LAN performance slow when WAN utilization high

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Nov 12th, 2008
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Throwing this issue out there, maybe someone has experienced the same problems. Here's the basic info, obviously there are a lot of factors, but any help is appreciated.


We have a fully meshed 11 site MPLS network through our carrier, HQ has 9mb and each office has a T-1. Cisco equipment throughout, except Riverbed boxes in each office. Microsoft for servers and clients. Some services (DHCP, some Exchange for example) are centralized and users do map drives and access files back to HQ. No WINS, DNS in each site.

Problem: When an offices WAN link gets high utilization, all users in that office pretty much lockup, even if they are accessing resources that are local. I have done packet tracing as well as Netflow locally and haven't see anything that stands out. The server team tells me that all sites have DC's and are global catalog servers.

Again, advise would be greatly appreciated.


Thanks


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Joseph W. Doherty Wed, 11/12/2008 - 12:55
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Since you only see the LAN access problem when the WAN is congested, either there's still something that needs to transit the WAN for LAN access you're unaware of, and/or depending how the Riverbed devices are set up within your infrastructure, then somehow they become a bottleneck for local LAN access when the WAN links are busy (such as checking local cache against remote cache).


If Riverbed traffic is mixed with non-Riverbed traffic, you might want to insure the two classes of traffic don't share the same queue, either MPLS ingress or egress, such that non-Riverbed traffic is impacted by high bandwidth Riverbed flows.

gizbri Thu, 11/13/2008 - 10:08
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Thanks for the response. All the traffic from the branches flows through the Riverbed. I am pretty sure it is due to the Windows client/server communications but that team isn't coming up with anything. I hate to throw out a generic "the lan is slow" question...

Joseph W. Doherty Thu, 11/13/2008 - 12:42
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So the Riverbed is only "in-line" with your WAN traffic, i.e. internal LAN traffic wouldn't transit it? If so, then it would seem some other across-the-WAN traffic was required, although you did note there's both local DNS and Windows DC, so it's not obvious what the other traffic might be.


Riverbeds can easily load your WAN links, and from you brief description, there could either be inbound and/or outbound congestion. Different QoS traffic treatment, both Riverbed and non-Riverbed, might help.

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