answered that it’s a globally unique address, you’re right! Trust me, this is going to save you time in addressing your host machines because they communicate with the router to make this happen.
To perform autoconfiguration, a host goes through a basic two-step process:
1. First, the host needs the prefix information (similar to the network portion of an IPv4 address) to configure its interface, so it sends a router solicitation (RS) request for it. This RS is then sent out as a multicast to each router’s multicast address. The actual information being sent is a type of ICMP message, and like everything in networking, this ICMP message has a number that identifies it. The RS message is ICMP type 133.
2. The router answers back with the required prefix information via a router advertisement (RA). An RA message also happens to be a multicast packet that’s sent to each node’s multicast address and is ICMP type 134. RA messages are sent on a periodic basis, but the host sends the RS for an immediate response so it doesn’t have to wait until the next scheduled RA to get what it needs.
These two steps are shown in Figure 1
Two steps to IPv6 autoconfiguration
By the way, this type of autoconfiguration is also known as stateless autoconfiguration because it doesn’t contact or connect and receive any further information from the other device. We’ll get to stateful configuration when we talk about DHCPv6 in a minute. Now let’s take a look at how to configure Cisco routers with IPv6.
Configuring Cisco Routers with IPv6
In order to enable IPv6 on a router, you have to use the ipv6 unicast-routing global configuration command:
Corp(config)#ipv6 unicast-routing
By default, IPv6 traffic forwarding is disabled, so using this command enables it. Also, as you’ve probably guessed, IPv6 isn’t enabled by default on any interfaces either, so we have to go to each interface individually and enable it.
There are a few different ways to do this, but a really easy way is to just add an address to the interface. You use the interface configuration command ipv6 address
<ipv6prefix>/ <prefix-length>[eui-64] to get this done.
Here’s an example:
Corp(config-if)#ipv6 address 2001:db8:3c4d:1:0260.d6FF.FE73.1987/64
You can specify the entire 128-bit global IPv6 address or you can use the eui-64 option. Remember, the eui-64 format allows the device to use its MAC address and pad it to make the interface ID. Check it out:
Corp(config-if)#ipv6 address 2001:db8:3c4d:1::/64 eui-64
As an alternative to typing in an IPv6 address on a router, you can enable the interface instead to permit the application of an automatic link-local address.
Note:Remember, if you only have a link-local address, you will only be able to communicate on that local subnet.
To configure a router so that it only uses link-local addresses, use the ipv6 enable interface configuration command:
Corp(config-if)# ipv6 enable
Okay, now let’s dive into stateful IPv6 by configuring a DHCP server for IPv6 use.
I’ll also demonstrate a device’s ability to automatically address itself—something called stateless autoconfiguration, plus another type of autoconfiguration known as stateful. Keep in mind that stateful autoconfiguration uses a DHCP server in a very similar way to how it’s used in an IPv4 configuration. I’ll also show you how Internet Control Message Protocol (ICMP) and multicast works for us on an IPv6 network.
Autoconfiguration
Autoconfiguration is an incredibly useful solution because it allows devices on a network to address themselves with a link-local unicast address. This process happens through first learning the prefix information from the router and then appending the device’s own interface address as the interface ID. But where does it get that interface ID? Well, you know every device on an Ethernet network has a physical MAC address, and that’s exactly what’s used for the interface ID. But since the interface ID in an IPv6 address is 64 bits in length and a MAC address is only 48 bits, where do the extra 16 bits come from? The MAC address is padded in the middle with the extra bits—it’s padded with FFFE.
For example, let’s say I have a device with a MAC address that looks like this: 0060.d673.1987.
After it’s been padded, it would look like this: 0260.d6FF.FE73.1987.
So where did that 2 in the beginning of the address come from? Another good question. You see, part of the process of padding (called modified eui-64 format) changes a bit to specify if the address is locally unique or globally unique. And the bit that gets changed is the seventh bit in the address. A bit value of 1 means globally unique, and a bit value of 0 means locally unique, so looking at this example, would you say that this address is globally or locally unique? If you answered that it’s a globally unique address, you’re right! Trust me, this is going to save you time in addressing your host machines because they communicate with the router to make this happen.
To perform autoconfiguration, a host goes through a basic two-step process:
1. First, the host needs the prefix information (similar to the network portion of an IPv4 address) to configure its interface, so it sends a router solicitation (RS) request for it. This RS is then sent out as a multicast to each router’s multicast address. The actual information being sent is a type of ICMP message, and like everything in networking, this ICMP message has a number that identifies it. The RS message is ICMP type 133.
2. The router answers back with the required prefix information via a router advertisement (RA). An RA message also happens to be a multicast packet that’s sent to each node’s multicast address and is ICMP type 134. RA messages are sent on a periodic basis, but the host sends the RS for an immediate response so it doesn’t have to wait until the next scheduled RA to get what it needs.
These two steps are shown in Figure 1
Two steps to IPv6 autoconfiguration
Configuring Cisco Routers with IPv6
In order to enable IPv6 on a router, you have to use the ipv6 unicast-routing global configuration command:
Corp(config)#ipv6 unicast-routing
By default, IPv6 traffic forwarding is disabled, so using this command enables it. Also, as you’ve probably guessed, IPv6 isn’t enabled by default on any interfaces either, so we have to go to each interface individually and enable it.
There are a few different ways to do this, but a really easy way is to just add an address to the interface. You use the interface configuration command ipv6 address
<ipv6prefix>/ <prefix-length>[eui-64] to get this done.
Here’s an example:
Corp(config-if)#ipv6 address 2001:db8:3c4d:1:0260.d6FF.FE73.1987/64
You can specify the entire 128-bit global IPv6 address or you can use the eui-64 option. Remember, the eui-64 format allows the device to use its MAC address and pad it to make the interface ID. Check it out:
Corp(config-if)#ipv6 address 2001:db8:3c4d:1::/64 eui-64
As an alternative to typing in an IPv6 address on a router, you can enable the interface instead to permit the application of an automatic link-local address.
Note:Remember, if you only have a link-local address, you will only be able to communicate on that local subnet.
To configure a router so that it only uses link-local addresses, use the ipv6 enable interface configuration command:
Corp(config-if)# ipv6 enable
Okay, now let’s dive into stateful IPv6 by configuring a DHCP server for IPv6 use.
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