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Marc Angelo
Beginner

Problem with DHCP and trunk between 3750G and 3750

Hi community,

 

I am setting up a "new" configuration of a 3750G 24PS and 3750 48PS.

There are various vlans which I have yet to use because I can seem to get DHCP to work.

The configuration is setup to lockdown the inter vlan comms once the basics are working - this explains the ACL "ALLOWTEMP" which should to allow everything for the moment.

The switches are currently connected via cat6 from 3750G port 24 to 3750 port 48.

I am able to ping to their respective vlan1 addresses from each other. I do not get a response from any other Vlan interface from either device (via their consoles.)

Scripts attached:

 

 

 

I can't readily see why DHCP isn't working on any vlan.

 

Thanks in anticipation,

 

Marc

 

2 ACCEPTED SOLUTIONS

Accepted Solutions
paul driver
VIP Mentor

Hello

switch  3750g-24ps

conf t

ip routing 

ip route 0.0.0.0 0.0.0.0 vlan 98 10.10.98.3

 

Also remove any L3 interfaces for the other switch this just need to new host switch with a default gateway pointing towards core switch mgt vlan

Lastly as suggested by georg apply stp portfast however not to just one access port but to ALL access ports on all switches

test again 

 

 



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future

View solution in original post

Marc

 

If I am understanding your post correctly you are configuring 2 switches and your immediate issue is that DHCP is not working. I suggest that we focus on 3750G, where the DHCP scopes are configured. When you have DHCP working there we can then look at the second switch and see if there are any problems there that are different from what we find on the first switch. Would you provide the output of these commands on the 3750G:

show ip interface brief looking to see which interfaces with IP addresses are up and which are down

show interface status  looking to verify vlan assignments and to see which ports are active and have connected devices

show interface trunk  looking especially at port 24 which connects to the second switch to verify which vlans are active there

 

Can you give us an example of a port on the 3750G which has a device connected, which is attempting to learn an address using DHCP, and is not learning an address?

 

After looking through the configs I do have some comments to offer:

- the 3750G has some access ports assigned to vlan 3. But there is no DHCP scope for vlan 3. Is that intentional?

- both switches have some access ports assigned to vlan 1. But there is no DHCP scope for vlan 1. Is that intentional?

- there is a DHCP scope for vlan 4 and for vlan 9. But I do not see ports on either switch in either vlan. Is that intentional?

- the 3750G has configured ip default-gateway. This is appropriate for a switch operating as a layer 2 switch. But when ip routing is enabled (as it is on the 3750G) then the default-gateway is ignore and the switch needs a default route. So the suggestion from @paul driver is good. Note that this has to do with functionality of the switch but is not related to the issue about DHCP. Also note that 3750 has neither default-gateway or default route. If you want it to be able to have layer 3 access to anything outside of vlan 1 then you need to configure a default-gateway for it.

- both switches have configured both enable secret and enable password. That is not necessarily a problem but Best Practices are that when enable secret is configured that enable password is not configured. The reason is that enable password is ignored when enable secret is configured. If it is going to be ignored then there is no good reason to configure it.

 

HTH

 

Rick

 

HTH

Rick

View solution in original post

7 REPLIES 7
lucasfreitas83
Beginner

Hi Marc,

 

Anyone vlans can get IP address?

How switch have this sympton, both?

Try ip default gateway in Switch 3750-48PS

 

lucasfreitas83
Beginner

In Switch server DHCP, try to put the command service DHCP enable.

Georg Pauwen
VIP Expert

Hello,

 

the problem might simply be that you need to configure 'spanning-tree portfast' on the access ports:

 

interface FastEthernet2/0/23
switchport access vlan 6
switchport mode access

spanning-tree portfast
!

paul driver
VIP Mentor

Hello

switch  3750g-24ps

conf t

ip routing 

ip route 0.0.0.0 0.0.0.0 vlan 98 10.10.98.3

 

Also remove any L3 interfaces for the other switch this just need to new host switch with a default gateway pointing towards core switch mgt vlan

Lastly as suggested by georg apply stp portfast however not to just one access port but to ALL access ports on all switches

test again 

 

 



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future

View solution in original post

Jaderson Pessoa
VIP Collaborator

well.. i think that your problem is simple.

try it:

show vlan brief ( check if L2 vlans are created on both switches ).

If not, create them and test again.

Regards,
Jaderson Pessoa
*** Rate All Helpful Responses ***

Marc

 

If I am understanding your post correctly you are configuring 2 switches and your immediate issue is that DHCP is not working. I suggest that we focus on 3750G, where the DHCP scopes are configured. When you have DHCP working there we can then look at the second switch and see if there are any problems there that are different from what we find on the first switch. Would you provide the output of these commands on the 3750G:

show ip interface brief looking to see which interfaces with IP addresses are up and which are down

show interface status  looking to verify vlan assignments and to see which ports are active and have connected devices

show interface trunk  looking especially at port 24 which connects to the second switch to verify which vlans are active there

 

Can you give us an example of a port on the 3750G which has a device connected, which is attempting to learn an address using DHCP, and is not learning an address?

 

After looking through the configs I do have some comments to offer:

- the 3750G has some access ports assigned to vlan 3. But there is no DHCP scope for vlan 3. Is that intentional?

- both switches have some access ports assigned to vlan 1. But there is no DHCP scope for vlan 1. Is that intentional?

- there is a DHCP scope for vlan 4 and for vlan 9. But I do not see ports on either switch in either vlan. Is that intentional?

- the 3750G has configured ip default-gateway. This is appropriate for a switch operating as a layer 2 switch. But when ip routing is enabled (as it is on the 3750G) then the default-gateway is ignore and the switch needs a default route. So the suggestion from @paul driver is good. Note that this has to do with functionality of the switch but is not related to the issue about DHCP. Also note that 3750 has neither default-gateway or default route. If you want it to be able to have layer 3 access to anything outside of vlan 1 then you need to configure a default-gateway for it.

- both switches have configured both enable secret and enable password. That is not necessarily a problem but Best Practices are that when enable secret is configured that enable password is not configured. The reason is that enable password is ignored when enable secret is configured. If it is going to be ignored then there is no good reason to configure it.

 

HTH

 

Rick

 

HTH

Rick

View solution in original post

Hi Paul, Rick and others,

I have accepted both your solutions because both were issues (and more too):

Paul's default route worked (also was Rick's comment too)
Rick's comments on the password setup were also very valid
Mostly Rick's comment to check that I has setup vlan interfaces highlighted that I had not (oops!) and this fixed the DHCP issue