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MDS9124 Buffer credits

Some documentation (CLI configuration guide) says that MDS 9124 has a pool of 64 buffer credits for a 4 ports group and 16 Buffers Credits in F Mode or 61 Buffers Credits in E Mode.

But the same documention for NX-OS 5.x says MDS 9124 has 16 Buffers. it doesn't speak about 61 Buffers Credits.

Does it means that MDS 9124 has just 16 Buffers (even for ISL) with NX-OS 5.x, while it could have 61 Buffers in the previous version of SAN-OS (SAN-OS 3.x and NX-OS 4.1x)?

I check on MDS 9124 with NX-OS 5.x and I could not setup more than 16 Buffers Credits for the ISL (FC1/1).

I didn't try to put the 3 other ports in the same port-group in "out-of-service" mode, but no documentation speak about it.



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New Member

MDS9124 Buffer credits

The Question isn't clear about what are you trying to acheive.

B2B is internal mechanisim and depends upon what the Module / Port / HBA is capable of how many buffers.

If you are intrested on MDS side which Module, Port has how many default Buffers.

Here is the Link.

New Member

MDS9124 Buffer credits

I am talking about MDS9124 BB_Credit Buffers when setting up an ISL between 2 MDS9124.

The document you refer tells that MDS9124 can have up to 61 BB_Credits Buffers in E mode (ISL), but I could not configure 61 BB_Credits on ISL port. It only accepts 16 BB_Credits. And the doc in Table15-16 says it is possible to configure 61 BB_Credits for an ISL (default is 16).

And the same document for NX-OS 5.x only talk about 16 BB_Credit Buffer.

The reason is that I am using an ISL on a long distance and I need more than 16 BB_Credits between both MDS9124.



New Member

Re: MDS9124 Buffer credits

Regarding credit Buffers

Extended BB_credits : allow for a very long haul Fibre connections between two MDS switches.

MDS9134(config-if)# switchport fcrxbbcredit ?

  <1-750>              Enter receive BB_credit

  default              Default receive BB_credit

  performance-buffers  Configure performance buffers for receive BB_credit

MDS9134(config-if)# switchport fcrxbbcredit

For Extended BB_Credits  Doc states:

Note Extended BB_credits are not supported on the Cisco MDS 9124 Fabric Switch, Cisco MDS 9134 Fabric Switch,the Cisco Fabric Switch for HP c-Class BladeSystem, and the Cisco Fabric Switch for IBM BladeCenter.

To facilitate BB_credits for long haul links, the extended BB_credits feature allows the user to configure the receive buffers above the maximum value on all Generation 2 and Generation 3 switching modules (see the "Buffer Credit Allocation" section). When necessary, you can reduce the buffers on one port and assign them to another port, exceeding the default maximum. The minimum extended BB_credits per port is 256 and the maximum is 4095.

In general, the user can configure any port in a port group to dedicated rate mode. To do this, you must first release the buffers from the other ports before configuring larger extended BB_credits for a port.

Note The ENTERPRISE_PKG license is required to use extended BB_credits on Generation 2 and Generation 3 switching modules. Also, extended BB_credits are not supported by ports in shared rate mode.

All ports on the Generation 2 and Generation 3 switching modules support extended BB_credits. There are no limitations for how many extended BB_credits you can assign to a port (except for the maximum and minimum limits). If necessary, you can take interfaces out of service to make more extended BB_credits available to other ports.


There is another type of buffers called Performance Buffers. that can improve switchport performance. Each MDS switchport supports upto 145 performance buffers, If normal BB_credits are exhauseted.


MDS9134(config-if)# switchport fcrxbbcredit performance-buffers ?

  <1-145>  Enter performance buffers for receive BB_credit

  default  Default performance buffers for receive BB_credit

Both Extended BB and Performance BB are used for different purposes.

New Member

Re: MDS9124 Buffer credits

I've found the solution for my problem:

I have to disabled (out-of-service) the other 3 ports from the same port-group to be able to configure 61 BB_Credit for 1 port. This way I could get 61 BB_Credits from the 64 BB_Credit pool allocated to the port-group.

Extended and Performance buffer are not available for MDS9124.


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