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I have two geographically separated DC and DR sites with hosts on the same subnet. The hosts on respective networks are connected with each other through respective switches at each site. Both the switches are GigaBit switches. I have a point to point link between the locations which is 5Mbps.

When I connect the networks with the point to point link, I get a ping latency of about 18 ms. However as soon as clients start accessing the DC, through a WAN link on a router in front of the switch, I experience serious degradation of connectivity (ping latency = 400 ms and 50% drops).

Could someone please tell me, what do I need to do, to alleviate the problem?

Here are the details:

  • Switches are GigaBit layer2 Ethernet switches .
  • Wan link is wireless Point To Point 5 MBps Link.

Topology is as below.

(DC) Servers ------>Switch----------->Router----Clients on ILL WAN Link
                      :
                      :
                      :
         WAN Point to point link (5Mbps)
                      :
                      :
                      :

(DC) Servers ------>Switch----------->Router----currently no clients

All servers have Gigabit NICs. All ports on both the switches are on "Auto" mode. Users connect to servers on each side through separate ILL links which terminate into respective routers.

The P2P link connects the respective switches at the DC and DR. There is no traffic on the P2P WAN link currently. Just the ping traffic, that i use to test. The ping connectivity/latency is fine when no users are connected to DC, but as soon as users get connected, it starts facing problems. Subnets on DC and DR are same.

Please let me know if you need any more details.

  • We need a lot more details, like what kind of switches, what kind of wan link, etc. A diagram would help too. The more details you can provide, the more we can help you. I will say, I'm not surprised to hear of your symptoms. – Ron Trunk Aug 6 '14 at 14:53
  • Thanks Ron. Here are the details: Switches are GigaBit layer 2 Ethernet switches. Wan link is wireless Point To Point 5 MBps Links. Topology is as below. Servers ---> Switch ---->Router <------WAN Link-------> Router<-----Switch<-----Servers All servers have Gigabit NICs. All ports on both the switches are on "Auto" mode. Users connect to servers on each side through separate ILL links which terminate into respective routers. Please let me know if you need any more details. – user1539909 Aug 7 '14 at 8:47
  • MB=megabytes while Mb=megabits... We know you meant Mb. – generalnetworkerror Aug 7 '14 at 16:27
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 10 '17 at 5:06
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Taking into consideration that you haven't posted your switches makes and configs, your WAN link could be your bottleneck.

While saving on some costs it is usually a half-duplex connection.

Recommendation:- For the desired bandwidth between your sites, it makes more sense to rent fibre (or sdsl) from a Telco/ISP provider as a point-to-point between your two sites (or install your own fibre).

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Knowing the brand/model of switch would be helpful too, but based on your description, the most likely problem is that your WAN link simply can't keep up with the traffic. The link is congested, and therefore you are dropping packets. If your wireless link is 802.11, you're lucky if you can get 2-3 Mbps actual throughput. You don't say how many servers you have, but it wouldn't take very many to completely saturate that link. Simply put, you need (a lot) more bandwidth.

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  • Thanks Ron. The earlier diagram was probably misleading. Sincere apologies about that. I have changed the diagram so that it depicts the actual and correct scenario. The P2P link terminates into the respective switches at the DC and DR. There is no traffic on the P2P WAN link currently. Just the ping traffic, that i use to test. The ping latency is fine when no users are connected to DC, but as soon as users get connected, the traffic on the P2P link starts facing problems. – user1539909 Aug 8 '14 at 9:03
  • I think you'll find there is more traffic than you think. You have a layer 2 connection between the data centers, and it won't take much to saturate that link. What brand of switches are they? – Ron Trunk Aug 8 '14 at 17:42

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