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DHCP Configuration Across Layer 3 Switch

By jayesh_nishane ·
I have 2 nos. layer 3 Core switches in the network. At present my network is flat i.e. is without any VLAN routing. I want to go for Layer 3 segmentation using VLANs. I will be having 26 VLAN's. I have 13 different segments connecting to both the Core Switches. I want to use 2 nos. of DHCP servers one on each of the Core Switch. What is the way I should configure the DHCP servers and the Cisco 6506 Core switch so that I can migrate smothly to Layer 3 Segmented Network from Flat Network.
Please help me.

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DHCP Configuration Across Layer 3 Switch

by mshavrov In reply to DHCP Configuration Across ...

What is a basics of Layer 3 switcing? Layer 2 switching lets you configure VLAN links by specifying groups of ports. A network station on one of these ports can send data to the other stations connected to ports in the same group, but they have no connection to stations on ports outside the port group. Layer 3 switching lets you connect stations on different VLANs using IP address and routing information. This is similar to connecting the VLANs using a router, but at a very high speed and without the added cost of an external router. Also, because both the switch and the router functionality are contained within the same box, the network is simpler. And becouse of that you should just add the port, where DHCP server resides to all VLANs which should receive IP Addresses (port should participate in multiply VLANs). But in addition, if you want to have different subnetwork addresses for your computers, you should define different address pools in DHCP server and configure workstations with different scopes. If you wanna to paly with it first, use small hubs and connect sniffers of protocol analyzers to look on the traffic.

Good luck.

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DHCP Configuration Across Layer 3 Switch

by jayesh_nishane In reply to DHCP Configuration Across ...

Simply I had to use IP HElper address as the address of the DHCP servers.

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DHCP Configuration Across Layer 3 Switch

by road-dog In reply to DHCP Configuration Across ...

Check your Email.. drawing attached

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DHCP Configuration Across Layer 3 Switch

by jayesh_nishane In reply to DHCP Configuration Across ...

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DHCP Configuration Across Layer 3 Switch

by Etherealmind In reply to DHCP Configuration Across ...

The most important thing will be to configure DHCP relay on the layer 3 switches in the routing interface. You will need to add the

ip helper-address xxx.xxx.xxx.xxx
ip helper-address xxx.xxx.xxx.xxx

to each VLAN you create.

To recap the DHCP process
- machine sends a broadcast to request an IP address on the local ethernet segment.
- if the DHCP server is local it will reply with a unicast packet to the workstation with an IP address and the appropriate options.
- if a DHCP relay agent is configured the broadcast will be forwarded to the DHCP server. The Relay Agent adds to the packet where the request came from so that DHCP server can reply with the IP address.

On Cisco 6500 you need to configure the "ip helper-address" command to setup DHCP relay. Note however, that this command is a universal type of command that is used to forward broadcasts for a number of protocols. One of these is NetBIOS broadcasts and if this is done your Network Neighborhood (which uses Microsoft Browser which relies in turn on UDP broadcasts) make sure you disable forwarding for these protcols. Here is the commands to do this

no ip forward-protocol udp tftp
no ip forward-protocol udp nameserver
no ip forward-protocol udp domain
no ip forward-protocol udp time
no ip forward-protocol udp netbios-ns
no ip forward-protocol udp netbios-dgm
no ip forward-protocol udp tacacs

If you have two ip helpers you can have two DHCP servers. But both of them will show an allocated address, the machine will use whichever packet it got first.

Hope this helps.

Greg

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DHCP Configuration Across Layer 3 Switch

by jayesh_nishane In reply to DHCP Configuration Across ...

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DHCP Configuration Across Layer 3 Switch

by jayesh_nishane In reply to DHCP Configuration Across ...

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