SMC Networks SMC8150L2 manual Dhcp Snooping

3 Configuring the Switch

CLI - This example displays all the resource records learned from the designated name servers.

Console#show dns cache			IP	TTL	4-238
NO	FLAG	TYPE			DOMAIN
0	4	CNAME	207.46.134.222	51	www.microsoft.akadns.net
1	4	CNAME	207.46.134.190	51	www.microsoft.akadns.net
2	4	CNAME	207.46.134.155	51	www.microsoft.akadns.net
3	4	CNAME	207.46.249.222	51	www.microsoft.akadns.net
4	4	CNAME	207.46.249.27	51	www.microsoft.akadns.net
5	4	ALIAS	POINTER TO:4	51	www.microsoft.com
6	4	CNAME	207.46.68.27	71964	msn.com.tw
7	4	ALIAS	POINTER TO:6	71964	www.msn.com.tw
8	4	CNAME	65.54.131.192	605	passportimages.com
9	4	ALIAS	POINTER TO:8	605	www.passportimages.com
10	4	CNAME	165.193.72.190	87	global.msads.net
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

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