Flexible IP addressing for IPoE/PPPoE v4 and v6 hosts is by default disabled. In other words, the subscriber hosts are instantiated in the BNG with ability to forward traffic only if their assigned IP addresses belong to one of the configured subnets/prefixes that are associated with the subscriber-interfaces. IPv4 and IPv6 cases are be examined separately:
IPv4:
By default, IPoE and PPPoE subscriber host creation fails in the following two cases:
The subscriber-interface does not have an IPv4 address configured, and therefore it is operationally down. This configuration is also known as unnumbered subscriber-interface.
The subscriber-interface does have an IPv4 address configured but the IPv4 address assigned to the subscriber host itself is outside of the subscriber-interface configured subnets. In such case, the host is instantiated, but the forwarding is disabled.
Subscriber host instantiation and forwarding can be explicitly enabled for both cases above with flexible IP addressing functionality.
For case 1, this can be achieved by borrowing an IP address for the subscriber-interface from any interface that is operationally up within the routing context. This functionality can be enabled with the configure service ies | vprn <service-id>subscriber-interface <ip-int-name> unnumbered <ip-int-name | ip-address> command.
To enable forwarding for the subscribers whose IP address falls outside of the configured subnet under the subscriber-interface (case 2), the configure service ies | vprn <service-id> subscriber-interface <ip-int-name> allow-unmatching-subnets command must be entered.
The above commands (unnumbered and allow-unmatching-subnets) are mutually exclusive. In addition, the unnumbered command can be configured only if the subscriber interface does not have an IP address already configured. Otherwise the execution of this command fails.
In both of these cases the host is installed in the routing table as /32.
IPv6:
For IPv6 there is a single command that enables flexible IP addressing for both cases:
IPv6 prefixes are not configured under the sub-if>ipv6 node
IPv6 prefixes are configured but the actual address or prefix assigned to the subscriber (by DHCP, LUDB or RADIUS) is outside any prefix that is configured under the sub-if>ipv6 hierarchy.
This single command is configure service ies | vprn <service-id> subscriber-interface <ip-int-name> ipv6 allow-unmatching-prefixes.
To summarize, the following scenarios are possible:
PPPoEv4
An IPv4 address under the subscriber-interface is configured
By default, hosts outside of the sub-intf subnet are instantiated but they are in a non-forwarding-state. Traffic is dropped.
allow-unmatching-subnets is configured. This command is allowed only if subscriber-interface has also configured its own IPv4 address(es). In this case the IP address for IPCP negotiation is one of the sub-intf addresses. Hosts outside of the sub-intf subnets are instantiated and forwarded.
The unnumbered <ip-address | ip-int-name> command is not allowed in this scenario.
An IPv4 address under the subscriber-interface is not configured
By default, the subscriber-interface is operationally down. Subscribers cannot be instantiated.
The allow-unmatching-subnets command has no effect because a subscriber-interface does not have an IPv4 address configured and is therefore operationally down. No subscribers can be instantiated.
The unnumbered <ip-address | ip-int-name> command is the only viable option in this case. The subscriber-interface borrows an IPv4 address from another interface that is operationally UP and consequently this allows subscribers to be instantiated. This command is mutually exclusive with allow-unmatching-subnets. In addition, this command can only be configured if the subscriber interface itself does not have explicitly configured an IPv4 address.
IPoEv4
Like the PPPoE case above.
IPoEv6 and PPPoEv6
The allow-unmatching-prefixes command is independent of any IPv4 command related to flexible IP address assignment (unnumbered or allow-unmatching-subnets). This command can always be enabled, regardless of the v6 prefixes configured under the sub-if>ipv6 hierarchy. Any subscriber, regardless of the subscriber interface prefix configuration is instantiated and forwarded.