DHCP over WAN

Answered Question
Jul 25th, 2010

DHCP server---R1-----Cloud-----R2------CAT3550---User’ PC(192.168.1.0/24)

I am wondering how the user gets DHCP.

1.Between R1 and R2, there is a GRE tunnel.

2.the interface of R2 connected to CAT 3550 is configured, no any ip helper command defined.

            ip address 1.1.1.1 255.255.255.0

            ip policy route-map test

           

The route-map identifies traffic,

Match ip 192.168.1.0 0.0.0.255 any,

Set ip next hop (tunnel interface at R1 side)

3. on Cat3550, the interface connects to R2 is access mode, on vlan 100.

It seems that users pcs do get DHCP.

Any idea?

Thanks,

I have this problem too.
0 votes
Correct Answer by gatlin007 about 6 years 5 months ago

It could be that R2 has been configured to serve DHCP if this device is the default gateway for Vlan100.  It would also be worth while to check the 3550, it could also be serving DHCP.

I've often found 'rouge' network devices at remote sites that serve DHCP, sometimes severely conflicting with the legitimate network.


Try executing an 'ipconfig/all' on the workstation, this data may be useful in tracking down the DHCP server.  I've found that if internet connection sharing is enabled on some windoze boxes they will begin serving DHCP.


In extreme cases you may need to write access layer switchport ACL's that prevent DHCP responses from unauthorized ports.


Chris
http://www.travelingtech.net

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Correct Answer
gatlin007 Mon, 07/26/2010 - 15:29

It could be that R2 has been configured to serve DHCP if this device is the default gateway for Vlan100.  It would also be worth while to check the 3550, it could also be serving DHCP.

I've often found 'rouge' network devices at remote sites that serve DHCP, sometimes severely conflicting with the legitimate network.


Try executing an 'ipconfig/all' on the workstation, this data may be useful in tracking down the DHCP server.  I've found that if internet connection sharing is enabled on some windoze boxes they will begin serving DHCP.


In extreme cases you may need to write access layer switchport ACL's that prevent DHCP responses from unauthorized ports.


Chris
http://www.travelingtech.net

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