Gbs servers to WAN connected branches

kwftide1 · ‎06-19-2003

We are discussing the pro's and con's of connecting multiple servers to the network using Gig NICs. We have 300 branches with 256K links connecting into 2 routers with full 45mbs DS-3s. Most of the server utilization will come from those WAN connected branches.

I am wondering if the gbs server connections will create a major bandwidth oversubscription problem. Let's say we connect the 6513 switch (that contains multiple gbs servers) to the WAN router with a 1 gbs link. It can only send those packets out at 45mbs (to the WAN). Somewhere along the way - either at the WAN router's input / output queues or the 6513 switch's input / output queues - there should be many dropped packets.

Am I right in assuming this? If so, what are the rules of thumb? Should gbs server NICs only be used when serving a campus network with a superfast backbone?

dixho · ‎06-20-2003

I hate to give this answer. It depends!

What protocols (for example TCP/IP, SPX/IPX, SNA, Appletalk) are used on the gigabits server?

In general, each protocol has a built-in flow control mechanism. If there is a congestion in the network, certain action is performed. For example, TCP uses a dynamic window to limit the amount of traffic sent on a TCP connection. The typical value is 20K to 64K. If you use window scaling, the TCP window size is even larger. If there is a WAN congestion, IP packets are dropped on the network. This causes TCP to retransmit the dropped packets, which slow down the data input rate into the WAN.

One more comment about your network. Since you are using asymmetric WAN (i.e. the bandwidth on the head end is not equal to the that of the remote sites), the head end router has no idea the bandwidth of the remote sites. If the head end pumps traffic greater than the bandwidth of the remote sites, the packets are dropped within the telco network without letting the head end router knows. There are a couple of features in Cisco routers to tackle this problem. Traffic shaping and LLQ.

To summarize,I think that this is not a problem to use gigabit server serving remote sites. On top of the built-in flow control mechanism by each protocol, Cisco routers do offer a number of QoS features. The tricky part is to identify mission critical applications; so that you can prioritize mission critical applications (like voice and SNA) or limit the bandwidth used by non-mission critical applications. Cisco routers do provide a number of features, like traffic shaping, LLQ, RSVP, and so on to address the concerns.

There is really too much to list all QoS related features on Cisco routers. The links below are good starting points:

http://www.cisco.com/cgi-bin/Support/browse/index.pl?i=Technologies&f=1387

http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/qos_vcg.htm