dhcp requests across MPLS

Answered Question
Sep 14th, 2008

we have to connect our remote offices with data center through MPLS connectivity.we will host our infastructure servers dhcp/dns at data center.

my question is how i would use the dhcp server at data center to provide IP's to remote clients.

what are the configuration involved both in Data center and Remote offices.

hope u got my question.

thanks.

I have this problem too.
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Correct Answer by Giuseppe Larosa about 8 years 2 months ago

Hrllo Sam,

your summarization is correct and complete

Best Regards

Giuseppe

ps. my name is Giuseppe, giuslar is my cisco account if you click on the nickname you can find the name (usually it is present) but no problem !

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Giuseppe Larosa Sun, 09/14/2008 - 22:36

Hello Sam,

if you are not using MPLS VPN you can proceed with the standard configuration for support of DHCP relay.

on the routers that act as the default gateways for the users on the interfaces where you want to provide DHCP support do:

int g0/0.500

ip helper-address

ip helper-address

...

multiple lines are allowed.

If the users and the DHCP servers are in different routing contexts VRFs additional steps are needed.

Hope to help

Giuseppe

sarsamivel Mon, 09/15/2008 - 00:28

Thanks for helping out giuslar.

I didn't get this.

If the users and the DHCP servers are in different routing contexts VRFs additional steps are needed.

can you explain a bit more.

actually i wanted to know what configs i'l be doing it at Data center end.MPLS at data center will terminate on the router and after that how the DHCP server will allocate the IP's to different locations based on its IP subnet.

Giuseppe Larosa Mon, 09/15/2008 - 01:55

Hello Sam,

thanks for your kind remarks.

if the users and the DHCP servers are in different VRF routing tables some form of leakage of communication is needed.

In your case I don't think that is needed.

How the DHCP relay works:

the DHCP server will be configured with so called DHCP scopes. A DHCP scope is the range of IP addresses that are available to DHCP clients within an IP subnet.

For example in our network we use /24 subnets and the first 32 ip addresses are left for infrastructure devices that have manually configured ip addresses.

the router that performs the DHCP relay action as per the ip helper-address command makes the following changes on the original DHCP request:

- the destination address is changed from 255.255.255.255 broadcast that cannot be routed to the address defined in the ip helper address.

- Inside the DHCP request there is a field gateway address where the router places the ip address of its interface that heard the DHCP request.

In this way the DHCP server can select the correct DHCP scope for answering the request and the request is sent back to the router using the same ip address as a destination.

No special config is needed on the data center you need to allow DHCP packets on the vlan where the DHCP servers are placed in and out.

The vlan ip subnet needs to be advertised in some routing protocol so that it can be reached by the routers in all the sites.

Hope to help

Giuseppe

sarsamivel Mon, 09/15/2008 - 22:24

Thanks for the explanation giuslar.I am bit more clear now.

but still have a question in mind.how the dhcp server allocate the IP subnets for different location.

Lets say I am able to reach the dhcp server from all locations,how does the server know which IP to allocate to a particular location.

whether i need to have seperate Vlan for each location.

sorry i would take some time of yours.

Giuseppe Larosa Mon, 09/15/2008 - 23:31

Hello Sam,

no problem I try to explain.

the DHCP server has no knowledge of network topology, it just looks at the gateway field and finds inside an ip address that address is the key used in a lookup in its database to find the right DHCP scope and then provides an ip address from that scope.

The DHCP scope has some attributes that have to be configured like the default gateway, DNS servers, other options that can be

option 150 : tftp servers for IP phones

option 43: for lightweight access points

and so on.

each DHCP scope has a name that can contain the location.

For example we do in this way:

the name is made of:

VL-towncode-sitecode-building-floor-number

the towncode are two letters that show the city or the whole name for sites abroad.

sitecode is the name of the street

for some campus we specify also the building letter, the floor, for Voip we add the voip code.

But this is useful when using the DHCP server to check if a client has got an ip address.

It is an info useful for the human users but when the request arrives the way it is handled is explained above

Hope to help

Giuseppe

sarsamivel Tue, 09/16/2008 - 03:52

Many thanks giuslar.You explained it well.

let me summarize what i understood.

1.all locations should have connectivity to DHCP server.

2.DHCP scope has tobe created for different locations.

3.IP helper addrss(dhcp IP addr) tobe configured on remote routers

DHCP server will provide the IP from dhcp scope for a particular location depending on the gateway IP in the request.

am i right,giuslar.

Correct Answer
Giuseppe Larosa Tue, 09/16/2008 - 12:57

Hrllo Sam,

your summarization is correct and complete

Best Regards

Giuseppe

ps. my name is Giuseppe, giuslar is my cisco account if you click on the nickname you can find the name (usually it is present) but no problem !

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