Switch Oversubscription Concept

[mmmartin823]

Hello all,

Can you help me understand the concept of Oversubscription for switches?  I heard this mentioned the other day and

[Reza Sharifi · Accepted solution]

Hi,

Have a look at this document;

Switch Oversubscription

Fabric switch oversubscription occurs when the overall switching bandwidth of the switch is less than the aggregate bandwidth of all ingress switch ports. This means that a subset of the total number of ports can run at full wire-rate simultaneously, but not all ports can. The important switch characteristic is to be able to balance the amount of bandwidth fairly among all the switch ports. Many Fibre Channel switches in the market today are oversubscribed at the switch level either intentionally or unintentionally, due to architectural inabilities. The Cisco MDS 9000 Series uses oversubscription on the DS-X9032 32-port switching module and on the Cisco MDS 9100 Series fabric switches to increase port density and reduce complexity and overall solution costs.

https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/storage-networking-solution/net_implementation_white_paper0900aecd800f592f.html

HTH

[Joseph W. Doherty · Accepted solution]

Reza describes oversubscription in relation to a switch fabric, but another common usage of switch oversubscription is when uplink port bandwidth is less than aggregation of edge port bandwidth. For example, a switch with 24 Gbps ports would slightly oversubscribe two 10 Gbps uplink ports (i.e. 24 to 20 Gb).

BTW, besides bandwidth oversubscription, sometimes switches don't have enough forwarding/switching capacity (in packet per second) either.