Maximum Number of IPv6 Prefixes Assigned to Clients Using Both DHPCv6 Local Server and Neighbor Discovery Router Advertisements

When both IPv6 Neighbor Discovery router advertisements and DHCPv6 Prefix Delegation methods are used to assign IPv6 prefixes to clients, either two or three host routes for IPv6 might be consumed from the routing table depending on the way in which the router advertisement prefix is determined. The following sections describe sample configuration scenarios to illustrate how a maximum of 48,000 subscribers can be handled for delegation of IPv6 prefixes, based on whether a unique IPv6 prefix is allocated to a client or the same IPv6 prefix is allocated to multiple clients:

• Delegation of a Unique IPv6 Prefix per Subscriber Example
• Delegation of the Same IPv6 Prefix for Multiple Subscribers Example

Delegation of a Unique IPv6 Prefix per Subscriber Example

Consider a scenario in which the RADIUS server is configured to assign a unique router advertisement prefix route to each IPv6 subscriber. In such a case, two routes are used for Neighbor Discovery and one IPv6 route is consumed for Prefix Delegation, which results in a total of three routes being utilized for each subscriber. If such a method for allocating prefixes to subscribers is configured, approximately 33,333 IPv6 bindings can be supported before the maximum IPv6 static route limit of 100,000 routes is reached. Therefore, in such a deployment, it is not possible to handle 48,000 subscribers for delegation of IPv6 prefixes using the DHCPv6 local server Prefix Delegation and Neighbor Discovery methods.

The following output of the show ipv6 route command displays how three routes are used by the same subscriber, as can be seen from the Interface field in the output. The routes are assigned using Prefix Delegation, Neighbor Discovery, and the access-internal route, such as the DHCP and AAA/PPP host route, which is a host route to directly connected clients. Access routes, also known as AAA framed routes, are sourced by AAA.

host1#show ipv6 route

Prefix/Length                      Type      Dst/Met     Interface 
-------------------------------- 	---------  --------   ---------------- 
1111:1111:1111:1111::/64          	Access     3/0 	      GigabitEthernet0/2.600.6
1111:1111:2222:2222::/64          	AccIntern  2/0 	      GigabitEthernet0/2.600.6 
1111:1111:2222:2222:21b:c0ff:fe4 	 AccIntern  2/0 	      GigabitEthernet0/2.600.6 b:9d00/128

Delegation of the Same IPv6 Prefix for Multiple Subscribers Example

Consider a scenario in which the same prefix with a length of /64 for ICMPv6 Neighbor Discovery router advertisements is assigned to all subscribers by configuring the prefix in the profile or by configuring the RADIUS server to send the same prefix in the Framed-IPv6-Prefix attribute (RADIUS IETF attribute 97) of the RADIUS-Access-Accept message. In such a topology, a unique /64 IPv6 route is not present per subscriber. Instead, one /64 prefix with multiple next-hops is assigned for all the subscribers.

If you use this method for allocating IPv6 prefixes of /64 length to subscribers, Neighbor Discovery consumes one IPv6 route and Prefix Delegation consumes one IPv6 route, which results in a total of two IPv6 routes per subscriber being used. Therefore, it is possible to scale up to a maximum of 48,000 subscribers for delegation of IPv6 prefixes.

The increased scaling limit of support for delegation of IPv6 prefixes using the DHCPv6 local server Prefix Delegation mechanism for 48,000 subscribers applies only to E120 and E320 routers and not to ERX14xx models, ERX7xx models, and the ERX310 router because the binding information is stored in the SRP modules of E120 and E320 routers. Also, a limitation exists on the number of IPv6 interfaces and the IPv6 routing table size supported by ERX routers that prevents the support for 48,000 subscribers for Prefix Delegation on DHCPv6 local servers running on those routers.

To enable support for 48,000 subscribers for IPv6 Prefix Delegation, about 5.5 MB of memory on the SRP module is consumed additionally.