Tuesday, April 5, 2016

Huawei S3700: DHCP Feature and Stacking Rules

DHCP Feature of Huawei S3700:
DHCP Client and DHCP Server
DHCP adopts the client/server mode, that is, the DHCP client sends request messages to the DHCP server. Then, the DHCP server returns the reply messages according to the address pool policy.
The DHCP server assigns an IP address to the client by using an address pool. When the client sends a DHCP request to the server, the DHCP server selects a proper address pool, finds an idle IP address from the pool, and delivers the IP address along with other related parameters, such as the gateway address, the DNS address and the address lease, to the client.
To dynamically allocate IP addresses to clients, you need to first configure the address pool range on the DHCP server. Currently, an address pool can be configured with only one address range and the address range is determined by the mask length.
DHCP Snooping
The S3700 can be deployed between the DHCP server and the DHCP client and it monitors the DHCP messages between the DHCP server and the DHCP client. The S3700 creates the IP +MAC+PORT+VLAN binding table according to the monitoring result to filter out invalid packets.
The S3700 also supports Option 82.
l After receiving a Request message from the DHCP client, the S3700 appends the Option
82 field to the Request message. The DHCP server enforces the IP address allocation policy according to the Option 82 field.
l The DHCP server appends the Option 82 field to a Response message. The S3700 analyzes the Option 82 field, determines a forwarding interface, removes the Option 82 field, and then forwards the message to a user.
Option 82 can be implemented in two modes on the S3700, Option 82 insert and Option 82 rebuild.
The Option 82 field contains the user circuit IDs. The user circuit IDs include user device name, outer VLAN ID, inner VLAN ID and port number etc. This can effectively prevent attackers from modifying the DHCP messages.

DHCP Relay
The DHCP client and the DHCP server send broadcast packets during the allocation of IP
addresses. Therefore, DHCP can be applied only when the DHCP client and DHCP server are in the same subnet. It is a waste of resource to deploy a DHCP server in each network segment.
The DHCP relay is introduced to solve this problem. Through DHCP relay, a DHCP client in a subnet can communicate with the DHCP server in another subnet and finally obtains an IP address. In this manner, the DHCP clients on different network segments can use the same DHCP server. This reduces costs and achieves centralized management.

Stacking means that the switches located in the same place are connected through the stacking cable or high-speed uplink interfaces, and thus the switches form a reliable switch group. In as switch group, the S3700s are connected through the stack interfaces multiplexed with uplink GE interfaces. Through stacking, the user can manage and maintain the switches uniformly; therefore, the stacking reduces the maintenance cost of the user.The stacked switches must be of the same type.
The stacked switches have three roles:
l Master switch
A stack has only one master switch. The master switch manages the entire stack system by assigning stack IDs to member switches, collecting information about the stack topology, and notifying all the member switches of the information.
l Backup switch
As the backup of the master switch, the backup switch becomes the master if the master
switch is faulty and takes over the work of the master switch.
l Slave switch
A slave switch only processes service traffic on the network and is managed by the master switch.
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