This task explains how to configure a new administrative distance for IPv6 IS-IS, configure the maximum number of equal-cost paths that IPv6 IS-IS will support, configure summary prefixes for IPv6 IS-IS, and configure an IS-IS instance to advertise the default IPv6 route (::/0). It also explains how to configure the hold down period between partial route calculations (PRCs) and how often Cisco IOS software performs the SPF calculation when using multitopology IS-IS.

You can customize IS-IS multitopology for IPv6 for your network, but you likely will not need to do so. The defaults for this feature are set to meet the requirements of most customers and features. If you change the defaults, refer to Cisco IPv4 configuration guide and IPv6 command reference to find the appropriate syntax.

Command syntax for customization of single topology IS-IS for IPv6:

To inject the default route into an IS-IS routing domain the following command may be entered in the address family ipv6 context of the corresponding IS-IS process:

             Router(config-router-af)# default-information originate [route-map map-name]

The route-map keyword and map-name argument specify the conditions under which the IPv6 default route is advertised.

If the route-map keyword is omitted, then the IPv6 default route will be unconditionally advertised at Level 2.

The following command defines an administrative distance for IPv6 IS-IS routes in the IPv6 routing table. Like the previous command it is entered in the address family ipv6 context.

           Router(config-router-af)# distance value

The value argument is an integer from 10 to 254 (values 0 to 9 are reserved for internal use).

As with RIP and BGP one may define the maximum number of equal-cost routes that IPv6 IS-IS can support:

          Router(config-router-af)# maximum-paths number-paths

Optionally one can allow a Level 1-2 router to summarize Level 1 prefixes at Level 2, instead of advertising the Level 1 prefixes directly when the router advertises the summary.

Router(config-router-af)# summary-prefix ipv6-prefix/prefix-length \\
                                                                                        [level-1 | level-1-2 | level-2]

The ipv6-prefix argument in the summary-prefix command must be in the form documented in RFC
2373 [RFC2373] where the address is specified in hexadecimal using 16-bit values between colons.
The argument is a decimal value that indicates how many high-order contiguous bits of the address
comprise the prefix (the network portion of the address). A slash mark must precede the decimal
value.