多选题Which two choices are equivalent？（）A16  4B16 / 2C16 * 4D16  2E16 / 22F16  2

正确答案：BD
B,D 解析：Explanation:
ISL and 802.1Q are the two trunking encapsulations that can be configured on a Cisco switch. ISL is Cisco proprietary and 802.1Q is the IEEE standard method.

Incorrect Answers:
A. VTP is the VLAN Trunking Protocol, which is used to carry VLAN information across the trunks. The question is asking for the encapsulation options for the trunk, which will be used by VTP.

C. LLC is the Logical Link Control, which is a sub-layer of the data link layer.

E. CDP is the Cisco Discovery Protocol, which is used by Cisco devices to discover information on neighboring Cisco devices.

F. PAP is the Password Authentication Protocol, which is used as an authentication mechanism on PPP links.

正确答案：CE
C,E 解析：Explanation:
The bridges elect a root bridge based on the bridge IDs in the BPDUs. The root bridge is the bridge with the lowest numeric value for the bridge ID. Because the two part bridge ID starts with the priority value, essentially the bridge with the lowest priority becomes the root.

For instance, if one bridge has priority 100, and another bridge has priority 200, the bridge with priority 100 wins, regardless of what MAC address was used to create the bridge ID or each bridge/switch.

正确答案：AB
A,B 解析：Explanation:
DHCP uses UDP as its transport protocol. DHCP messages from a client to a server are sent to the DHCP server port (UDP port 67), and DHCP messages from a server to a client are sent to the DHCP client port (UDP port 68).The client broadcasts a DHCPDISCOVER message on its local physical subnet. The DHCPDISCOVER message may include options that suggest values for the network address and lease duration. BOOTP relay agents may pass the message on to DHCP servers not on the same physical subnet.

Each server may respond with a DHCPOFFER message that includes an available network address in the "ipaddr" field (and other configuration parameters in DHCP options). Servers need not reserve the offered network address, although the protocol will work more efficiently if the server avoids allocating the offered network address to another client. The server unicasts the DHCPOFFER message to the client (using the DHCP/BOOTP relay agent if necessary) if possible, or may broadcast the message to a broadcast address (preferably 255.255.255.255) on the client's subnet.

Incorrect Answers:
C. DHCP messages are broadcast to the "all hosts" address. IP multicast addresses are not used.

D. UDP is used, not TCP.

E. Since DHCP is used so that a client can obtain an IP address, a layer two destination address must be used, as the layer 3 IP address does not yet exist on the client for the return traffic.

F. DHCP, along with nearly every other type of traffic, requires the use of a transport layer protocol.

正确答案：BD
B,D 解析：Explanation:
Trunks are used to carry traffic belonging to multiple VLANs between devices over the same link. A device can determine which VLAN the traffic belongs to by its VLAN identifier. The VLAN identifier is a tag that is encapsulated with the data. ISL and 802.1q are two types of encapsulations used to carry data from multiple VLANs over trunk links.

ISL is a Cisco proprietary protocol for interconnecting multiple switches and maintaining VLAN information as traffic goes between switches. ISL provides VLAN trunking capabilities while maintaining full wire speed performance on Ethernet links in full-duplex or half-duplex mode. ISL operates in a point-to-point environment and will support up to 1000 VLANs.

In ISL, the original frame. is encapsulated and an additional header is added before the frame. is carried over a trunk link. At the receiving end, the header is removed and the frame. is forwarded to the assigned VLAN. .ISL uses Per VLAN Spanning Tree (PVST) which runs one instance of Spanning Tree Protocol (STP) per VLAN.

PVST allows for optimal root switch placement for each VLAN and supports load balancing of VLANs over multiple trunk links. 802.1Q is the IEEE standard for tagging frames on a trunk and supports up to 4096 VLANs. In 802.1Q, the trunking device inserts a four-byte tag into the original frame. and re-computes the Frame. Check Sequence (FCS) before sending the frame. over the trunk link. At the receiving end, the tag is removed and the frame. is forwarded to the assigned VLAN. 802.1Q does not tag frames on the native VLAN.

It tags all other frames transmitted and received on the trunk. While configuring a 802.1 trunk, you must make sure that the same native VLAN is configured on both sides of the trunk. IEEE 802.1Q defines a single instance of spanning tree running on the native VLAN for all the VLANs in the network which is called Mono Spanning Tree (MST). This lacks the flexibility and load balancing capability of PVST available with ISL. However, PVST+ offers the capability to retain multiple Spanning Tree topologies with 802.1Q trunking.

正确答案：C

正确答案：AC
A,C 解析：Explanation:
The network uses a 28bit subnet (255.255.255.240). This means that 4 bits are used for the networks and 4 bits for the hosts. This allows for 14 networks and 14 hosts (2n-2). The last bit used to make 240 is the 4th bit (16) therefore the first network will be 192.168.15.16.

The network will have 16 addresses (but remember that the first address is the network address and the last address is the broadcast address). In other words, the networks will be in increments of 16 beginning at 192.168.15.16/28. The IP address we are given is 192.168.15.19. Therefore the other host addresses must also be on this network. Valid IP addresses for hosts on this network are: 192.168.15.17-192.168.15.30.

Incorrect Answers:
B. This is not a valid address for this particular 28 bit subnet mask. The first network address should be 192.168.15.16.

D. This is the network address.

E. This is the broadcast address for this particular subnet.