10

I'm helping a client through the process of configuring a Windows 2012 DHCP server while migrating from Linux ISC DHCP.

In the Windows New Scope Wizard dialog, there is an option to:

...specify the routers, or default gateways (plural), to be distributed by the scope.

My customer asked when it would make sense to distribute multiple default gateways to DHCP clients...

I didn't have a good answer. So I'm curious when/if this functionality is appropriate, and does it hinge heavily on certain clients systems (e.g. Windows PCs)? I'd like to hear about practical scenarios.

enter image description here

7
  • 2
    In a properly designed network, you don't.
    – MDMarra
    Nov 6, 2014 at 12:52
  • 1
    This seems to be a prominent dialog box that I've clicked-through many times. Are there any real use cases for it?
    – ewwhite
    Nov 6, 2014 at 12:53
  • 1
    Only if you have multiple default gateways on the same subnet, which isn't the right way to handle gateway redundancy
    – MDMarra
    Nov 6, 2014 at 13:00
  • Personally I don't see anything particularly wrong with this kind of setup. If you only care about outbound internet access and you've got two independent internet connections from two different providers then you can deploy multiple default gateways via DHCP and let Dead Gateway Detection take care of it in the event of a failure of one of the internet connections. That seems like a pretty good "poor man's" implementation of providing redundant internet connections.
    – joeqwerty
    Nov 6, 2014 at 14:59
  • 2
    Any given connection defines a gateway as non-operational (dead) when a packet sent to the gateway must be retransmitted more than half of the number of times specified in the value of the TcpMaxDataRetransmissions entry. The connection switches to the next gateway in the list in the DefaultGateway or DhcpDefaultGateway entries. The system defines a gateway as dead when more than 25 percent of its connections have switched to the next default gateway in the list. Source: technet.microsoft.com/en-us/library/cc960464.aspx
    – Chris S
    Nov 11, 2014 at 19:18

6 Answers 6

1

If you got multiple subnet linked from a switch in exemple and your main router as your gateway CAN'T do ICMP Redirect, you better give the default gW, or route 0.0.0.0 to the router with the internet behind and to do static rule in the computer to make the computer talk to other router to go somewhere else (like another site)

Only time I seen it can be usefull to have two GW it's on layer 3 switch&router scenario. What I seen in the past was that;

[PC]---1 GW (switch vlan ip)---[SWITCH LYR 3]----- GW#1--[RTR-1]---[WAN1]
                                              -----GW#2--[RTR-2]---[WAN2]

So, is it usefull, no IMO, as nowaday router are cheap and allow multiple WAN, not like in the past.

1

This is an old post but for anyone else with this question:

If you're using HSRP or VRRP for L3 high availability, you can load balance across the two L3 switches by creating two HSRP/VRRP groups and using one switch as the active forwarder for one group and standby for the other (vice versa on the other switch). Some client gateways would be pointed to the first groups virtual IP and some client gateways would be pointed to the second groups virtual IP.

See the image below:

enter image description here

0
1

I am a CCIE, the answer to your question depends on the topology and your business parameters. In fact, where most people leverage HSRP or VRRP you can probably get away with just distributing multiple default gateways via DHCP. Failure can be a little more seamless via HSRP or VRRP and if any stateful devices in the path also happen to record or track the mac then you'll run into problems (some firewalls do in internal tables as well as load balancers). Keep in mind all devices on the network would need to support multiple routers for holistic redundancy. You may run into problems on things like print servers and IoT type devices. Outside those instances, you'd be fine. Whenever possible, keep it simple

0

This only makes sense if you have a company structure that is split into many subnets but every network has its own gateway with different IPs:

Consider your organisation has three different networks. In your office you have three systems, each from individual network. you may have only one laptop. You need to work on these three networks. Instead of connecting one network at one time, you can access all three networks at once.

http://www.whatvwant.com/lan-multiple-default-network-gateway/

1
  • 1
    This doesn't work and in Win8.1 and newer it will not let two default routes on the same interface have the same metric.
    – Chris S
    Nov 6, 2014 at 16:06
0

If you're using a First Hop Redundancy Protocol, especially Multigroup HSRP, on your local network then you want to split half your users to default out Router1 and the other half out of Router2.

So to explain you have 2 routers at a site with redundancy in mind. You want to load balance traffic out both of the routers. Each router acts as a gateway to the user. To balance the traffic you create a DHCP scope that sets half of your users to use Router1 as their default gateway and the other half to use Router2 as their default gateway.

0

To answer the question of why this might be needed, consider the scenario with network access from a foreign country. Normal activity would all be from NAT'd addresses from your firewall/router all of which would come from the foreign IP cluster of your provider. The firewall provides private IP addresses (say 192.168.1.xxx) to devices on the network gateway'd to the "foreign access" address set.

For specific devices by MAC address dhcp can provide another set of private IP addresses (say 10.0.0.xxx) that are gateway'd to a VPN that terminates in the United States and provides NAT'd to US IP addresses.

There are lots of reasons to do this, use your imagination.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .