为什么交换机支持ipv6却不能配置？

测试设备是一台Catalyst3750，从思科官网查看Catalyst3750对IPv6的支持情况：

Table 7 Catalyst 3750, 3560, 3560-C, 2960, 2960-S and 2960-C Switches and Cisco EtherSwitch Service Module Features and the Minimum Cisco IOS Release Required

当前版本是Version 15.0(2)SE11，显然也是支持ipv6的。但是无法配置。

Cisco IOS Software, C3750 Software (C3750-IPSERVICESK9-M), Version 15.0(2)SE11, RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2017 by Cisco Systems, Inc.
Compiled Sat 19-Aug-17 09:28 by prod_rel_team
HULC:Collect Entropy thread starting on box...
Initializing flashfs...

<snip>

配置尝试：没有找到ipv6的配置命令。

Switch(config)#ip?
ip  ipc  

Switch(config)#ipv6 uni
Switch(config)#ip ro   
Switch(config)#ip routi
Switch(config)#ip routing 
Switch(config)#ipv
Switch(config)#?  
Configure commands:
  aaa                         Authentication, Authorization and Accounting.
  access-list                 Add an access list entry
  access-session              Access Session Global Configuration Commands
  alias                       Create command alias
  archive                     Archive the configuration
  arp                         Set a static ARP entry
  authentication              Auth Manager Global Configuration Commands
  auto                        Configure Automation
  banner                      Define a login banner
  beep                        Configure BEEP (Blocks Extensible Exchange
                              Protocol)
  boot                        Modify system boot parameters
  bridge                      Bridge Group.
  buffers                     Adjust system buffer pool parameters
  call-home                   Enter call-home configuration mode
  cdp                         Global CDP configuration subcommands
  cef                         Cisco Express Forwarding
  cisp                        Set CISP parameters
  class-map                   Configure CPL Class Map
  clns                        Global CLNS configuration subcommands
  clock                       Configure time-of-day clock

解决方法：修改sdm template，然后重启设备。

Switch(config)#
Switch(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
Switch(config)#end
Switch#sho 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 IPv6 multicast groups:                  0
  number of IPv6 unicast routes:                    0
    number of directly-connected IPv6 addresses:    0
    number of indirect IPv6 unicast routes:         0
  number of IPv4 policy based routing aces:         0
  number of IPv4/MAC qos aces:                      0.5K
  number of IPv4/MAC security aces:                 1K
  number of IPv6 policy based routing aces:         0
  number of IPv6 qos aces:                          20
  number of IPv6 security aces:                     25
 
 On next reload, template will be "desktop IPv4 and IPv6 routing" template.
Switch#wr
Building configuration...
[OK]
Switch#reload
Proceed with reload? [confirm]

*Mar  1 00:17:37.996: %SYS-5-RELOAD: Reload requested by console. Reload Reason: Reload command.
Boot Sector Filesystem (bs) installed, fsid: 2
<snip>

设备重启完毕之后，发现可以配置ipv6：

Switch#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)#ipv6 unicast-routing 
Switch(config)#

Switch#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)#int vlan10
Switch(config-if)#ipv6 add
Switch(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
  dhcp                Obtain a ipv6 address using dhcp