Two DHCP servers in separate buildings

[swips88]

What is best way to increase speed and separate two building's networks.

Building A has firewall/router which is gateway to internet. DHCP currently DISABLED on this box.
about 75 hosts ip addresses in building not all in use at one time.
20 port switch in same building with 5 WIFI access points also.

500' COAXIAL connection to Building B using DSL converter boxes

Building B has Cisco switch and two smaller switches for printers and a few shop computers.
Windows Server in this building is configured as DHCP Server providing both buildings with IP range 192.168.1.x

Another 50-75 or so ip host addresses in this building.

Most network traffic stays in building B other than internet access. Most network traffic in Building A stays in building A occasionally accessing another application server and a file server in building B.

Having the Windows DHCP server giving out IP addresses to building A is not very efficient considering the coaxial link between buildings.

I am thinking there should be two DHCP servers with a subnet in building B keeping existing Windows server for all DHCP addresses in Building B

Configure Router to be DHCP server for building A with different ip range 192.168.2.x
Does router LAN port get new address for gateway in new subnet ip range and Windows DHCP Server points Router and gateway to this address?

thanks in advance for any suggestions.

[Joseph W. Doherty]

From what you've described, you would probably benefit very little from a second DHCP server.  However, splitting one subnet into two subnets, one per building, with a "router" at each end of inter building link, could be of benefit (BTW, you may not need a second DHCP server for that too).

[M02@rt37]

Hello @swips88,

You can still configure the primary Windows Server DHCP in Building B to handle DHCP for both subnets while following the subnet division and router setup. Devices in Building A can receive their IP addresses and DHCP settings from the Windows Server in Building B, which simplifies the DHCP management.

The key is to ensure that the router at Building A (the gateway for Building A's subnet) properly routes traffic to the Windows Server in Building B for DHCP requests and general network communication.

[swips88]

What I wanted to avoid is network traffic from Bldg A having to travel down the slow coax link to bldg B just to resolve ip's at DHCP Server.

Most of traffic in bldg A is just Internet traffic and that is where firewall/router with gateway is located.

Most of traffic in Bldg B is just inter-office system traffic. Workstations / Servers / Printers.

[Joseph W. Doherty]

Again, you're concern, over DHCP being a network performance issue is very unlikely (unless, perhaps, your DHCP lease time is very, very short).

". . . just to resolve ip's at DHCP Server."

Strictly speaking, that's not a function of DHCP, but the same server might support host name resolution.  Is your server doing that too?

If so, that's rarely a network bottleneck performance issue either.

How "slow" is the inter-building co-ax link?

BTW, I have years of experience optimizing network performances where offices were thousands of miles apart and link capacity was (often small) fractions of a T-1 or E-1.  For such, a host using DHCP, with a RTT of 200+ ms and with just 64 Kbps, worked fine (for DHCP or host name resolution).  (Other stuff, like remote file access, email access or web access, that's it's own issue.)

[swips88]

OK perhaps I am thinking wrong about this then. The link between buildings is an old 10BASE2 coax cable but we have been using VDSL converters on either end to speed up this link supposedly to 100mbs.

The server giving out ip's in Bldg B is a Windows SB Server also running Active Directory and inter-office email Exchange Server.

Bldg A is mostly Internet and guest WIFI useage. In reality it should be on separate subnet anyway. I was under impression that the link to the building B where Server resides would slow down internet connections in Bldg A also.

[Joseph W. Doherty]

Possibly your thinking isn't 100% wrong.  Again, DHCP and/or name resolution usually require little from a network.  However, if there is concurrent other traffic on same link, enough of which to cause drops, etc., that can be detrimental to other traffic, like DHCP.

Splitting off local services, avoids the contention on the congested link, but IMO, QoS is much better approach.

In ye olden times, when shared L2 was the norm, segmentation via L3 was the way to go.  With switched L2, L3 segmentation, for performance, loss much of its benefits.  (Laugh, how technology evolves - in another recent posting, the question was, Cisco SDN design guide recommending /16 subnets really a good idea?)

From what you've described, although splitting buildings into their own subnets is "good", your "best" option might be "management" of your inter-building link.

[MHM Cisco World]

I dont get it ?
if you have DHCP server in SiteB then make all use in that site use DHCP
for siteA you dont need split the subnet but you need 
config any L3 device as local DHCP server and make server in siteB as backup. 

Thanks A Lot
MHM

[swips88]

There is some limited access required from Bldg A to Bldg B needed. A few Workstations/Laptops a backup storage station all which need access to Network/Servers in Bldg B. However most of the hosts in Bldg A are 5 WIFI Access Points, IP cameras, a multitude of IP smart devices, video streaming etc. These I want to keep off Bldg B network for speed and security. I was trying to limit the broadcast traffic from these devices going down the coax link to network in Bldg B.

If we do the separate DHCP for Bldg A, I would still need to create routing to other network for the few devices I want connected. Is this not the same as making a subnet?

[MHM Cisco World]

Friend, 
if you decided to separate the subnet in SiteA and SiteB there is no need to run trunk and make DHCP broadcast across the two sites.
only in SiteA config DHCP relay to SiteB DHCP server. 
this make each Site have its subnet 
and host in SiteA use DHCP server in SiteB

Thanks A Lot
MHM

[Joseph W. Doherty]

You can have multiple DHCP servers for the same network.

Normally, though, if two DHCP servers are handing out IPs for the same network, you have a "race" condition.  I.e. first DHCP server to get IP offer to host is the "winner".

This though, doesn't preclude DHCP broadcast from still going across your inter building link.

Again, DHCP tends to usually have barely noticeable impact to network bandwidth consumption.

The way you've described you inter-building link, it could be a bottleneck, but it should be the focus for analysis with possible mitigation.

For example, you wrote the inter-building link is "speed up this link supposedly to 100mbs", but is it or isn't it?

What's the interface stats for the interfaces that connect to our VDSL converters?  What's those interface device kind?

[swips88]

Bldg A has the router/gateway where internet is located for both buildings to access. We use a sonicwall firewall/router and CISCO switches.

When I do a speedtest at Bldg B, I get ~ 50-60 mbs. At Bldg A where gateway is, we get 90-94 mbs on wired connections.

[Joseph W. Doherty]

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@swips88 wrote:

Bldg A has the router/gateway where internet is located for both buildings to access. We use a sonicwall firewall/router and CISCO switches.

When I do a speedtest at Bldg B, I get ~ 50-60 mbs. At Bldg A where gateway is, we get 90-94 mbs on wired connections.

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That might indicate your inter-building link isn't capable of 100 Mbps, but it might not too, as we don't know what else is crossing that link at the same time.

Again, what devices/ports connect to the VDSL converters?  Can you obtain stats from those ports?  If so, easy test, you use a traffic generate to push 100 Mbps stream across the Interbuilding link and see whether other side receives 100 Mbps.

[swips88]

Both media converters connect to a switch on each end.

I do not know how to generate and monitor a 100mbs stream to test?

When I ping each switch from a workstation in Bldg B i get slightly slower to the Bldg A switch

Switch in Bldg A:
Pinging 192.168.1.178 with 32 bytes of data:
Reply from 192.168.1.178: bytes=32 time=10ms TTL=64
Reply from 192.168.1.178: bytes=32 time=6ms TTL=64
Reply from 192.168.1.178: bytes=32 time=7ms TTL=64
Reply from 192.168.1.178: bytes=32 time=8ms TTL=64

Switch in Bldg B:
Pinging 192.168.1.68 with 32 bytes of data:
Reply from 192.168.1.68: bytes=32 time=6ms TTL=64
Reply from 192.168.1.68: bytes=32 time=1ms TTL=64
Reply from 192.168.1.68: bytes=32 time=1ms TTL=64
Reply from 192.168.1.68: bytes=32 time=1ms TTL=64

when I log into the switch and use ping utility:

[Joseph W. Doherty]

The test stream is generated on a PC using an application.

Monitoring the test stream would be done looking at switch port stats.

What specific model switch on each end and version of software it's using?