DHCP relay secnario.....Not getting IP address

soshomile · ‎01-06-2009

Dear All;

I have problem while configuring Intervlan Routing + DHCP relay by using 3750 switch. by the experts help my IVR is working fine. when i configure my ip-helper address on core 3750 if my client is directly attached to 3750 then it would get ip from DHCP server which is behind the 3750 switch haviing different Vlan. for more detail please see this example.

2950 switch having Vlan 10 & 20......Uplink to 3500 switch having Vlan 10 & 20....then Uplink to 3750 switch in whcih i configure the Vlan with IP address on V10 & V10 which are the gw for the users those are behind the 2950. configuration in 3750 looks like;

!

interface Vlan10

ip address 10.20.10.1 255.255.255.0

ip helper-address 10.25.0.6

!

interface Vlan20

ip address 10.20.20.1 255.255.255.0

ip helper-address 10.25.0.6

!

Interface Vlan100

ip address 100.1.1.1 255.0.0.0

my question is why my clients are not getting IP addres.

please help me in this regards.

soshomile

mahmoodmkl · ‎01-07-2009

Hi

What is the type of link between 3550 and 3750 on which u have configured IVR ensure that all the links are trunk link.

Thanks

Mahmood

soshomile · ‎01-07-2009

Dear Mehmood;

All links between switches are trunk links. encap is 802.1q

regards

mahmoodmkl · ‎01-07-2009

Hi

Have checked weather IP Routing is enabled may it sounds stupid.

Thanks

Mahmood

soshomile · ‎01-07-2009

ip routing is enabled.....even when i assing manual ip to client then it pings to server which is on different VLAN it means 3750 switch is routing the packets. IVR is working fine. even now i skip 3500 switch from whole secnario now 2950 is directly connected to 3750 on 802.1q trunk link. but the behaviour is the same. i think 3750 is blocking bootp request of client....i m not sure....

regards

mahmoodmkl · ‎01-07-2009

Hi

Just to confrim have configured a superscope on the DHCP or individual scopes for specific vlans.

make sure that u have created individual scopes not a superscope.

Thanks

Mahmood

soshomile · ‎01-07-2009

Dear Mehmood;

you are right.....but when i will connect any host directly to 3750 then regardless of any scope it will assign IP to cleint's request from first scope....i think this behaviour must be the same....means DHCP (Linux) server should assing ip address from first scop if scope is not correctly define. in current secnario i have 3 scope in server. here i would have 1 question that if scope is not correctly define then rely would not work?

regards

Giuseppe Larosa · ‎01-07-2009

Hello Soshomile,

verify if the C3750 is configured for DHCP snooping.

in that case you need to trust the uplink ports of the 2950 or subsequent DHCP requests will be treated as attack attempts

see

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_46_se/configuration/guide/swdhcp82.html

when you connect the client directly on a port of c3750 only one dhcp request is seen on that port.

on an uplink multiple requests can be seen

Hope to help

Giuseppe