Cisco Sg3008 Manual

							IP Configuration
IPv4 Management and Interfaces
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-Delayed—The assignment of the IP address is delayed for 60 second if 
DHCP Client is enabled on startup in order to give time to discover DHCP 
address.
-Not Received—Relevant for DHCP Address. When a DCHP Client starts 
a discovery process, it assigns a dummy IP address 0.0.0.0 before the 
real address is obtained. This dummy address has the status of “Not 
Received”.
STEP  2Click Add.
STEP  3Select one of the following fields:
•Interface—Select Port, LAG, or VLAN as the interface associated with this 
IP configuration, and select an interface from the list.
•IP Address Type—Select one of the following options:
-
Dynamic IP Address—Receive the IP address from a DHCP server.
-
Static IP Address—Enter the IP address.
STEP  4SelectDynamic Address or Static Address.
STEP  5If Static Address was selected, enter the IP Address for this interface, and enter 
one of the following:
•Network Mask—IP mask for this address.
•Prefix Length—Length of the IPv4 prefix.
STEP  6Click Apply. The IPv4 address settings are written to the Running Configuration 
file.
IPv4 Routes
When the device is in Layer 3 system mode, this page enables configuring and 
viewing IPv4 static routes on the device. When routing traffic, the next hop is 
decided on according to the longest prefix match (LPM algorithm). A destination 
IPv4 address may match multiple routes in the IPv4 Static Route Table. The device 
uses the matched route with the highest subnet mask, that is, the longest prefix 
match.  
						

							IP Configuration
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To define an IP static route:
STEP 1Click IP Configuration > IPv4 Management and Interfaces > IPv4 Routes.
STEP  2Click Add.
STEP  3Enter values for the following fields:
•Destination IP Prefix—Enter the destination IP address prefix.
•Mask—Select and enter information for one of the following:
-Network Mask—The IP route prefix for the destination IP.
-Prefix Length—The IP route prefix for the destination IP.
•Route Type—Select the route type. 
-
Reject—Rejects the route and stops routing to the destination network 
via all gateways. This ensures that if a frame arrives with the destination 
IP of this route, it is dropped. 
-
Remote—Indicates that the route is a remote path.
•Next Hop Router IP Address—Enter the next hop IP address or IP alias on 
the route. 
NOTEYou cannot configure a static route through a directly-connected IP 
subnet where the device gets its IP address from a DHCP server.
•Metric—Enter the administrative distance to the next hop. The range is 1–
255. 
STEP  4Click Apply. The IP Static route is saved to the Running Configuration file.
ARP
The device maintains an ARP (Address Resolution Protocol) table for all known 
devices that reside in the IP subnets directly connected to it. A directly-connected 
IP subnet is the subnet to which an IPv4 interface of the device is connected. 
When the device is required to send/route a packet to a local device, it searches 
the ARP table to obtain the MAC address of the device. The ARP table contains 
both static and dynamic addresses. Static addresses are manually configured and 
do not age out. The device creates dynamic addresses from the ARP packets it 
receives. Dynamic addresses age out after a configured time.  
						

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IPv4 Management and Interfaces
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NOTEIn Layer 2 mode, the IP, MAC address mapping in ARP Table is used by the device 
to forward traffic originated by the device. In Layer 3 mode, the mapping 
information is used for Layer 3 routing as well as to forward generated traffic. 
To define the ARP tables:
STEP 1Click IP Configuration > IPv4 Management and Interfaces > ARP.
STEP  2Enter the parameters.
•ARP Entry Age Out—Enter the number of seconds that dynamic addresses 
can remain in the ARP table. A dynamic address ages out after the time it is 
in the table exceeds the ARP Entry Age Out time. When a dynamic address 
ages out, it is deleted from the table, and only returns when it is relearned.
•Clear ARP Table Entries—Select the type of ARP entries to be cleared from 
the system.
-
All—Deletes all of the static and dynamic addresses immediately.
-
Dynamic—Deletes all of the dynamic addresses immediately.
-
Static—Deletes all of the static addresses immediately.
-
Normal Age Out—Deletes dynamic addresses based on the configured 
ARP Entry Age Out time.
STEP  3Click Apply. The ARP global settings are written to the Running Configuration file.
The ARP table displays the following fields:
•Interface—The IPv4 Interface of the directly-connected IP subnet where the 
IP device resides.
•IP Address—The IP address of the IP device.
•MAC Address—The MAC address of the IP device. 
•Status—Whether the entry was manually entered or dynamically learned.
STEP  4Click Add.
STEP  5Enter the parameters:
•IP Version—The IP address format supported by the host. Only IPv4 is 
supported.
•VLAN—In Layer 2, displays the management VLAN ID.  
						

							IP Configuration
IPv4 Management and Interfaces
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For devices in Layer 2 mode, there is only one directly-connected IP subnet, 
which is always in the management VLAN. All the static and dynamic 
addresses in the ARP Table reside in the management VLAN.
Interface—For devices in Layer 3 system mode, an IPv4 interface can be 
configured on a port, LAG or VLAN. Select the desired interface from the 
list of configured IPv4 interfaces on the device.
•IP Address—Enter the IP address of the local device.
•MAC Address—Enter the MAC address of the local device.
STEP  6Click Apply. The ARP entry is saved to the Running Configuration file.
ARP Proxy
The Proxy ARP technique is used by the device on a given IP subnet to answer 
ARP queries for a network address that is not on that network. 
NOTEThe ARP proxy feature is only available when the device is in L3 mode. 
The ARP Proxy is aware of the destination of traffic, and offers another MAC 
address in reply. Serving as an ARP Proxy for another host effectively directs LAN 
traffic destination to the host. The captured traffic is then typically routed by the 
Proxy to the intended destination by using another interface, or by using a tunnel.
The process in which an ARP-query-request for a different IP address, for proxy 
purposes, results in the node responding with its own MAC address is sometimes 
referred to as publishing.
To enable ARP Proxy on all IP interfaces:
STEP 1Click IP Configuration > IPv4 Management and Interfaces > ARP Proxy.
STEP  2Select ARP Proxy to enable the device to respond to ARP requests for remotely-
located nodes with the device MAC address.
STEP  3Click Apply. The ARP proxy is enabled, and the Running Configuration file is 
updated. 
						

							IP Configuration
IPv4 Management and Interfaces
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UDP Relay/IP Helper
The UDP Relay/IP Helper feature is only available when the device is in Layer 3 
system mode. Switches do not typically route IP Broadcast packets between IP 
subnets. However, if this feature enables the device to relay specific UDP 
Broadcast packets, received from its IPv4 interfaces, to specific destination IP 
addresses.
To configure the relaying of UDP packets received from a specific IPv4 interface 
with a specific destination UDP port, add a UDP Relay:
STEP 1Click IP Configuration > IPv4 Management and Interfaces > UDP Relay/IP 
Helper.
STEP  2Click Add.
STEP  3Select the Source IP Interface to where the device is to relay UDP Broadcast 
packets based on a configured UDP destination port. The interface must be one of 
the IPv4 interfaces configured on the device. 
STEP  4Enter the UDP Destination Port number for the packets that the device is to relay. 
Select a well-known port from the drop-down list, or click the port radio button to 
enter the number manually.
STEP  5Enter the Destination IP Address that receives the UDP packet relays. If this field 
is 0.0.0.0, UDP packets are discarded. If this field is 255.255.255.255, UDP packets 
are flooded to all IP interfaces.
STEP  6Click Apply. The UDP relay settings are written to the Running Configuration file.
DHCPv4 Snooping/Relay
DHCPv4 Snooping
DHCP snooping provides a security mechanism to prevent receiving false DHCP 
response packets and to log DHCP addresses. It does this by treating ports on the 
device as either trusted or untrusted. 
A trusted port is a port that is connected to a DHCP server and is allowed to 
assign DHCP addresses. DHCP messages received on trusted ports are allowed 
to pass through the device.  
						

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An untrusted port is a port that is not allowed to assign DHCP addresses. By 
default, all ports are considered untrusted until you declare them trusted (in the 
DHCP Snooping Interface Settings page).
DHCPv4 Relay
DHCP Relay relays DHCP packets to the DHCP server. 
DHCPv4 in Layer 2 and Layer 3
In Layer 2 system mode, the device relays DHCP messages received from VLANs 
on which DHCP Relay has been enabled. 
In Layer 3 system mode, the device can also relay DHCP messages received from 
VLANs that do not have IP addresses. Whenever DHCP Relay is enabled on a 
VLAN without an IP address, Option 82 is inserted automatically. This insertion is in 
the specific VLAN and does not influence the global administration state of Option 
82 insertion.
Transparent DHCP Relay
For Transparent DHCP Relay where an external DHCP relay agent is being used, 
do the following:
•Enable DHCP Snooping.
•Enable Option 82 insertion.
•Disable DHCP Relay.
For regular DHCP Relay:
•Enable DHCP Relay.
•No need to enable Option 82 insertion.
Option 82
Option 82 (DHCP Relay Agent Information Option) passes port and agent 
information to a central DHCP server, indicating where an assigned IP address 
physically connects to the network. 
The main goal of option 82 is to help to the DHCP server select the best IP subnet 
(network pool) from which to obtain an IP address.
The following Option 82 options are available on the device: 
						

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•DHCP Insertion - Add Option 82 information to packets that do not have 
foreign Option 82 information.
•DHCP Passthrough - Forward or reject DHCP packets that contain Option 
82 information from untrusted ports. On trusted ports, DHCP packets 
containing Option 82 information are always forwarded.
The following table shows the packet flow through the DHCP Relay, DHCP 
Snooping, and Option 82 modules:
The following cases are possible:
•DHCP client and DHCP server are connected to the same VLAN. In this 
case, a regular bridging passes the DHCP messages between DHCP client 
and DHCP server.
•DHCP client and DHCP server are connected to different VLANs. In the 
case, only DHCP Relay can and does broadcast DHCP messages between 
DHCP client and DHCP server. Unicast DHCP messages are passed by 
regular routers and therefore if DHCP Relay is enabled on a VLAN without 
an IP address or if the device is not a router (Layer 2 device) then an 
external router is needed. 
DHCP Relay and only DHCP Relay relays DHCP messages to a DHCP server 
Interactions Between DHCPv4 Snooping, DHCPv4 Relay and 
Option 82
The following tables describe how the device behaves with various combinations 
of DHCP Snooping, DHCP Relay and Option 82. 
The following describes how DHCP request packets are handled when DHCP 
Snooping is not enabled and DHCP Relay is enabled. 
DHCP Relay
VLAN with IP AddressDHCP Relay
VLAN without IP Address
Packet arrives 
without Option 
82Packet arrives 
with Option 
82Packet arrives 
without 
Option 82Packet arrives 
with Option 
82 
						

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The following describes how DHCP request packets are handled when both 
DHCP Snooping and DHCP Relay are enabled:  Option 82 
Insertion 
DisabledPacket is sent 
without Option 
82Packet is sent 
with the 
original 
Option 82Relay – 
inserts Option 
82
Bridge – no 
Option 82 is 
insertedRelay – 
discards the 
packet
Bridge – 
Packet is sent 
with the 
original 
Option 82
Option 82 
Insertion 
EnabledRelay – is sent 
with Option 82
Bridge – no 
Option 82 is 
sentPacket is sent 
with the 
original 
Option 82Relay – is sent 
with Option 
82
Bridge – no 
Option 82 is 
sentRelay – 
discards the 
packet
Bridge – 
Packet is sent 
with the 
original 
Option 82
DHCP Relay
VLAN with IP AddressDHCP Relay
VLAN without IP Address
Packet arrives 
without Option 
82Packet 
arrives with 
Option 82Packet arrives 
without 
Option 82Packet arrives 
with Option 
82
Option 82 
Insertion 
DisabledPacket is sent 
without Option 
82Packet is 
sent with the 
original 
Option 82Relay – inserts 
Option 82
Bridge – no 
Option 82 is 
insertedRelay – 
discards the 
packet
Bridge – 
Packet is sent 
with the 
original 
Option 82 DHCP Relay
VLAN with IP AddressDHCP Relay
VLAN without IP Address 
						

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The following describes how DHCP Reply packets are handled when DHCP 
Snooping is disabled: Option 82 
Insertion 
EnabledRelay – is sent 
with Option 82
Bridge – Option 
82 is added
(if port is 
trusted, 
behaves as if 
DHCP Snooping 
is not enabled)Packet is 
sent with the 
original 
Option 82Relay – is sent 
with Option 82
Bridge – 
Option 82 is 
inserted
(if port is 
trusted, 
behaves as if 
DHCP 
Snooping is 
not enabled)Relay –
discards the 
packet
Bridge – 
Packet is sent 
with the 
original 
Option 82
DHCP Relay
VLAN with IP AddressDHCP Relay
VLAN without IP Address
Packet arrives 
without 
Option 82Packet arrives 
with Option 
82Packet arrives 
without 
Option 82Packet arrives 
with Option 
82 DHCP Relay
VLAN with IP AddressDHCP Relay
VLAN without IP Address 
						

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Option 82 
insertion 
disabledPacket is sent 
without 
Option 82Packet is sent 
with the 
original 
Option 82Relay – 
discards 
Option 82
Bridge – 
Packet is sent 
without 
Option 82Relay – 
1.  I f  r e p l y  
originates in 
device, 
packet is sent 
without 
Option 82
2. If reply 
does not 
originate in 
device, 
packet is 
discarded
Bridge – 
Packet is sent 
with the 
original 
Option 82
Option 82 
insertion 
enabledPacket is sent 
without 
Option 82Relay – 
Packet is sent 
without 
Option 82
Bridge – 
Packet is sent 
with the 
Option 82Relay – 
discards 
Option 82
Bridge – 
Packet is sent 
without 
Option 82Relay – 
Packet is sent 
without 
Option 82
Bridge – 
Packet is sent 
with the 
Option 82 DHCP Relay
VLAN with IP AddressDHCP Relay
VLAN without IP Address