04-03-2009 08:08 AM - edited 03-06-2019 04:58 AM
Hi all,
I have a couple of 3550's running c3550-ipservicesk9-mz.122-25.SED which are displaying odd DHCP relay behaviour when multiple ip helper-address's are defined on any VLAN. For example:
interface Vlan4
ip address 192.168.4.1 255.255.255.224
ip helper-address 192.168.2.11
ip helper-address 192.168.2.12
no ip redirects
no ip unreachables
no ip proxy-arp
end
In this configuration, the DHCP unicast packets (relayed by the switch) only reache 192.168.2.11 and NOT 192.168.2.12. The behaviour is incontary to how my understanding of how multiple ip helper-address's should function - I expect to receive a the unicast packets (sourcing from the switch) at both DHCP servers. Extensive sniffing has confirmed that only the *1st* host defined in an ip helper-address statement receives the relayed packets.
I can confirm that relayed packets can reach both DHCP servers if I define only one ip helper-address under the VLAN interface, i.e.
interface Vlan4
ip address 192.168.4.1 255.255.255.224
ip helper-address 192.168.2.11
no ip redirects
no ip unreachables
no ip proxy-arp
end
OR:
interface Vlan4
ip address 192.168.4.1 255.255.255.224
ip helper-address 192.168.2.12
no ip redirects
no ip unreachables
no ip proxy-arp
end
I've spent hours pulling my hair out over this very strange problem. Please can someone assist?
Kind regards,
Tom Ranson
04-03-2009 08:42 AM
Tom,
I believe the behavior is if the first helper responds, it won't roll over to the next. The second one is used for failover.
HTH,
John
04-03-2009 08:49 AM
John
Sorry to have to contradict you but the L3 device should send unicast packets to all ip helper-addresses at the same time. The first to respond is usually the one used by the client.
Jon
04-03-2009 08:51 AM
Hi Jon,
I agree - this is my understanding of how this should function.
Are you able to advise me as to why the behaviour I am seeing may be occuring?
Kind regards,
Tom
04-03-2009 09:01 AM
Tom
"Are you able to advise me as to why the behaviour I am seeing may be occuring?"
Unfortunately no. Could be a bug perhaps. Have you tried another version of software to see if you get the same issue.
Perhaps if you posted the config something might jump out altho it's unlikely.
Jon
04-03-2009 08:54 AM
Jon,
This is how we learn :)
rated.
John
04-03-2009 08:50 AM
Hi John,
Many thanks for your prompt reply.
This goes against what Cisco state to be expected behaviour. Can you please point me towards any documentation to support this theory?
Kind regards,
Tom
04-03-2009 09:10 AM
Tom
I am finding lots of references in Cisco documentation that specify that you can have multiple helper addresses. But I have not yet found one that specifies the expected result. But I certainly agree with Jon that the expected result is forwarding to each specified helper.
Since you seem to have a repeatable behavior I would suggest that you open a case with Cisco TAC about this.
HTH
Rick
04-03-2009 09:40 AM
Tom
I now have found Cisco documentation that does describe the expected behavior of multiple helper addresses:
Cisco routers forward the DHCPDISCOVER message to all the helper addresses mentioned for that interface.
here is the link:
http://www.cisco.com/en/US/tech/tk648/tk36/technologies_tech_note09186a00800f0804.shtml
HTH
Rick
04-04-2009 03:59 AM
Hi Rick,
Many thanks for your input - I'm positive that I'm not going mad now. It looks like I may have a bug on my hands; I'll add another '1' to my long list of open TAC cases :-/
Cheers,
Tom
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide