Cisco IOS allows the configuration of the following router parameters in router advertisements:
• The time interval between periodic router advertisement messages;
• The “router lifetime” value, which indicates the usefulness of a router as the default router (for use by all nodes on a given link);
• The network prefixes in use on a given link;
• The time interval between neighbor solicitation message retransmissions (on a given link);
• The amount of time a node considers a neighbor reachable (for use by all nodes on a given
link).
Enabling Router Advertisement Messages
The configured parameters are specific to an interface. The sending of router advertisement messages (with default values) is automatically enabled on Ethernet and FDDI interfaces when the ipv6 unicastrouting global configuration command is configured. This means that a router will automatically send out router advertisements for the prefix corresponding to the (global) addresses configured on an interface belonging to the particular link. That is, if there is a statement of the form:
Router (config-if)# ipv6 address address/prefix-length
Within the interface configuration context the router will automatically send router advertisements for the prefix this address was built from. However, for stateless autoconfiguration to work properly, the advertised prefix length in router advertisement messages must always be 64 bits.
For other interface types, the sending of router advertisement messages must be manually configured by using the no ipv6 nd suppress-ra global configuration command. The sending of router advertisement messages can be disabled on individual interfaces by using the ipv6 nd suppress-ra interface configuration command.
Example
If an interface is configured in the following way, the router will automatically send out router advertisements for the prefix 2001:638:500:101::/64 on the link this interface is attached to.
interface Ethernet0/0
ipv6 address 2001:638:500:101::/64 eui-64
If the interface was additionally configured as follows, the prefix list in router advertisements on this link would include prefix 2001:638:102::/64.
ipv6 address 2001:638:500:102:aaaa:bbbb:cccc:dddd/64
Customizing Prefix Information in Router Advertisement Messages
Aside from the prefixes to be advertised on a link Cisco IOS allows the configuration of the following router parameters in router advertisements:
• If more than one address is configured on an interface the network prefixes to be advertised on a given link. It is also possible to advertise prefixes not configured on the interface itself .
• The time interval between periodic router advertisement messages. This parameter is set for the interface in general. That means all prefixes to be advertised on the interface will be advertised at the same time albeit possibly with different lifetimes.
The “router lifetime” value, which indicates the usefulness of a router as the default router (for use by all nodes on a given link). This means that routers can theoretically advertise prefixes for use with stateless address autoconfiguration while not acting as default router for hosts on.
• The time interval between neighbor solicitation message retransmissions (on a given link).
• The amount of time a node considers a neighbour reachable (for use by all nodes on a given link).
Command Syntax:
Router (config-if)# ipv6 nd prefix ipv6-prefix/prefix-length \\
| default [[valid-lifetime preferred-lifetime] \\
| [at valid-date preferred-date] | infinite \\
| no-advertise | off-link | no-autoconfig]]
This command allows control over the individual parameters per prefix, including whether or not the prefix should be advertised at all.
As mentioned above, prefixes configured as addresses on an interface using the ipv6 address command are advertised in router advertisements by default. However, if you configure prefixes for advertisement using the ipv6 nd prefix command, then only these prefixes are advertised.
The default keyword can be used to set default parameters for all prefixes.
With the at valid-date preferred-date option a date can be set to specify the expiration of a prefix. The valid and preferred lifetimes are counted down in real time. When the expiration date is reached, the prefix will no longer be advertised.
If not otherwise specified all prefixes configured on interfaces that originate IPv6 router advertisements are advertised with a valid lifetime of 2592000 seconds (30 days) and a preferred lifetime of 604800 seconds (7 days).
When onlink is “on” (by default), the specified prefix is assigned to the link. Nodes sending traffic to such addresses that contain the specified prefix consider the destination to be locally reachable on the link.
When autoconfig is “on” (by default), it indicates to hosts on the local link that the specified prefix can be used for IPv6 autoconfiguration.