Configure IP helper-address with Cisco 2911 and 3COM 4400

[Tommy Svensson]

Hi.

I am setting up a network using a Cisco 2911 router and a couple of 3COM 4400 switches. I am having some trouble with DHCP-forward through the switches. I have the network set up like this:

Cisco 2911 with 2 VLAN interfaces with the following IP-addresses 10.10.10.1 & 10.10.20.1

Connected to the router is a 3COM 4400 switch (Lets call this one 4400-1). It is trunking VLANs to the router on port 24 and trunking the same two VLANs to another 3COM 4400 on port 23.

Connected to port 23 on 3COM 4400 is another 3COM 4400, lets call this one 4400-2.

My problem is that when i connect a host to 4400-1 the DHCP works fine, but when i connect it to 4400-2 it wont work at all. I have looked around for some sort of IP helper-address command in the 3COM switches whitout any result. I can find it in the router on my interfaces but i cant find it on the switches. Does it need do be configured on the switches?

Regards Tommy Svensson

[cadet alain · Accepted solution]

Hi,

If your switches are L2 switches then you don't need to configure a dhcp relay agent on them.

Could you post 2911 config and also tell us more about the 2 hosts you tried dhcp on as well as put a diagram of your network.

Regards.

Alain.

[Latchum Naidu]

Hi,

You need to configure ip helper address on the cisco 2911 on both interfaces where your 3 com switches connected.
Make sure ip helper configured on port 23 whre 3com 4400-2 is connected.

And if your 3 com switches are L2 switches then you no need to configure ip helper address on these.

Please rate the helpfull posts.
Regards,
Naidu.

[Tommy Svensson]

Hi.

Both of the 3COM 4400 switches are layer 2. The Cisco 2911 is working as DHCP. Im attaching a schema over my network setup aswell as a running config of my Cisco 2911 router.

Regards Tommy Svensson

Running config:

R1(config-subif)#do show run
Building configuration...

Current configuration : 4992 bytes
!
! Last configuration change at 09:44:56 PCTime Mon Feb 28 2011 by asdasd
!
version 15.0
no service pad
service tcp-keepalives-in
service tcp-keepalives-out
service timestamps debug datetime msec localtime show-timezone
service timestamps log datetime msec localtime show-timezone
service password-encryption
service sequence-numbers
!
hostname R1
!
boot-start-marker
boot-end-marker
!
logging buffered 51200
logging console critical
enable secret 5 $1$x5i9$recbh8hCB5SBR5V7Tn/Yc/
!
no aaa new-model
!
!
!
clock timezone PCTime 1
clock summer-time PCTime date Mar 30 2003 2:00 Oct 26 2003 3:00
!
no ipv6 cef
no ip source-route
ip cef
!
!
ip dhcp excluded-address 10.10.10.1 10.10.10.49
ip dhcp excluded-address 10.10.10.251 10.10.10.254
ip dhcp excluded-address 10.10.20.1 10.10.20.49
ip dhcp excluded-address 10.10.20.251 10.10.20.254
!
ip dhcp pool ccp-pool1
network 10.10.10.0 255.255.255.0
domain-name tedact.local
dns-server 192.168.98.2
default-router 10.10.10.1
!
ip dhcp pool ccp-pool2
import all
network 10.10.20.0 255.255.255.0
domain-name tedact.local
dns-server 192.168.98.2
default-router 10.10.20.1
!
!
no ip bootp server
ip domain name tedact.local
ip name-server 192.168.98.2
!
multilink bundle-name authenticated
!
!
!
crypto pki trustpoint TP-self-signed-530346110
enrollment selfsigned
subject-name cn=IOS-Self-Signed-Certificate-530346110
revocation-check none
rsakeypair TP-self-signed-530346110
!
!
crypto pki certificate chain TP-self-signed-530346110
certificate self-signed 01
30820245 308201AE A0030201 02020101 300D0609 2A864886 F70D0101 04050030
30312E30 2C060355 04031325 494F532D 53656C66 2D536967 6E65642D 43657274
69666963 6174652D 35333033 34363131 30301E17 0D313130 32323430 37323030
315A170D 32303031 30313030 30303030 5A303031 2E302C06 03550403 1325494F
532D5365 6C662D53 69676E65 642D4365 72746966 69636174 652D3533 30333436
31313030 819F300D 06092A86 4886F70D 01010105 0003818D 00308189 02818100
CCC4DE13 6476A2A0 B05D718B BDA1BB42 953FED57 2F37490D BEF58A1B 8F4774A0
17F52B83 A48A59BC 46F1BFBA 68D2BBE3 66A40219 8B6FB14E 96424551 4AFB4598
C2F0E9DA 53946559 767A6468 88253DDF B42DEFA3 EF0693F2 E2B77B24 2EFD3F6C
620E1F33 3B994749 A9C1F5A9 63821FD4 0A0C808F DF3D70D7 9C1E813E D78E79C7
02030100 01A36F30 6D300F06 03551D13 0101FF04 05300301 01FF301A 0603551D
11041330 11820F52 312E7465 64616374 2E6C6F63 616C301F 0603551D 23041830
16801439 3E27ECCE E810B254 66EA1C16 3213546A 2C345230 1D060355 1D0E0416
0414393E 27ECCEE8 10B25466 EA1C1632 13546A2C 3452300D 06092A86 4886F70D
01010405 00038181 001AE204 00263DC0 F478478D 94CD33B9 CFCC4685 16D3EC89
0EE17A28 709F7B2A 7060A2C1 C851D34C 4A5A5E82 428E5101 2CF2E90D FFBAC276
81B09ADF BDA33EC5 E6EB5F38 13613C88 15D43E93 F40F6C53 2C92AE4E 0F169075
0964F08C DB2A0F71 BFAC9BF0 C51A92BC CC7B93A3 D6AEEBAF 50AEBF71 E3F8BFAE
E9FB1AB8 726902D1 78
quit
license udi pid CISCO2911/K9 sn xxxxxxxxxxxxx
!
!
username iosoft privilege 15 secret 5 $1$v5OF$R3yjpXGxwxc9RajjsyciT.
username admin privilege 15 secret 5 $1$y/t2$OMcn.CBnCLZDxWsREnyRH.
!
redundancy
!
!
ip tcp synwait-time 10
ip ssh authentication-retries 5
!
!
!
!
!
!
!
interface GigabitEthernet0/0
description NOT USED
no ip address
no ip redirects
no ip unreachables
no ip proxy-arp
ip flow ingress
ip nat inside
ip virtual-reassembly
duplex auto
speed auto
no mop enabled
!
!
interface GigabitEthernet0/0.1
description VLAN 10 TRUNK
encapsulation dot1Q 10
ip address 10.10.10.1 255.255.255.0
ip nat inside
ip virtual-reassembly
no cdp enable
!
interface GigabitEthernet0/0.2
description VLAN 20 TRUNK
encapsulation dot1Q 20
ip address 10.10.20.1 255.255.255.0
ip nat inside
ip virtual-reassembly
no cdp enable
!
interface GigabitEthernet0/1
no ip address
no ip redirects
no ip unreachables
no ip proxy-arp
ip flow ingress
shutdown
duplex auto
speed auto
no mop enabled
!
!
interface GigabitEthernet0/2
description $ETH-WAN$
ip address 192.168.98.205 255.255.255.0
no ip redirects
no ip unreachables
no ip proxy-arp
ip flow ingress
ip nat outside
ip virtual-reassembly
duplex auto
speed auto
no mop enabled
!
!
ip forward-protocol nd
!
ip http server
ip http authentication local
ip http secure-server
ip http timeout-policy idle 60 life 86400 requests 10000
!
ip nat pool with_overload 192.168.98.205 192.168.98.205 prefix-length 24
ip nat inside source list 7 pool with_overload overload
ip route 0.0.0.0 0.0.0.0 192.168.98.254
!
logging trap debugging
access-list 7 permit 10.10.10.0 0.0.0.255
access-list 7 permit 10.10.20.0 0.0.0.255
!
no cdp run

!
!
!
!
!
control-plane
!
!
banner exec ^C HOHO ^C
banner login ^CAuthorized access only!
^C
!
line con 0
login local
transport output telnet
line aux 0
login local
transport output telnet
line vty 0 4
privilege level 15
login local
transport input telnet ssh
line vty 5 15
privilege level 15
login local
transport input ssh
!
scheduler allocate 20000 1000
end

R1(config-subif)#

[Latchum Naidu]

Hi,

Please check the ports config at 3Com switches end. Make sure the trunk ports at 3COM switches end port24 and port23 are same. Because in your scenario DHCP is working fine through port 24.
And also check the default gateway in switch (port 24 trunking) is same like switch (port 23 trunking)

Please rate the helpfull posts.
Regards,
Naidu.

[cadet alain · Accepted solution]

Hi,

according to your diagram the 2 hosts are in the same vlan and so they should both be getting an IP address via DHCP

without any helper-address. Are they really in the same VLAN?

Can you do a debug ip dhcp server events and debug ip dhcp server packet on your 2911 while trying to get your IP from the host

having a problem.

Regards.

Alain.

[Tommy Svensson]

You are right, the trunk is not working for some reason.

switch 4400-1 and switch 4400-2 has both port 10 member och VLAN 10 and port 20 member of VLAN 20. 4400-1 has port 23 and 24 working as trunk ports. port 24 to the router and port 23 to 4400-2. The routing between the VLANs work just fine on 4400-1 but not on 4400-2. Im guessing i dont need to set any additional settings in the switch like default gateway that was suggested, it is after all just layer 2 switches.

Regards Tommy Svensson

[cadet alain · Accepted solution]

Hi,

Now that you know the trunk between the switches is not working, you need to find out why.

But you don't need to configure anything special on the switches, the default gateway is only for their management.

Can you post the running from the 2  switches.

Regards.

Alain.

[Tommy Svensson]

I got it working. Thank you for all your help.

Regards Tommy Svensson

[Latchum Naidu]

Hi Tommy,

Please rate the helpfull posts so that the people will get encouraged.

Regards,
Naidu.

[Robert R]

Hi Tommy,

Glad you got everything working, just wanted to make a posting suggestion, when you post you're running config, delete the enable secret and any usernames from the config file before you post it.

They can easily be decrypted using readily available tools.  I would suggest changing any passwords that you have listed in the config.

Rob