DHCP server on 2 Cisco ISR Gateways

[Roland Theisen]

Hello, 3 years I started to implement VoIP in my company with 2 Cisco Callmanager server (version 7.1.5), 1 ISR 2811 gateway and some 300 IP Phones. The gateway has a DHCP pool on it and makes the link to a PBX (via an E1 connection). As I will take of my PBX system and for redundancy reasons, I have added today a second gateway (ISR 2921) to my environment. My questions is, how should I configure the DHCP on my second gateway (the ISR 2921) in case that my first gateway (ISR 2811) dies? All the devices are in the same network (VLAN) and the default gateway is the interface on our firewall. Best regards, Marc Hoffmann

[acampbell]

Hi Marc,

Can you provide the existing DHCP config section from the 2811 router.

Also is this the default gateway for the VLANS/subnets that is servers as the DHCP server.

You may want/need to consider HSRP aswell if this is all about redundancy

Alex

[Roland Theisen]

hello Alex, Here is my DHCP configuration from the 2811: ip dhcp excluded-address 172.16.0.1 172.16.0.20 ip dhcp pool net-VoIP   network 172.16.0.0 255.255.248.0   default-router 172.16.0.1   option 150 ip 172.16.0.11   domain-name mycompany.com We have a Nokia Firewall cluster in our productive environment. This Nokia box is the default gateway/router/firewall for our VoIP network, the computer network and our guest network. Each network has a dedicated VLAN ID. So for the VoIP, the IP address "172.16.0.1" is on the interface of the firewall. Do I need to put a second dhcp pool on the my second gateway? Or shall I keep the existing DHCP pool and split it up into two parts? Dou have a configuration example for HSRP configuration? best regards, Marc

[acampbell · Accepted solution]

Marc,

As the router is not the default gateway then HSRP between the routers is not required.

I would slit the the IP scope pool

Router 1

ip dhcp excluded-address 172.16.0.1 172.16.0.20

ip dhcp excluded-address 172.16.4.0 172.16.7.254

ip dhcp pool net-VoIP

network 172.16.0.0 255.255.248.0

default-router 172.16.0.1

option 150 ip 172.16.0.11

domain-name mycompany.com

Router 2

ip dhcp excluded-address 172.16.0.1 172.16.3.255

ip dhcp pool net-VoIP
network 172.16.0.0 255.255.248.0
default-router 172.16.0.1
option 150 ip 172.16.0.11
domain-name mycompany.com

Hope this help

Alex

[Roland Theisen]

Hi Alex, Yes, this helps. Thanks for your answer. best regards, Marc

[James Hawkins]

Hi Marc,

Part of the DHCP specification is that the server should try and ping the address it is about t allocate before issuing the address to make sure it is not in use - see link below for more details.

http://www.cisco.com/en/US/docs/ios/12_1/iproute/configuration/guide/1cddhcp.html#wp1001176

This means that you should be able just to replicate the DHCP config on your old router on the new one and have them both running simultaneously without worrying about duplicate IP addresses being issued.

In practice it is probably best to  split the DHCP pool between the two routers using excludes as shown above. The only issue with this would be if the number of phones exceeded the number of IP addresses that a single router had available to allocate you would run into problems if one of the routers failed and was out of service for a while.

[Santosh Mohite]

Hi,

I have similar kind of setup with almost 2500 devices. I would suggest to use lease command. With dhcp lease set to 7 or 8 days, even if your router dies, the phones will continue to carry the same IP address.

Plus, you can load balance the networks across 2 routers. Example: 172.20.1.0/ 24 on one router and 172.20.2.0 on another router.

Thanks,

Santosh