Dhcp Snooping - SMC Networks SMC8126L2 Management Manual

3
Configuring the Switch
CLI - This example displays all the resource records learned from the designated
name servers.
Console#show dns cache
NO FLAG
0 4
1 4
2 4
3 4
4 4
5 4
6 4
7 4
8 4
9 4
10 4
Console#

DHCP Snooping

DHCP snooping allows a switch to protect a network from rogue DHCP servers or
other devices which send port-related information to a DHCP server. This
information can be useful in tracking an IP address back to a physical port.
Network traffic may be disrupted when malicious DHCP messages are received
from an outside source. DHCP snooping is used to filter DHCP messages received
on a non-secure interface from outside the network or firewall. When DHCP
snooping is enabled globally and enabled on a VLAN interface, DHCP messages
received on an untrusted interface from a device not listed in the DHCP snooping
table will be dropped.
When enabled, DHCP messages entering an untrusted interface are filtered based
upon dynamic entries learned via DHCP snooping.
Filtering rules are implemented as follows:
• If the global DHCP snooping is disabled, all DHCP packets are forwarded.
• If DHCP snooping is enabled globally, and also enabled on the VLAN where the
DHCP packet is received, all DHCP packets are forwarded for a trusted port. If the
received packet is a DHCP ACK message, a dynamic DHCP snooping entry is also
added to the binding table.
• If DHCP snooping is enabled globally, and also enabled on the VLAN where the
DHCP packet is received, but the port is not trusted, it is processed as follows:
• If the DHCP packet is a reply packet from a DHCP server (including OFFER,
ACK or NAK messages), the packet is dropped.
• If the DHCP packet is from a client, such as a DECLINE or RELEASE message,
the switch forwards the packet only if the corresponding entry is found in the
binding table.
• If the DHCP packet is from a client, such as a DISCOVER, REQUEST, INFORM,
DECLINE or RELEASE message, the packet is forwarded if MAC address
verification is disabled. However, if MAC address verification is enabled, then
3-186
TYPE IP
CNAME 207.46.134.222
CNAME 207.46.134.190
CNAME 207.46.134.155
CNAME 207.46.249.222
CNAME 207.46.249.27
ALIAS POINTER TO:4
CNAME 207.46.68.27
ALIAS POINTER TO:6
CNAME 65.54.131.192
ALIAS POINTER TO:8
CNAME 165.193.72.190
TTL DOMAIN
51 www.microsoft.akadns.net
51 www.microsoft.akadns.net
51 www.microsoft.akadns.net
51 www.microsoft.akadns.net
51 www.microsoft.akadns.net
51 www.microsoft.com
71964 msn.com.tw
71964 www.msn.com.tw
605 passportimages.com
605 www.passportimages.com
87 global.msads.net
4-238