Cisco Cat 6509 + DHCP relay + Unnumbered SVI

[Konstantin Vinogradov]

Hello!

I have some problems with dhcp-relay feature in unnumbered vlan interface.

Now I have two vlan-interface with clients PCs:

interface Vlan120

description TEST-CLIENTS

ip unnumbered Loopback120

ip helper-address 192.168.0.100

shutdown

end

and loopback:

interface Loopback120

description TEST-LOOPBACK

ip address 192.168.100.1 255.255.255.0

no ip redirects

end

and vlan-interface for connecting to DHCP-server:

interface Vlan776

description TEST-DHCP

ip address 192.168.0.1 255.255.255.0

end

There is no connected routes for clients because interface Vlan120 is unnumbered. If I create static route for client PC (for example ip route 192.168.100.100 255.255.255.255 Vlan 120) all fine: PC can get ip address from DHCP and all ok. But if I remove static route (if I understand correctly router must create static route for dhcp-clients automatically) - no anything works. DHCP server recived request from client, but can't get address for it. And there are no any routes in show ip route dhcp.

Could you tell me: what is the problem? May be I have wrong understanding of dhcp-relay functions?

[Vladimir Troitskiy]

Hi, Konstantin!

Have you enable dhcp snooping for vlan 120?

Check the "show ip dhcp snooping" output.

[Konstantin Vinogradov]

Hi, Vladimir!

But there are no any "snooping" commands in my "sh ip dhcp..." context.

c6509#sh ip dhcp ?

  binding   DHCP address bindings

  conflict  DHCP address conflicts

  database  DHCP database agents

  import    Show Imported Parameters

  relay     Miscellaneous DHCP relay information

  server    Miscellaneous DHCP server information

[Rolf Fischer]

Command History
 12.2(18)SXE 
 Support for this command was introduced on the Supervisor Engine 720.
 12.2(33)SRA 
 This command was integrated into Cisco IOS Release 12.2(33)SRA.

Never mind. It's a feature for edge-ports and should only be used on access-switches anyway. But it was worth verifying it.

Best regards

Rolf

[Rolf Fischer · Accepted solution]

Hi,

there's a "shutdown" in the interface vlan 120 config you've posted.
For troubleshooting I'd start with
show ip int brief
debug ip dhcp server packets

Also make sure that service dhcp is enabled:

show processes cpu | include DHCPD

I always thought you cannot assign ip unnumbered to multiaccess interfaces, I'll do some readings on this.

Best regards
Rolf

Sent from Cisco Technical Support Android App

[Vladimir Troitskiy]

Hi, Rolf!

We use absolutely the same scheme with the topicstarter and everything works fine.

[Konstantin Vinogradov]

Hi, Rolf.

Thanks for pointing out my mistake with "shut" vlan-interface.

But the reason of my problems was wrong DHCP-server (I don't understand HOW?!?!?! REALLY!!). For my quick test I was using Tiny DHCP for Windows.

After I had changed TinyDHCP to Open DHCP Server (for Windows too) this scheme started to work correctly.

I really don't understand how dhcp-servers version can impact in this case, but now test scheme works fine. I hope new surprises don't wait me in production.

Thanks for all!