xStack
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DGS-3400 Series Layer 2 Gigabit Ethernet Managed Sw itch
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Strict and Loose State
Other than ACL and ARP mode, users can also configure the state on a port for granular control. There are two states: Strict and
Loose, and only one state can be selected per port. If a port is set to Strict state, all packets entering the port are denied (dropped)
by default. The switch will continuously compare all IP and ARP packets it receives on that port with its IMPB entries. If the IP-
MAC pair in the packet matches the IMPB entry, the MAC address will be unblocked and subsequent packets sent from this client
will be forwarded. On the other hand, if a port is set to Loose state, all packets entering the port are permitted (forwarded) by
default. The switch will continuously compare all ARP packets it receives on that port with its IMPB entries. If the IP-MAC pair
in the ARP packet does not match the IMPB white list, the MAC address will be blocked and subsequent packets sent from this
client will be dropped.
DHCP Snooping Option
If DHCP snooping is enabled, the switch learns IP-MAC pairs by snooping DHCP packets automatically and then saves them to
the IP-MAC-Port Binding white list. This enables a hassle-free configuration because the administrator does not need to manually
enter each IMPB entry. A prerequisite for this is that the valid DHCP server’s IP-MAC pair must be configured on the switch’s
IMPB while list first; otherwise the DHCP server packets will be dropped. DHCP snooping is generally considered to be more
secure because it enforces all clients to acquire IP through the DHCP server. Additionally, it makes IP Information auditable
because clients cannot manually configure their own IP address.
An example of DHCP snooping in which PC-A and PC-B get their IP addresses from a DHCP server is depicted below.
The
switch snoops the DHCP conversation between PC-A, PC-B, and the DHCP server. The IP address, MAC address, and connecting
ports of both PC-A and PC-B are learned and stored in the switch’s IMPB white list. Therefore, these PCs will be able to connect
to the network. Then there is PC-C, whose IP address is manually configured by the user. Since this PC’s IP-MAC pair does not
match the one on Switch’s IMPB white list, traffic from PC-C will be blocked.
192.168.1.1
00E0-0211-111
Port 1
192.168.1.2
00E0-0211-222
Port 2
Figure 6 - 6 DHCP Snooping Example
ARP Inspection
ARP spoofing can attack hosts, switches, and routers connected to a Layer 2 network by “poisoning” their ARP caches. As the
figure below shows, Host C can “poison” the ARP caches of Host B by broadcasting forged ARP responses with bindings (IP B,
MAC C). As a result, Host C intercepts the traffic sent to Host B. IMPB v3.8 was developed to prevent this kind of ARP spoofing
(including Netcut and Netcut restore attacks).
IMP Binding Enabled
Address Learning
192.168.1.2
00E0-0211-2222
192.168.1.1
00E0-0211-3333
(IP manually configured by user)
DHCP Server
PC-B
PC-C
Doesn’t match the
White list, block PC-C
White List
PC-A
192.168.1.1
00E0-0211-1111
(IP assigned by DHCP for
PC-A and PC-B)
Содержание xStack DGS-3427
Страница 134: ...xStack DGS 3400 Series Layer 2 Gigabit Ethernet Managed Switch 125 Figure 2 148 Port Speed Utilizing the Tool Tip ...
Страница 215: ...xStack DGS 3400 Series Layer 2 Gigabit Ethernet Managed Switch 206 Figure 3 68 LLDP Local Port Brief Table window ...
Страница 354: ...xStack DGS 3400 Series Layer 2 Gigabit Ethernet Managed Switch 345 Figure 6 81 JWAC Global State Configuration window ...
Страница 404: ...xStack DGS 3400 Series Layer 2 Gigabit Ethernet Managed Switch 395 Example topology ...
Страница 406: ...xStack DGS 3400 Series Layer 2 Gigabit Ethernet Managed Switch 397 ...