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--- cisco:switch:recommended_levels_for_storm_control [2022/01/16 19:35] – aperez
+++ cisco:switch:recommended_levels_for_storm_control [2022/01/16 20:15] (current) – aperez
@@ Line 27: / Line 27: @@
 . Clear the counters
-  - switch#clear counters
+  switch#clear counters
 . Leave the switch working during 24 hours
-. Port by port (physical interfaces) check the amount of broadcast input
+. Port by port (physical interfaces) check the amount of broadcast input packets, multicast input packets and the total input packets.
-packets, multicast input packets and the total input packets.
+  Switch#Show interfaces // look for
+  packets input value (TPI)
-  * Switch#Show interfaces //look for
+  Received broadcasts value (BPI)
-  * packets input value (TPI)
+  (multicast) value (MPI)
-  * Received broadcasts value (BPI)
-  * (multicast) value (MPI)
 . Let's do some mathematics:
-  - To get unicast packets you do TPI - BPI, TPI - BPI = UPI
+  To get unicast packets you do TPI - BPI, TPI - BPI = UPI
-  - Normal percentage of broadcast = (BPI/TPI) * 100
+  Normal percentage of broadcast = (BPI/TPI) * 100
-  - Normal percentage of multicast = (MPI/TPI) * 100
+  Normal percentage of multicast = (MPI/TPI) * 100
-  - Normal percentage of unicast = (UPI/TPI) * 100
+  Normal percentage of unicast = (UPI/TPI) * 100
 That could give you an idea of the daily unicast, multicast and broadcast percentage on your network and could help you to set the proper threshold.
-Now the formulas that I am giving you will give a general idea, just to have a projection, nevertheless the error range is great.
+Now the formulas that I am giving you will give a general idea, just to have a projection, nevertheless, the error range is great.
 In order to know the real values, you will require to monitor the traffic during a month or 2 getting the same statistics and perform and statistical analyze based on average and variance to get a closer real-life value. Also probably your network experienced seasons that some times could be on a low
@@ Line 58: / Line 56: @@
 Please do not think the formula is the best way to determine the threshold, they are just to give a general idea but a deeper research should be done to determine that properly."
+----
+I have seen customer networks working fine with 1 % of broadcast storm control on access ports just to accomodate for ARP request traffic.
+The greater is the IP subnet the more broadcast traffic is needed but for /24 or more specific 1% of broadcast storm-control should work well on access ports.
+About multicast traffic the first question is your network is using multicast streams of any form ? if the answer is yes you need to accomodate space for this legitimate traffic . If you use multicast a reasonable threshold for multicast can be 10%.
+Unicast storm-control 60% or more
+We have to remember that:
+- the feature works on received traffic on the port not on outbound traffic on 1 second time intervals
+- the feature is not smart and it is not able to discriminate good traffic and bad traffic
+With low / aggressive thresholds storm control on access ports can make the difference between the capability to access remotely the distribution switches at the beginning of a bridging loop to shut down some links or the need to have someone on site to remove cables or even power off a distribution switch in an attempt to break the loop.
+This happened before introduction of VSS many years ago.
+On a L2 uplink trunk port of course thresholds cannot be so low as they carry traffic for multiple VLANs
+B 15%
+M 30%
+U  60%
+----