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Aussiewan_19227's avatar
Aussiewan_19227
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Mar 16, 2015

F5 DHCP Relay in multi-hop configuration issue

Hi all,

 

We have 2x F5 BIG-IP 4000 units with APM Base, GTM-DNS and LTM Base licenses, running version 11.6.0 (build 3.0.412). We are having trouble with a multi-hop DHCP relay config where the F5 does not have a presence in the client VLAN. The reason we are wanting the F5 in the mix is so that we can take advantage of Server 2012 R2 DHCP redundancy/failover features and switch DHCP servers far quicker than updating 200 devices at once. We do not want or need the F5 to service DHCP broadcast requests hitting it's own interfaces.

 

We have been using these documents to try get this running:

 

We have the following setup:

 

Client network config:

 

  • Client on VLAN20
  • Cisco switch (C4500X with iOS XE 03.04.05) with VLAN20 configured with 10.1.0.1/24 and an IP Helper pointing to 10.2.0.5 which is 3 hops away
  • A single laptop with a wired connection to a switchport configured for VLAN20.

F5 config:

 

  • F5 Virtual Server configured with Type DHCP, Source Address 0.0.0.0/0, Destination (Other) 10.2.0.5, listening only on the VLAN where 10.2.0.5 is present.
  • DHCPv4 Profile configured using default settings, but tried in both Relay and Forward mode, with Idle Timeout set to Indefinite, Max Hops set to 30, Default TTL set to 2000, and TTL Decrement Amount set to 0, with no change in result.
  • Member Pool with 1x Windows Server 2012 R2 member on 10.2.0.6:67 running MS DHCP with a DHCP pool for 10.1.0.0/24. Also tried with a Windows Server 2008 R2 member.
  • No iRules applied.
  • SNAT rule created to map 10.1.0.1 to 10.1.0.1, but tried with and without this in place
  • Tried with and without a Self IP on 10.2.0.5 (not sure if it's required with a unicast DHCP packet instead of broadcast)

DHCP Server config:

 

  • Windows Server 2012 R2 with DHCP service installed
  • Scope created for 10.1.0.0/24 with gateway, DNS and domain name set
  • Static route added for 10.1.0.1/32 to go via 10.2.0.5

What we're seeing:

 

  • Virtual Server stats show traffic in and out, including recognising the DHCPDISCOVER packet, however it shows that all requests are timing out.
  • Pool stats show zeroes across the board for bits, packets, connections etc
  • Wireshark on the Windows DHCP server monitoring UDP port 67 and destination IP of 10.2.0.5. We see the DHCP health checks appear as malformed BOOTP packets, but no "real" DHCP packets.

Other thoughts I've had:

 

  • Cisco IOS doesn't seem to allow adding/editing a Max Hops value on IP Helper.
  • In DHCP mode, a Virtual Server forwards DHCP packets to all members at the same time. This may prevent the stats from reporting in an expected way.

Any tips or help that anyone can provide would be greatly appreciated.

 

Regards, Philip