Mailing List Archive: Wishing for clarification on how dhcp-relay works with multiple server addresses

Wishing for clarification on how dhcp-relay works with multiple server addresses

Jun 10, 2020, 10:40 PM

Post #1 of 5 (1147 views)

[. I posted this same question to Juniper forums as well - sorry for
repetition ]

Hello,

We have a setup where one set of DHCP servers deliver IP configuration
to clients and another set of DHCP servers deliver the PXE options. This
setup works only if we configure dhcp-relay without forward-only option.
The issue is that this eats up licences and this is something we would
like to avoid. We have tried the setup with forward-only-replies option
as well. When I monitor traffic on the client switch port I see only the
DHCP packets providing IP configuration.

I've tried to go through documentation but I haven't found an
explanation how the DHCP relay works with different configurations. The
observed behaviour suggests that only the DHCP packets from the first
responder are let through and others are dropped. I wish I could change
this behaviour so that all DHCP packet would be allowed to flow through.

Could anyone give me more details how the DHCP relay actually works with
different configuration? The router is MX10003 and it's running Junos
18.2R3.4.

Cheers,

Matti
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Re: [EXT] Wishing for clarification on how dhcp-relay works with multiple server addresses [ In reply to ]

cra at WPI

Jun 11, 2020, 4:27 AM

Post #2 of 5 (1144 views)

Permalink

On Thu, Jun 11, 2020 at 08:40:23AM +0300, Matti Saarinen wrote:
> We have a setup where one set of DHCP servers deliver IP configuration
> to clients and another set of DHCP servers deliver the PXE options. This

Don't do that. Clients do not aggregate DHCP options from different
responses--they pick ONE DHCP server to bind to and use the info from
that one only. That's how the DHCP spec is written.
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Re: [EXT] Wishing for clarification on how dhcp-relay works with multiple server addresses [ In reply to ]

mjsaarin at cc

Jun 11, 2020, 10:44 PM

Post #3 of 5 (1138 views)

Permalink

Chuck Anderson <cra@WPI.EDU> wrote:

> On Thu, Jun 11, 2020 at 08:40:23AM +0300, Matti Saarinen wrote:
>> We have a setup where one set of DHCP servers deliver IP configuration
>> to clients and another set of DHCP servers deliver the PXE options. This
>
> Don't do that. Clients do not aggregate DHCP options from different
> responses--they pick ONE DHCP server to bind to and use the info from
> that one only. That's how the DHCP spec is written.

Actually, this setup has been working for years. I suppose the PXE code
is more flexible in that matter. In any case, it worries me that we have
been relying on a feature that may change without any notice when NIC
firmwares are updated.

Back to my question:

Based on the forum responses[1] I'd say we have to live with the
situation where we need to run dhcp-relay without forward-only on
interfaces connecting networks needing PXE. The annoying issue is that
every interface without forward-only eats one scale-subsrciber licence.

Cheers,

Matti

[1] https://forums.juniper.net/t5/forums/v3_1/forumtopicpage/board-id/IProuting/message-id/21843
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Wishing for clarification on how dhcp-relay works with multiple server addresses [ In reply to ]

cra at WPI

Jun 12, 2020, 4:03 AM

Post #4 of 5 (1137 views)

Permalink

On Fri, Jun 12, 2020 at 08:44:48AM +0300, Matti Saarinen wrote:
> Chuck Anderson <cra@WPI.EDU> wrote:
>
> > On Thu, Jun 11, 2020 at 08:40:23AM +0300, Matti Saarinen wrote:
> >> We have a setup where one set of DHCP servers deliver IP configuration
> >> to clients and another set of DHCP servers deliver the PXE options. This
> >
> > Don't do that. Clients do not aggregate DHCP options from different
> > responses--they pick ONE DHCP server to bind to and use the info from
> > that one only. That's how the DHCP spec is written.
>
> Actually, this setup has been working for years. I suppose the PXE code
> is more flexible in that matter. In any case, it worries me that we have
> been relying on a feature that may change without any notice when NIC
> firmwares are updated.
>
> Back to my question:
>
> Based on the forum responses[1] I'd say we have to live with the
> situation where we need to run dhcp-relay without forward-only on
> interfaces connecting networks needing PXE. The annoying issue is that
> every interface without forward-only eats one scale-subsrciber licence.

You can try using the legacy helpers configuration, but I'm not sure it works on MX10003:

set forwarding-options helpers bootp server x.x.x.x
set forwarding-options helpers bootp server y.y.y.y
set forwarding-options helpers bootp server z.z.z.z
set forwarding-options helpers bootp maximum-hop-count 16
set forwarding-options helpers bootp client-response-ttl 20
set forwarding-options helpers bootp interface xe-x/x/x.nnnn broadcast
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Re: [EXT] Wishing for clarification on how dhcp-relay works with multiple server addresses [ In reply to ]

juniper-nsp at puck

Jun 12, 2020, 7:56 AM

Post #5 of 5 (1137 views)

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