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I have the following setup:

  • Two modems/router for two DSL lines (different ISPs)
  • Cisco 1941 router
  • HP 1920 managed switch
  • Server with dual ethernet ports
  • workstations

I would like to bond both lines for load balancing, while the 1941 router has only two ports. Is it possible to connect everything to switch? Or shall I get HWIC card with ethernet port? In any case, there will be two DHCP scopes, right? Shall the managed switch be responsible on internal DHCP?

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    You do not want to bond both circuits. You will get packets out of order and significantly reduced performance.
    – Ron Trunk
    Commented Oct 3, 2017 at 12:16

2 Answers 2

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With ISP load balancing, not only will packets be delivered out of order (out-of-order packet delivery will actually slow TCP, but it will completely kill real-time traffic like VoIP or video), as Ron Trunk mentions, but using NAT, packets sent will have two different source addresses, and that will not work. With different ISPs you really need to either set up an Active/Standby (you could configure some hosts to primarily use one ISP, and some to primarily use the other ISP), or you need provider-independent addressing and run BGP to exchange routes with your ISPs (unlikely, but not impossible on a 1941 router).

Also, to actually be able to bond two lines, you would need both lines to terminate in the same device on both ends, and you cannot do that with two different ISPs because you will have two different devices on the ISP side.

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  • Replace 1920 with a router with failover (LRT224 or TL-er5120) can avoid packets going out of order?
    – Kareem
    Commented Oct 3, 2017 at 14:15
  • Two different ISPs will have different paths and latency to a destination. You really can't even avoid out-of-order packet delivery with dual links on a single ISP. Only with something like Active/Standby where you only use the other ISP in a failover situation will you avoid that. You can set up so that some hosts on your network use one ISP as their primary, and the others use the other ISP as their primary. That will help to balance the traffic across the ISPs without trying to load balance the individual packets.
    – Ron Maupin ♦
    Commented Oct 3, 2017 at 14:23
  • I think I misuse the word bonding.
    – Kareem
    Commented Oct 3, 2017 at 14:47
  • Also, the HWIC slots are designed for use on your WAN circuits (the "W" stands for WAN). The router is really designed so that the WIC slots are to connect to your ISPs, and the built-in interfaces are for your LAN. You can use the built-in interfaces for the WAN, but you only have two, so the that would limit you to one WAN and one LAN connection. Ideally, you would use tow WIC slots for your two ISPs, and the built-in interfaces for your LAN.
    – Ron Maupin ♦
    Commented Oct 3, 2017 at 14:52
  • Yes, I consider that, thought it was not clear in my question. Thanks
    – Kareem
    Commented Oct 3, 2017 at 15:17
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You don't say what kind of service the server is giving. Although not very fine-grained, you can sometimes get good-enough balancing by configuring the server with two distinct IP addresses as if they are two different servers:

Idea 1. on different DNS names (eg mail.example.com and www.example.com); Idea 2. alternatively as two A records for one name and let DNS's round-robin behaviour randomly allocate to clients.

My starting point would be to partition the switch into a top-side and a bototm-side. Best if you have proper modems (not routers) and you configure two Dialer interfaces on your router (via PPPoA). Do all DHCP on the router not the switch. Configure the server as if it's two servers.

Get router to send traffic according to which server its for.

modem1    modem2
  |         |
==+=========+=====+===
                  |
               router
                  |
==+=========+=====+===
  |         |
server1  server2

Just to be clear: if it's resilience you're after, one goes about it a different way (two routers, two switches, two power distributions, only joining at unremovable single point of failure.)

Hope that helps.

Jonathan.

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