1

Cisco IOS switch dropping legit DHCP requests

view story
linux-howto

http://serverfault.com – We have Cisco Catalyst 4507R running IOS v12.2 that appears to be dropping valid DHCP requests. Checking for the requests using tcpdump on the DHCP server and on a machine connected to a monitoring port shows that only after 30s (and on the 8th request) does the request make it to the server. The request packets appear to not have any indicators that would trigger the DHCP control features on the switch that I know of, and are from uBoot doing a network boot (but also show when other machines are booting) Below is the DHCP snooping config on the switch, I do notice that the restrictions on h (HowTos)

taskli's picture
Submitted by taskli on 12/09/2012

Stories similar to Cisco IOS switch dropping legit DHCP requests

I have been trying to PXE boot a machine. However, it is not working. I suspect that this is due to the fact that this is a server network and all the machines on that network use static IPs so the switch was not configured to relay DHCP requests.

I have checked our DHCP logs and I do not see any requests coming from the machine in question.

My school had an Ubuntu 10.04 LTSP cluster which was PXE booting thin clients ok.

Then we changed to new Cisco switching gear... The initial DHCP works and the TFTP pulls the boot image. The image then appears to make another DHCP request--with an option 61 and a zero length payload--which Cisco then drops!!! Client stalls...

I want to configure DHCP server in a way that it puts "regular" smartphones and tablets into a separate subnet.

I would like to establish trunk between Cisco router 1812 and Catalyst 2960 switch. What I would like to have is that router gets ip address on his fe0 interface and on fe2 provides trunk with 3 VLANs with DHCP server all NAT-ed to fe0.

Here is my shortened current config for with vlan: VLAN 20

ROUTER

!

Many ARP requests in TCP dump? 1 year 23 weeks ago

I have been experiencing very slow upload speeds on my cable network lately. I ran a tcpdump and found many ARP requests.

I'm running a KVM instance inside of OpenStack, and it isn't getting an IP address from the DHCP server.

Using tcpdump, I can see the request and reply packets on vnet0 of the compute host:

# tcpdump -i vnet0 -n port 67 or port 68 tcpdump: WARNING: vnet0: no IPv4 address assigned tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on vnet0, link-type EN10MB (Ethe

I have isc dhcpd v3 serving a local subnet. I have installed a rogue dhcp server detector (dhcp_probe) which constantly sends dhcp requests.

Cannot find the DHCP Server 16 weeks 14 hours ago

The network was fine yesterday and as far as I know nothing has been changed or added. This morning some computers are connected to our domain but most of them are reporting limited connectivity. On checking our DHCP server we are getting this message.

Cannot find the DHCP Server:

The DHCP server you specified cannot be located.

eth dropping from dhcp loss 47 weeks 5 days ago

I took down my DHCP server to change some stuff, and after bringing it back up (down for 15 mins or so), I noticed that four other Ubuntu 12.04 servers set to use DHCP were unreachable. Running ifconfig on them returned just lo, so the eth0 and eth1 had been disabled.

I'm assuming the DHCP server missing caused them to shutdown the interface?