You can encounter congestion at any point where you oversubscribe bandwidth.
Take an example where we have hosts connected to your switch at 1,000 Mps (gig), a WAN router connected to it at 100 Mbps (FastEthernet), and the WAN router having a 1 Mbps link. With this setup, even a single host can easily push data faster than either the WAN link can handle, or the WAN router's connection to the switch can handle.
What often happens, though, is most traffic runs over TCP. TCP starts by only sending a couple of packets and waits to see if the host received them. If so, the TCP sender will double the number of packets, and see if they are received. Without going into all the details of TCP flow control, TCP may keep doing this packet transmission doubling until it sees drops. When it sees drops, it decreases the number of packets it sends and starts to increase them again, although in the latter case, usually by a packet at a time.
What's happening, TCP will try to use all the minimal bandwidth along the path. What you might see is the 1 Mbps link hits 100%, but you don't see the 100 Mbps link hit more than an average of 1%. Your WAN link will become badly saturated, but not the Ethernet link to the router.
Saturation isn't always bad. TCP will deliver its data at the reduced bandwidth rate that's available to it.
However, to avoid any saturation on the WAN link, you would need to size it (and the router connection to the switch) to handle the concurrent number of gig flows you expect. If your 72 gig ports were only busy about 10%, your WAN path might avoid any congestion with 8 to 10 gig of bandwidth.
With TCP traffic, you can size bandwidth for what you want the effective bandwidth to be. Choice is yours.
With non-TCP traffic, things might change. Non-TCP might not have any flow control and would require the 8 to 10 gig of WAN bandwidth to work well or it might not work at all.
For example, you might run a 100 GB backup over a 5.6 Kbps dial-up modems (will take some time!!!) sucessfully, but might need a DS-3 to support Cisco TelePresense.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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