NP交换考试题怎么解，求大神指教。。

1812286006

QUESTION 106
A switch is added into the production network to increase port capacity. A network engineer is configuring
the switch for DHCP snooping and IP Source Guard, but is unable to configure ip verify source under
several of the interfaces. Which option is the cause of the problem? (choose four)
A. The local DHCP server is disabled prior to enabling IP Source Guard.
B. The interfaces are configured as Layer 3 using the no switchport command.
C. No VLANs exist on the switch and/or the switch is configured in VTP transparent mode.
D. The switch is configured for sdm prefer routing as the switched database management template.
E. The configured SVIs on the switch have been removed for the associated interfaces.
Correct Answer: B       为什么答案只有一个？上面说选四个，他选B是因为除了B其他的答案（ACDE）是真正的答案吗？ 跪求指导
Section: (none)
Explanation
IP source guard is a security feature that restricts IP traffic on nonrouted, Layer 2 interfaces by filtering
traffic based on the DHCP snooping binding database and
on manually configured IP source bindings. You can use IP source guard to prevent traffic attacks caused
when a host tries to use the IP address of its neighbor.
You can enable IP source guard when DHCP snooping is enabled on an untrusted interface. After IP
source guard is enabled on an interface, the switch blocks all
IP traffic received on the interface, except for DHCP packets allowed by DHCP snooping. A port access
control list (ACL) is applied to the interface. The port ACL
allows only IP traffic with a source IP address in the IP source binding table and denies all other traffic.
The IP source binding table has bindings that are learned by DHCP snooping or are manually configured
(static IP source bindings). An entry in this table has an IP
address, its associated MAC address, and its associated VLAN number. The switch uses the IP source
binding table only when IP source guard is enabled.
IP source guard is supported only on Layer 2 ports, including access and trunk ports. You can configure IP
source guard with source IP address filtering or with
source IP and MAC address filtering.

1812286006

求指导，

G_star

同求，此问！！感谢！！

fengyueshusheng

考过的人出来解释一下，同求。

fulltimes

这是单选题   后面  (choose four)多余了