DHCP SNOOPING TO THE SWITCH OF CISCO SMALL BUSINESS SF200-48

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  Nice day

  What are the automatic installation options available for the Managed Switch series of Cisco Small Business 300?

  It seems there are not, the operating system is not the standard IOS I used to.

  I would use with ability to bridge the Infrastructure first Cisco PnP.

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  Stone

  First Infrastructure use and assume an agent CNS (Cisco Network Services) on the other side, when you enable the 'plug & play '. Series 300/500 do not run IOS and does not include the CNS agent, but there is a way to activate the automatic configuration. Switches supported by the automatic DHCP configuration (using DHCP options 66, 67, etc.) in which case they automatically download their config to a TFTP server at boot time. This allows an administrator to put the configs for all switches on a TFTP server and simply start a default switch factory in order to obtain config.

  Seal a document shows how this is done.

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• Configuration of SNMP on Cisco Small Business 300 Series managed switch

  Please can someone help me configure SNMP on Cisco Small Business 300 series switch?

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  Hi Anton,.

  Don't forget to activate the service SNMP is first of all: Security > Services TCP/UDP.

  http://sbkb.Cisco.com/CiscoSB/UKP.aspx?VW=1&docid=3b13278d9ef9402a8fef57...

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  Laurent.

  Hi Laurent, I am pleased that the 1.2.5.70 has solved this problem.

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• Cisco Small Business Equipment VLAN security issue

  Hi, I have a RV220W router and a switch SG200-18. I'm trying to set up my network to be as secure as possible...

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  Port 18 (access): manage (unlabeled) * used to configure and manage the switching and routing of a pc

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  In what concerns the vlan tag/UNTAG Yes. You must take into account the limitation of the router.

  -Tom
  Please mark replied messages useful

• How many licenses are delivered with the package of creative small business cloud?

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• No small business SF200 smart switches command line interface?

  Hi all, we just got 5 new smart switches SF200-48 for small businesses. I noticed that the only way I can set them up is to use the web gui. Is there a way to allow the good old CLI?

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• Remote Desktop and Cisco Small Business RV042

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  Hi Felipe

  Look forward to hearing how you go.

  If you reset the factory default switch and the switch is NOT connected to a server, the switch restarts with a default IP 192.168.1.245 address according to the Quick start guide .  If your PC has a static address 192.168.1.2, you should be able to access the graphical interface of the switch.

  http://www.Cisco.com/en/us/docs/switches/LAN/CSBMs/sf30x_sg30x/Quick_Start/78-19252-01.PDF

  If the switch is connected to a DHCP server, the switch Gets an IP via DHCP.

  I've had problems using hyperterminal on my Windows 7 box, that I have overcome by restarting my PC in order to free the COM port.  Settings of the console are;

  • 115200 bits per second
  • 8 data bits
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  • 1 stop bit
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  If you really have problems, and you think that the switch is faulty, you have the possibility to use the services of small business Support Center (HWC) in order to identify if the switch is good or bad.

  The URL that watch phone numbers are lower;

  www.Cisco.com/go/SBSC

  Best regards, Dave

• Help the VLANS on Cisco SG200 - 08 p switch voice and data

  Hi all

  I'm faced with a problem of configuration on the Cisco SG200 - 08 p.

  We use Cisco SG200 - 08 p on a mobile carriage which will go from classroom to classroom who will have computer and phone VoIP cisco plugged into it. The question is that each of our closets are in different VLANS (1 voice and 1 data... Let's say data vlan 20 and vlan voice 2025 for conversation) and which move towards every closet.

  It would be great if I could just create a vlan voice dynamically pick up this switch upstream has however and generic data, it seems that I was failed to do.

  So far, I can pass the data Vlan no probably. The upstream switch port is set to access port and a switch port access voice vlan (these are x 3750 switches)

  If the above is not possible, I guess I'll take what I can get. Should I create data vlan 20 and vlan2025 of the voice on the Cisco SG200 - 08 p and make a port on the Cisco SG200 - 08 p and a trunk trunk on the 3750 x? Is there an option on the Cisco SG200 - 08 p to tag voice traffic?

  I am also concerned about DRIFTING and I did not see an area in the Cisco SG200 - 08 p to set it as a customer and a transparent mode.

  Thanks for any help,

  Dan

  Playne,

  first the bad news, the switches for small businesses currently do not support VTP, they support the GVRP Protocol which is like VTP, but there is no State that the VLAN used it will not automatically learn as VTP.

  You should be able to configure the 3750 as a trunk with a vlan 1U or unmarked and vlan 2025 as the tag for the voice. configuration of the port on the switch to small businesses the same way to its home port. All ports which have only phones will be 2025U of access and all ports only PC would be access 1U. All ports of phone and computer would be trunk 1u, 2025T

  Cisco Small Business Support Center

  Randy Manthey

  CCNA, CCNA - security

• DHCP Snooping

  Scenerio:

  As part of a modernization project of tech in a big campus and because of several problems caused by users (l) connect the routers on the network and causing DHCP issues, I'm looking to turn on DHCP snooping. During the tech switches access update will be updated first and then the kernel. The new access switches are 4510R + E/Sup7, running the latest IOS XE base license and just passing through. New carrots are 6509 Sup 720's configured as a cluster VSS, manage all routing for VIRTUAL local area networks and have the statements of support IP. The DHCP server which takes care of all the VLAN is a Windows 2008 server that is directly connected to the base.

  I also read all the info I could find on DHCP snooping, but I'm still a little fuzzy on if it changes the way that the DHCP server handles requests.

  Issues related to the:

  • Because the access switches pass only, they only need monitoring DHCP enabled (in the world and on VIRTUAL local area networks) and their uplinks to the core set as being approved, right? In particular, they only declarations of support IP or Layer-3 interfaces for all of their VIRTUAL local networks, right?
  • While I understand that DHCP snooping will be ineffective if it is not lit on the kernel, there is no reason I can't deploy it first to the access layer without touching the basic configurations to avoid large amounts of documents of change control, right? Then, when the kernel is put at level and DHCP snooping successfully activated that will work.
  • I got that on the layer to access the switches uplink to the core are approved, but I'm not 100% on the question of whether the same interfaces are approved on the carrots. I don't think but want to be sure. Carrots of course trust the real interface on that server DHCP is plugged
  • The most confusing part is all the stuff from the Option-82. As near as I can tell its option for the server to use the information from the Option-82. I think that if all I do is enable DHCP snooping on worldwide and on the right VIRTUAL LANs the DHCP relay between the core and the DHCP server will continue to work as it is today, is that correct?

  Is there really this traps or in my case I really just need to turn it on in the world and by vlan, trust the uplinks on the access switches and the DHCP server on the kernel interface and call it a day?

  Thank you

  Nathan Spitzer

  SR Network Communications analyst.

  Lockheed Martin

  Hello Nathan,.

  Given that the access switches are only switching, they only need DHCP snooping turned on (both globally and on the VLANS) and their uplinks to the core set as trusted, right?

  Fix.

  In particular they dont need IP helper statements or layer-3 interfaces for all of their VLANS, right?

  Fix. The statement of support ip address would only be necessary if switches performed routing inter - VLAN and the DHCP server is located in a VLAN different.

  While I understand that DHCP snooping will only be marginally effective if it is not turned on on the core, there is no reason I cannot deploy it first at the access layer without touching the core configurations to avoid large amounts of change-control paperwork, right? Then when the core is upgraded and DHCP snooping properly enabled it will work. 

  To my knowledge, the opposite is true. DHCP Snooping is a service of access protection layer - is it not in the core of the network. It has nothing to protect in the kernel once DHCP messages have beein properly disinfected at the edge of the network. For some inexplicable reason, many people think that the DHCP Snooping should be enabled on the network. The fact is that the DHCP Snooping protects against

  • DHCP messages are sent to ineligible devices
  • Ineligible devices posing as DHCP servers

  From this it naturally follows that it is the limit of the network, or the layer of access, where such protection is the most effective. So in your case, I believe that the activation of the DHCP Snooping only on the access layer is actually what you want to do.

  I got that on the access layer switches the uplinks to the core are trusted, but I am not 100% on whether the same interfaces are trusted on the cores. I dont think so but want to be sure. Of cource the cores do trust the actual interface the DHCP server is plugged in on

  If you enable the DHCP Snooping on the basic features and uplink between the access switches and core would have to be configured as confidence both on the basic switches and access. Otherwise, the base ports would pass DHCP messages received from customers because the access layer switches running DHCP Snooping insert DHCP Option 82 in the DHCP messages sanitized and ports untrustred delete all DHCP messages including 82 of the present Option.

  2960 Configuration Guide to

  http://www.Cisco.com/en/us/docs/switches/LAN/catalyst2960/software/release/12.2_55_se/configuration/guide/swdhcp82.html#wp1078853

  The switch removes a DHCP packet when one of these situations occurs:

  • Comes from a packet to a DHCP server, for example a DHCPOFFER, DHCPACK, DHCPNAK or DHCPLEASEQUERY package, outside the network or firewall.

  • A packet is received on an interface that is not reliable, and do not match the source MAC address and hardware address of the DHCP client.

• The switch receives a message DHCPRELEASE or DHCPDECLINE with a MAC address in the DHCP snooping database binding, but the information in the database of linking interface does not correspond to the interface on which the message was received.

• A DHCP relay agent sends a DHCP packet that includes a relay agent IP address which is not 0.0.0.0 or relay agent transmits a packet that includes information of option-82 to an untrusted port.

As I have indicated, however, I personally discourage running DHCP Snooping on the basic devices - I see no reason for this. Please correct if I am wrong!

The most confusing part is all the Option-82 stuff. As near as I can tell its optional for the server to use the Option-82 information. I believe that if all I do is turn DHCP snooping on globally and on the right VLANS the DHCP relaying between the core and the DHCP server will continue working just like it is today, is that correct?

LOL, my favorite on the DHCP Snooping things is the Option 82 interesting how much this topic brings confusion...

The Option 82 was created to provide DHCP relay agent the ability to identify itself and the customer who sent the original message from DHCP unmodified. The DHCP server can then use this information to perform certain policies of customer trust. The format of the Option 82 is not strictly specified, only its basic structure is fixed. You can read more on this and the whole reason to be in the RFC 3046. One of the key points to remember here, however, is that the DHCP server may or may not recognize the Option 82, but apart from that, to copy the value of the Option 82A received in the message to a DHCP client for all its replies sent to this client.

DHCP Snooping uses the Option 82 differently. He didn't expect and doesn't require that the DHCP Server includes the Option of 82 or manages a special way. The Option 82 is inserted by switches access performing DHCP Snooping and it contains two important parts:

• The Circuit ID that identifies the port to which the client is connected (VLAN and the location of the physical port in a switch)
• The remote ID that identifies the access switch to which the client is connected (by the MAC address of the switch)

See http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_55_se/configuration/guide/swdhcp82.html#wp1105589

Now, when an access switch performing DHCP Snooping receives a message from DHCP client on an untrusted port, this will happen:

• The switch will insert the 82 Option in the message of the DHCP client. The Option 82 will identify the specific switch and the port to which the client is attached
• The switch will forward the DHCP message according to its MAC address of destination (i.e. in a completely normal way)
• The server receives the DHCP message containing the Option 82. It is not relevant for DHCP Snooping if the server takes into account the value of the Option 82. However, when the server replies, it will insert the original value of the Option of 82 to the answer.
• Access switch will finally receive the DHCP response. Looking at the Option 82, he knows exactly in which port is the message transmitted to the customer - and only the customer - even if the answer was broadcast!

Note that the Option 82 contributes enormously to identify exactly the access switch and its port where the client is attached. If other switches with DHCP Snooping has received this DHCP message (in reason of the flood or address broadcast requested by the client), they would pass this message because they understand once glancing at the 82 Option that the customer is attached elsewhere. The 82 Option allowing to ensure DHCP communication between a particular client and the DHCP server doesn't leak not to other customers.

There is a hunt for witches, associated with the Option 82. A switch run DHCP Snooping inserts the Option 82 messages DHCP clients. However, each DHCP message contains a field named GIADDR where the IP address of the relay agent is registered, where the DHCP message was relayed. Clearly, when a DHCP message goes through a switch DHCP Snooping, it is not relayed (drawn from one VLAN and rerouted to another), so an access switch does not change the GIADDR that remains set to 0.0.0.0. However, at least the implementation of server DHCP Cisco IOS performs a validation on a test received DHCP messages and it drops DHCP messages containing the Option 82, but which the GIADDR field is set to 0.0.0.0 (i.e. unitialized). This can be seen in the output of the debug ip dhcp server packet :

Router# debug ip dhcp server packet

*Sep 9 01:59:40: DHCPD: inconsistent relay information.

*Sep 9 01:59:40: DHCPD: relay information option exists, but giaddr is zero

Under normal circumstances, such a mental health check makes sense - how is it that a DHCP message contains the Option 82 (i.e. the Relay Agent Information Option DHCP) when there is no DHCP relay identified in the GIADDR? However, with DHCP Snooping on the access layer switches, DHCP messages are normal and expected. Therefore, it is essential to disable this check of mental health on the Cisco box that is running the DHCP server configuration using global ip dhcp relay confidence all information or only is selected routed (i.e. L3) interfaces with command level interface ip dhcp relay reliable information.

To summarize:

• The 82 Option is A Good Thing (TM) because it allows to deliver DHCP messages only to the client for which they are intended. Any suggestions to disable the insertion of the Option 82 on access DHCP Snooping Switches are useless 82 Option is inserted by DHCP Snooping Switches in DHCP messages by default - no additional configuration is necessary.
• Through the easiest way - when you deploy DHCP Snooping, does not initially change anything about the Option 82. Make sure that your customers can receive their config IP via DHCP. If yes then there is nothing to resolve. If not, go further.
• If you run a DHCP server on a Device IOS base (router, switch), you may need to use the command ip dhcp relay information confidence-everything (global config) or ip dhcp relay reliable information (level interface) to allow the DHCP messages with the Add Option 82 and unitialized field GIADDR to be accepted. These commands are required only on the device where the DHCP server is running, not on the access layer switches. You may want to first perform debugging as I suggested previously, and only if you see that packets are dropped, add these commands to the configuration.
• I don't know if these commands should be added also to a DHCP relay function efficient switch - I can check that tomorrow in a laboratory.
• If you are using another DHCP server you have to try experimentally whether happy with the DHCP messages with 82 Option present and unitialized GIADDR field

Sorry for the long answer... I hope that I do not bore you to death. We invite you to ask for more! I'll try to be more concise next time

Best regards

Peter

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