does asymmetrical routing affect dhcp?

[get_rthym]

Hello experts,

Does asymmetrical routing affect dhcp?
Ip-phone is not getting ip from dhcp server located in remote site?
We have helper-address configured. There are many ip phones. Some get ip and some time out. I checked n found somehow asymmetrical path.
I work I big ISP at csc, so I won't be able to post configs because we r not allowed.
I can share the topology.
We have 2 edge routers, primary and secondary, behind that is 2 l3 switches n they are connected via ether channel.
We have 2 helper address configured on both l3sws. When we shut primary wan, dhcp works through secondary but currently we are supposed to get address through primary by design.
Any thoughts???

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[Richard Burts]

There is not enough information here for us to figure out what the fundamental problem is. But my thought is that it is not likely that the problem is due to asymmetric routing. If the request gets to the DHCP server and the response gets back to the client it should not matter whether they take the same path or not. If some phones are not able to get addresses then either something is preventing the request getting to the server or something is preventing the response getting back to the client.

HTH

Rick

[milan.kulik]

Hi Rick,

the response from DHCP server should be delivered to the L3 interface on which the ip helper-address was active and forwarded the (encapsulated) client request to the server, shouldn't it?

So it's possible when the response is deliverewd to the secondary router , it is not able to deliver it to the primary switch L3  LAN interface due to some routing issue?

Or the primary switch is dropping it for some reason (like uRPF checking activated while the packet is coming from incorrect direction)?

Best regards,

Milan

[Richard Burts]

Milan

Thanks for joining the discussion. I am finding it difficult to think of a routing issue that would impact DHCP. But your mention of uRPF is very interesting. I have indeed seen instances where a response coming back was routed to the standby member of an HSRP pair, and when it forwarded the response to the primary that uRPF on the primary did reject the response.

As I said in my previous response, we need additional information about the situation to be able to understand the problem and to suggest solutions. Part of the information that we need would include whether uRPF is configured on the primary, and whether uRPF is dropping packets.

HTH

Rick

[John Blakley]

This can also happen if your dhcp server is on a trunked port and doesn't need it. Some phones will get an address and some won't. You only need the dhcp server on your data vlan, and the dhcp request can handle the rest for tagging, etc.

HTH,
John

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