Issue with DHCP relay on C1131X-8PLTEPWE

[markus_penner]

Hello and good day community,

I would need your help with an DHCP relay problem on a Cisco C1131X-8PLTEPWE with 5G WAN connection.

Let me inform you, before I tell you my problem, I´ve got the same setup, also with the same Vlan confiuration and same subnet on a C921-4PLTEGB router and this works fine and as expected.

Now to my problem ......

This 5G router has a Vlan (Vlan1) and a /28 network configured. All switchports (only one is in use - GigabitEthernet0/1/0) are configured for access this vlan1 ->

VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------

1 default active Gi0/1/0, Gi0/1/1, Gi0/1/2, Gi0/1/3, Gi0/1/4, Gi0/1/5, Gi0/1/6, Gi0/1/7, Wl0/1/8

On the vlan interface, I´ve configured two ip helper addresses for the central DHCP server. I can ping this server, I can start a traceroute to this server and I can also make a telnet to this server, for example with TCP port 53, all this with source interface vlan1. We can see all this working connections on the central firewall, where the DHCP server is located behind.

When I start debugging (debug ip dhcp server packet detail) on the router, I can see the DHCP BOOTREQUEST going out to the server with the source IP from the vlan interface, but we can not see any packets on the firewall for this DHCP request.

If we configure the same on the the C921-4PLTEGB and start the DHCP process, packets are visible on the central firewall, we receive the unicasting BOOTREPLY to client and the router is creating the arp entry for the client and everything is fine.

So the way over the WAN is the same for both routers but it is not working with the C1131X-8PLTEPWE. I found a resolved problem for IOS-XE 17.2.x.x with DHCP under this firmware, having some troubles with unicasts. It looks like, that the resolution for this problem was to configure the command "ip dhcp client broadcast-flag clear". I´ve tried this also, but without success.

Did anyone of you also had an issue like this with this routertype or an other C1100? At the moment, I´ve have the IOS-XE version 17.15.03a in use, this is one of the latest MD´s from Cisco.

Thank you in advance for taking your time and looking at my problem

BR

Markus

[pieterh]

it is a 5G router, but i do not see you mention a VPN connection on the 5G interface
you first need the VPN connection to your main site operational before DHCP requests can traverse
can you specify more details (e.g. share the running configs) ?

another question: do you really mean you are using the same subnet on vlan1 on the C921 the  C1113xXat the same time ?
that will not work if both routers are operational
or do you mean you are replacing the C921 with the C1113X ?

[markus_penner]

Hi,

in that case, I do not need a VPN connection, because I´m using a private APN from the mobile provider with a private IP address range. The SIM card is coming online and get´s an IP address assigned wich is configured on a central management tool.

No, I do not use the same subnet on both routers at the same time. I´ve configured and tested it on the 4G router and afterwards I deleted the configuration from the 4G router and took over the config to the 5G router. So the config is active either on the 4G or the 5G router. Never at the same time.

[pieterh]

good, both my concerns (vpn and router instance) are covered.

4G and 5G at your provider are different networks, not the same network just using a different frequency.
let your provider check that the 5G network correctly processes your routers DHCP request to your private IP range

[markus_penner]

I´m going to have a troubleshooting call on monday with a technician from our mobile provider.

I give you an update afterwards ....

[markus_penner]

Hello again!

So I had a call with our mobile provider. They can see alls packets in the backbone coming from my 5G router except the DHCP packets. I started a ping and a telnet conneciton and they saw the connetions.

The expression from the technician was, that there is no difference between 4G and 5G in the backbone, except the type of radio connection.

So from my point of view, there must be some issue for this router model itself, that it is not sending out this DHCP request over the WAN.

[pieterh]

is it possible to attach a (sanitized) copy of the routers config?