Catalyst 4500-X DHCP relay secondary subnets

[NetworKing16]

We are going to be putting in a Cat 4500-X as a new core switch/router. At least for right now we need to replicate our current setup which has one VLAN interface with 5 secondary addresses. I believe the primary address set on it is the WAN address, the rest are internal subnets and one for a VPN subnet. (Public I.Pv4 address space NO NAT/PAT).

Right now we have all kinds of separate DHCP servers, my goal is to eventually setup a a couple systems of VMs with server 2012 or 2016 and use DHCP fail-over on these servers. The issue though is setting DHCP relay parameters on the 4500-X. I was hoping I could put in the normal ip helper-address on the VLAN interface and then have it relay all of the subnets to the same server. 

As per my understanding of DHCP and DHCP relay I would think you could just pass all the DHCP to one server, the server would then go though scope 1,2,3,... In our case though because DHCP only works if the address is reserved every system would be associated with a particular subnet and would then get it's proper address. 

I looked into this some, and it seems I need DHCP smart relay enabled, but no idea how this gets pointed to the server. 
I also saw that "ip helper-address" is more versatile. But there is an option too for "ip dhcp relay". I hope to use this in the future, but I would rather test it now before we go live with this unit. 

[Reza Sharifi]

no idea how this gets pointed to the server. 

ip dhcp smart-relay is a global config command and not per interface.  See example:

Since the 4500 is capable of having multiple SVIs, you can simply create 5 SVIs for your 5 vlans and add multiple DHCP address to each SVI.  This way, there is no need for secondary addresses.

http://www.cisco.com/en/US/docs/ios/12_4t/ip_addr/configuration/guide/htdhcpre.html#wp1090022

HTH

[NetworKing16]

As near as I can tell we need subinterfaces as all of these subnets are on a single VLAN I would guess roughly 1000 nodes on this VLAN.

Unless there is a way to do int VLAN x then do subinterfaces and have all of those map to the same VLAN

[Reza Sharifi]

On the 4500x, you can have many vlan interfaces:

interface vlan 10

ip address 

inter vlan 20 

ip address 

int vlan 30

ip address

etc...

HTH

[NetworKing16]

I know that, but this is our issue:
Our default VLAN has 4 class C subnets. All of these subnets on this SAME VLAN need to route. 

In our case say vlan 100 is the default vlan for all of these subnets. In thiscase they are public addresses, but I will use the 192 private range for this example. 

We would have int vlan 100
ip address 192.168.0.1 255.255.255.0 (WAN address)
ip address secondary 192.168.1.1 255.255.255.0 (LAN#1)
ip address secondary 192.168.2.1 255.255.255.0 (LAN#2)
ip address secondary 192.168.3.1 255.255.255.0 (LAN#3)
ip address secondary 192.168.4.1 255.255.255.0 (LAN#4)
ip address secondary 192.168.5.1 255.255.255.128 (VPN network)

As near as I can tell I can only set one DHCP relay per VLAN interface and I need to make LAN#s 2,3,4 all relay over to a server on LAN#1. 

I know you can do sub interfaces and routing on a stick, but I I don't know if I could do that with only one VLAN. Maybe like int vlan1.1 for LAN1 on VLAN1, int vlan 1.2 for LAN2 on VLAN 1, etc. 

[paul driver]

Hello

My understanding of Smarty-relay is it is used when you have multiple dhcp servers available and you wish to utilize them in away when the primary dhcp server associated with a certain scope isnt available or its ip allocation is exhausted.

So let say for instance Dhcp Srvr1 Scope 1 has depleted its allocation but is still active, smart relay will then try the next dhcp server listed for allocation of that subnet, Without this feature if the primary server is still available but its scopes allocation is depleted the router wont try the next server listed for that same scope.

We have this feature enabled in some of our trusts and it works accordingly.

res
Paul

[NetworKing16]

If that is the case then maybe not what I am looking for. I want one server or to use DHCP failover for all of our DHCP. I want the seconadry subnets the server isn't on to be able to get DHCP relayed to them. 

[paul driver]

Hello

Well this could still applicable to do-

1) create you new dhcp servers

2)have the same scopes on each server so each server is serving only half of ecah scope for resiliency 

3) enable smart-relay and the dhcp helpers as usual 

4) drop you lease trimings on the old server and when you ready to migrate activate the new server dhcp scopes and deactive the old ones.

This should give you dhcp some resiliency should either one of your server dies or becomes unavailable 

res

paul