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DEMO: Setting RA Flags

Router Advertisement (RA) flags control how IPv6 clients obtain addressing information, directing them to use SLAAC, DHCPv6 for additional options, or DHCPv6 for full address assignment. This content covers how to configure the O flag and M flag on a Cisco switch virtual interface to define client behavior on a VLAN.

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About this video

IPv6 Router Advertisements do more than announce a default gateway — they carry flags that tell clients exactly how to proceed with address configuration. The two primary flags covered here are the O flag (Other Configuration) and the M flag (Managed Address Configuration). The O flag is used alongside SLAAC, signaling that the client should auto-configure its own IP address but reach out to a DHCPv6 server for additional parameters such as DNS. The M flag shifts full address assignment responsibility to DHCPv6, instructing the client to request an IP address from a DHCPv6 server rather than generating one through stateless autoconfiguration. Configuration takes place on a Cisco Layer 3 switch acting as the default gateway for multiple VLANs, with changes applied at the SVI level. The command ipv6 nd other-config-flag enables the O flag on a given interface, and ipv6 nd managed-config-flag enables the M flag. Both can be removed by prepending no to the respective command. The demonstration uses interface VLAN 10 as the target SVI, showing how each flag is toggled independently and how they interact — notably, enabling the M flag automatically suppresses SLAAC-based auto-configuration. Understanding these flag relationships is essential for designing IPv6 addressing schemes that correctly coordinate client behavior with available DHCPv6 infrastructure.

What you'll learn

What's covered

IPv6 Router Advertisement Flags

Aligned to

Cisco CCNA
1.8 Configure and verify IPv6 addressing and prefix
1.9 Describe IPv6 address types
4.3 Explain the role of DHCP and DNS within the network
4.6 Configure and verify DHCP client and relay

Key terms

Dynamic Host Configuration Protocol
DHCP
A network protocol that automatically assigns IP addresses and other configuration parameters to devices on a network.
Router Advertisement
RA
An ICMPv6 message periodically broadcast by IPv6-enabled routers to announce their presence, advertise network prefixes, and provide default gateway information for SLAAC. Hosts also request RAs immediately on startup by sending a Router Solicitation.
Stateless Address Autoconfiguration
SLAAC
An IPv6 mechanism that allows devices to automatically generate a globally unique IPv6 address using the network prefix from a Router Advertisement combined with a locally generated interface identifier, without requiring a DHCP server. SLAAC uses the EUI-64 process or a random interface ID to create the host portion of the address.
Managed Configuration Flag
An IPv6 router advertisement flag (M flag) that, when set, instructs clients to obtain their IP address from a stateful DHCPv6 server.
Other Configuration Flag
An IPv6 router advertisement flag (O flag) that, when set, instructs clients to use DHCPv6 to obtain additional configuration information such as DNS addresses, while still self-assigning their IP address via SLAAC.
Switch Virtual Interface
SVI
A virtual layer 3 interface on a switch associated with a VLAN. On a layer 2 switch a single SVI provides the management IP address; on a layer 3 switch, SVIs also route between VLANs.

Topics

Ipv6 Router Advertisement Slaac Dhcpv6 Neighbor Discovery Cisco Ios Networking

Transcript

When a router sends out a router advertisement, it will send out information about the default gateway, about itself, to the client. But it also will send out information on what that client should do next: should it reach out to a DHCP version 6 server for additional options, or should it reach out for an IP address, or should it create its own IP address?

Where we are making the change

We are going to make these configuration changes on switch one. Switch one is the default gateway for all of our different VLANs, so it is the appropriate spot to be setting this up on, at least for this demo.

What we are going to be doing is setting it up so that the router advertisements that are sent out are going to tell the client what it is going to do next. If we configure SLAAC, it would set the auto configure flag to one, letting that client know that it is going to auto configure an IP address for itself - it is stateless. Or what we could do is set it to SLAAC plus DHCP version 6. So we are going to turn on the other flag on here, so that way it will reach out to a DHCP version 6 to get some additional information.

I will also show you how to turn on the manage flag, so that way we turn DHCP version 6 on, so then it would actually reach out to a DHCP version 6 to get its IP address as well.

One thing to note is that I am not actually going to turn on SLAAC at this point in time. We are just going to take a look at how to adjust these flags, so in essence, at the end of this demo we will not have this all up and running and working. I am just going to be showing you how to turn it on - and in fact, this one right here just gets turned on when you turn SLAAC on. So we are going to take a look at these two configurations right here, of how to set up these flags on the network, and then you can take this and apply it as we talk further about turning on these different options.

Setting the O flag

Here I am on my switch, and I need to make these changes on the individual interfaces. So I am going to get onto the interface, and in this case it is going to be our switch virtual interface. I am going to jump onto our switch virtual interfaces, our SVIs, and for this particular one let us take a look at VLAN 10 and do this for VLAN 10.

I am going to get into global configuration mode first, and look at interface VLAN 10, so we get into that switch virtual interface. Then I am going to type in ipv6 nd other-config-flag.

If I had turned SLAAC on, the flag for the auto configuration would already be turned on. This would then turn the O flag on, and I would hit that and the O flag would then be turned on. To remove it, it is as simple as the same command with the no at the front, and then we just go back to straight SLAAC. So this is how to turn on SLAAC plus DHCP version 6, and then turn the second part off, so now it is just SLAAC at this point.

Setting the manage flag

Then I can also do the same thing if I want to just point it and say, hey, go find your IP address from a DHCP version 6. I am - dash flag - hit enter, and now the clients on this network will just go reach out to a DHCP version 6 for their IP address.

This would also turn the auto config flag off and also turn off the other flag, but I want those flags to be on, so I am going to type in no ipv6 nd manage-config-flag.

There you have it. That is how to turn on the different flags in the router advertisement. If the router advertisement was going out, then those flags would be turned on.

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