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DHCP Redundancy - IP Helper Address point to HSRP

My actual Scenario

1 x 4500 and 1 x 3560

They are gateways of 8 Vlans

They are doing HSRP in each of those Vlans

The 4500 is the Active

There is a DHCP Pool for each of those Vlans on both gateways

Using "ip dhcp excluded-address" I ensured that the range of provided ips by each DHCP server will not be overlapped

Obs.: Reducing the lease time, I ended with the calls bringing related problems.

OK, every thing is blue, every thing is fine.

But the network diagram is realy complex(41 switchs, 89 uplinks), and depending of how is the network flow, one or other server answer first or latter.

For many reasons I would like that the secondary DHCP server would answer only if the primary DHCP server goes down.

To me, the bigger reason is that DHCP database would be only in one DHCP server.

But there is other reasons.

I passed by many frustrated solutions:

Try to force a delay on the answer on one of the servers. - Impossible.

Try to disable DHCP server, and, using EEM, enable it only if router became active in HSRP. - I couldn't do It.

What I'm thinking now is use the HSRP resource to resolve it.

On both routers I would put a "ip helper-address" pointing to an Virtual_HSRP_IP.

And depending on which router is the active, him will answer the request.

My first doubt is:

Would it work?

The second doubt is:

Could I use the same Virtual_HSRP_IP that exists on that Vlan(see example 1),

or I would need to point it to a Virtual_HSRP_IP in a different Vlan(see example 2)?

Example 1

        -----------------------------------

        |              4500               |

        -----------------------------------

        interface Vlan1

         ip address 10.10.0.2 255.255.0.0

         ip helper-address 10.10.0.1

         standby 1 ip 10.10.0.1

         standby 1 priority 110

         standby 1 preempt

         standby 1 track 10 decrement 30

        ip dhcp pool VLAN1

           network 10.10.0.0 255.255.0.0

           default-router 10.10.0.1

           domain-name anything.com

           dns-server 8.8.8.8

           lease 0 1

        -----------------------------------

        |              3560               |

        -----------------------------------

        interface Vlan1

         ip address 10.10.0.3 255.255.0.0

         ip helper-address 10.10.0.1

         standby 1 ip 10.10.0.1

         standby 1 priority 90

         standby 1 preempt

         standby 1 track 10 decrement 30

        ip dhcp pool VLAN1

           network 10.10.0.0 255.255.0.0

           default-router 10.10.0.1

           domain-name anything.com

           dns-server 8.8.8.8

           lease 0 1

Example 2

        -----------------------------------

        |              4500               |

        -----------------------------------

        interface Vlan1

         ip address 10.10.0.2 255.255.0.0

         ip helper-address 10.99.0.1

         standby 1 ip 10.10.0.1

         standby 1 priority 110

         standby 1 preempt

         standby 1 track 10 decrement 30

        interface Vlan99

         ip address 10.99.0.2 255.255.0.0

         standby 1 ip 10.99.0.1

         standby 1 priority 110

         standby 1 preempt

         standby 1 track 10 decrement 30

        ip dhcp pool VLAN1

           network 10.10.0.0 255.255.0.0

           default-router 10.10.0.1

           domain-name anything.com

           dns-server 8.8.8.8

           lease 0 1

        -----------------------------------

        |              3560               |

        -----------------------------------

        interface Vlan1

         ip address 10.10.0.3 255.255.0.0

         ip helper-address 10.99.0.1

         standby 1 ip 10.10.0.1

         standby 1 priority 90

         standby 1 preempt

         standby 1 track 10 decrement 30

        interface Vlan99

         ip address 10.99.0.3 255.255.0.0

         standby 1 ip 10.99.0.1

         standby 1 priority 110

         standby 1 preempt

         standby 1 track 10 decrement 30

        ip dhcp pool VLAN1

           network 10.10.0.0 255.255.0.0

           default-router 10.10.0.1

           domain-name anything.com

           dns-server 8.8.8.8

           lease 0 1

4 REPLIES

DHCP Redundancy - IP Helper Address point to HSRP

You cannot use ip helpers when the dhcp server is directly on the switch which is the L3 for the vlans where the requests originate from.

The kind of solution proposed here will likely break dhcp completely.

Best solution is what you have: to stay with a scope per switch, the alternative is to install a dedicated (linux) dhcp server.

Linux is free and runs on virtually everything; it wont cost a fortune.

regards,

Leo

Re: DHCP Redundancy - IP Helper Address point to HSRP

Well, I don't know...

Maybe I'm in right direction.

Searching by "Cisco ip helper-address" I found some thing about VGR(Virtual Router Group), that is related to HSRP.

Here is an example that is similar with what I want.

http://www.cisco.com/en/US/docs/ios-xml/ios/ipapp/configuration/12-4t/iap-udp-vrg.html

Tomorrow, depending on my enthusiasm, I will test this config.

DHCP Redundancy - IP Helper Address point to HSRP

The only problem is the feature is in 12.4T, your switches are at 12.2...

New Member

Hi Douglas,

Hi Douglas,

I would believe the solution you suggested with a separate vlan  with VRRP/HSRP to be used as the IP address of your DHCP server would work.  Did this not work for you?

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