How to Enable IPv6 Support on a Cisco Catalyst 3560 Switch?

S1#show sdm prefer

 The current template is “desktop default” template.

 The selected template optimizes the resources in

 the switch to support this level of features for

 8 routed interfaces and 1024 VLANs.

  number of unicast mac addresses:                  6K

  number of IPv4 IGMP groups + multicast routes:    1K

  number of IPv4 unicast routes:                    8K

    number of directly-connected IPv4 hosts:        6K

    number of indirect IPv4 routes:                 2K

  number of IPv4 policy based routing aces:         0

  number of IPv4/MAC qos aces:                      0.75K

  number of IPv4/MAC security aces:                 1K

To see the list of available SDM templates, go to config mode and type sdm prefer? You can see dual stack ipv4/ipv6 support is one of the templates.

S1(config)#sdm prefer ?

  access              Access bias

  default             Default bias

  dual-ipv4-and-ipv6  Support both IPv4 and IPv6

  ipe                 IPe bias

  routing             Unicast bias

  vlan                VLAN bias

So, if you try to configure an IPv6 address on an interface without enabling the correct SDM, you get nothing:

S1(config-if)#ipv6 ?

% Unrecognized command

How do I enable IPv6 support on a Cisco 3560 switch?

To enable IPv6 support on the Cisco 3560 switch, you simply change the SDM template and reload. Start by enabling the dual-ipv4-and-ipv6 template with one of three sub-options:

S1(config)#sdm prefer dual-ipv4-and-ipv6 ?

  default  Default bias

  routing  Unicast bias

  vlan     VLAN bias

As you can see, you can decide to give more resources to routing or more to vlans when enabling IPv6 support. Since I’m using this switch in a lab to practice IPv6 routing protocols, I’m going to select the routing bias.

S1(config)#sdm prefer dual-ipv4-and-ipv6 routing

Changes to the running SDM preferences have been stored, but cannot take effect

until the next reload.

Use ‘show sdm prefer’ to see what SDM preference is currently active.

After a reload, here’s the resulting template change:

S1#show sdm prefer

 The current template is “desktop IPv4 and IPv6 routing” template.

 The selected template optimizes the resources in

 the switch to support this level of features for

 8 routed interfaces and 1024 VLANs.

  number of unicast mac addresses:                  1.5K

  number of IPv4 IGMP groups + multicast routes:    1K

  number of IPv4 unicast routes:                    2.75K

    number of directly-connected IPv4 hosts:        1.5K

    number of indirect IPv4 routes:                 1.25K

  number of IPv6 multicast groups:                  1.125k

  number of directly-connected IPv6 addresses:      1.5K

  number of indirect IPv6 unicast routes:           1.25K

  number of IPv4 policy based routing aces:         0.25K

  number of IPv4/MAC qos aces:                      0.75K

  number of IPv4/MAC security aces:                 0.5K

  number of IPv6 policy based routing aces:         0.25K

  number of IPv6 qos aces:                          0.5K

  number of IPv6 security aces:                     0.5K

And now if you try to configure an IPv6 address on an interface, the command will show up:

S1(config-if)#ipv6 address ?

  WORD                General prefix name

  X:X:X:X::X          IPv6 link-local address

  X:X:X:X::X/<0-128>  IPv6 prefix

  autoconfig          Obtain address using autoconfiguration

S1(config-if)#ipv6 address dead:beef:cafe::1/64

Finally, don’t forget to enable IPv6 unicast-routing globally if you want IPv6 packets to be forwarded.

S1(config)#ipv6 unicast-routing

Source: https://blog.router-switch.com/2013/06/how-to-enable-ipv6-support-on-a-cisco-catalyst-3560-switch/