Help understanding DHCP Snooping and Dynamic ARP Inspection

Please help me to understand DHCP Snooping and Dynamic ARP Inspection.

HI Ezra,
In simple words:
DHCP Snooping is a feature which is available on switches. This feature is used to prevent rogue dhcp server attacks.
In the diagram, a valid dhcp server is connected to the network. The computers are suppose to receive dynamic ip addresses from the valid server. An attacker implants a rogue dhcp server on the network as shown in the diagram. The following steps are followed for a client to receive an ip address from a dhcp server.
When a client (computer) is connected to the switch and is configured to receive a dynamic ip address from a dhcp server, the dhcp service on the client, sends out a DHCP Discover packet, searching for servers on the network. This packet is broadcast in nature. DHCP servers on the network, would respond to the DHCP Discover packet sent from the client. In the example, both the DHCP servers would respond to the DHCP discover packet. The client would process the first packet it receives. If the response send by the rogue dhcp server reaches the client first, then the computer would have an ip address provided by the rogue dhcp server.
To prevent this, dhcp snooping is configured on the port on which the valid dhcp server is connected to. After the configuration is performed, no other ports on the switch would be able to respond to DHCP Discover packets from the clients. So even through the attacker has set up a rogue dhcp server, the port on the switch to which the attacker has connected would not be allowed to respond to DHCP discover packets. Thus dhcp snooping thwarts the attempt from the attacker in setting up a rogue dhcp server.
DAI:
Please read the expalined version from here: http://ciscocertstudyblog.blogspot.de/2010/06/ciscoblogpics.html
More about DHCP snooping and DAI: Please read this attached document with some detailed explanation.
Hope it helps.
Regards
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Sg200-50 support dhcp snooping and dynamic arp inspection?

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dynamic arp inspection
?? thanks

HI d.pennington,
SG200 is L2 switch only. so this mean switch not support dhcp snooping. Switch support IGMP snooping, Switch support dynamic arp table. You can management switch with web page GUI only (CLI) not supported.
Thanks,
Moh

Do sg200-50 support dhcp snooping or dynamic arp inspection (DAI) ?

do the sg200-50 switches support:
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dynamic arp inspection
?? thanks

HI d.pennington,
SG200 is L2 switch only. so this mean switch not support dhcp snooping. Switch support IGMP snooping, Switch support dynamic arp table. You can management switch with web page GUI only (CLI) not supported.
Thanks,
Moh

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Good Day to everybody.
I had implemented DHCP Snooping & Dynamic ARP inspection feature to mitigate ARP spoofing attacks to one of customer location where we have mix of Windows vista & XP systems. By default DAI feature rate limit ARP packets on un-trusted ports to 15 Packets per second. With this value I was facing some issue to access file shares where port will go in error-disabled state due to ARP broadcast from system was crossing 15 PPS limit of DAI. For the same, I had increased the DAI limit to 64 & after that we had not facing this problem from windows XP systems, but windows vista systems are still giving problem. Also this probem is very random in nature & not all the windows Vista system will face same issue even though they are accessing same file share & are configured with same DAI rate limit.
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Hello bensyseng,
check out this thread.
As topmahof said already it could correlate with a wrong Intel driver.
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How config dynamic arp inspection for 300 or 500 series ?

Hi Cisco Expert ,
How config dynamic arp inspection for 300 or 500 series ? Do you have clearly document for this solution ? Could you please to share ?
i find in admin guide it's no simple to do
Thank you for kindly support.

Hi Siriphan, using the command line is the easiest way to deal with this.
You need to understand the difference between trusted and untrusted interfaces. The untrusted interfaces are the ports that will be inspected and if not specified within the arp entry list then will get dropped.
Any port you do not want arp inspection to be a part of, you need to trust that port.
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configure terminal
interface fe1
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configure terminal
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ip 192.168.100.3 mac-address 64:31:50:1c:50:a1
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Lastly, you need to enable the arp inspection globally. You DO NOT want to toggle the arp inspection without establishing your interfaces or bind list. If you do not establish your trust interfaces and list first, you will lock down any connection through the switch and essentially brick it.
To toggle the global arp inspection
configure terminal
ip arp inspection
Once you're done, save your running config to the start up config.
-Tom
Please mark answered for helpful posts

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Is Dynamic ARP inspection possible to be done on wifi routers? I'm asking because I can't find any model with that feature. I would especially be interested in some cheaper models for home or small business use (maybe Linksys).

You could be better served posting this on the SOHO forum. Speaking to enterprise gear like the cisco WLC yes.
DAI for Wireless Access
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http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Mobility/emob73dg/emob73/ch4_Secu.html#pgfId-1019449

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>sh ver
Cisco Internetwork Operating System Software
IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(22)EA8a, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2006 by cisco Systems, Inc.
Compiled Fri 28-Jul-06 15:16 by weiliu
Image text-base: 0x80010000, data-base: 0x8056A000
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information DHCP Snooping information
vlan DHCP Snooping vlan
<cr>
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% Unrecognized command

Hi Alain,
Thanks for this info! I've read you're CCNA Security.
Just curious, are you gonna write your CCNP Security soon?
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Sent from Cisco Technical Support iPad App

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Hello,
There is no know problem with Jumbo/Giant frame support on 3750 platform other than the bug you reported.
I have verified that Jumbo/Giant frame support works on 12.2(25)SED in stack configuration.
Facts
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http://www.cisco.com/cgi-bin/Software/Iosplanner/Planner-tool/printdefer.pl?platform=CAT3750&majorRel=12.2&release=12.2.25-SEB2&data_from=&file=12.2.25-SEB2.CAT3750.c.html
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Can someone point me to step-by-step configuration guide of how to enable DAI on Cisco Catalyst 6500 Series Switches.
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HI d.pennington,
SG200 is L2 switch only. so this mean switch not support dhcp snooping. Switch support IGMP snooping, Switch support dynamic arp table. You can management switch with web page GUI only (CLI) not supported.
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Hi All,
I have Configured DHCP snooping and IP source guard and Dynamic arp inspection on my 3560 and 3750 Network Switches,
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check the following link for configuration of DHCP snooping
http://packetlife.net/blog/2010/aug/18/dhcp-snooping-and-dynamic-arp-inspection/
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/snoodhcp.html

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Thanks for any comments.
Regards,
Yi

check the following link for configuration of DHCP snooping
http://packetlife.net/blog/2010/aug/18/dhcp-snooping-and-dynamic-arp-inspection/
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/snoodhcp.html

Help understanding DHCP Snooping and Dynamic ARP Inspection

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