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June 24th, 2008 04:00
How to change buffer to buffer credits (B2B)
Dear all,
I just connecteed an MDS9124 over 63km to another 9124. The link is up on 2Gb (we choose a 2Gb SFP, since the B2B credits aren't enough to be able to run the ISL on 4Gb).
If possible I'd like to run this link on 2Gb, but if it's not possible 1Gb is ok as well.
But how do I change the b2b credits on this port fc1/13 ? I tried the command:
- show interface bbcredit
This one shows
When I run "switchport fcrxbbcredit 63 mode E" I get an error "fc1/13: (error) Invalid rxb2b credit for desired port-mode and rate-mode". I've tried 30 instead of 63, but I'm getting the same error.
How do I change the b2b credits for this E port to 63 ?
How do I config the port to run at 2Gb (if possible) ?
I just connecteed an MDS9124 over 63km to another 9124. The link is up on 2Gb (we choose a 2Gb SFP, since the B2B credits aren't enough to be able to run the ISL on 4Gb).
If possible I'd like to run this link on 2Gb, but if it's not possible 1Gb is ok as well.
But how do I change the b2b credits on this port fc1/13 ? I tried the command:
- show interface bbcredit
This one shows
fc1/13 is trunking
Transmit B2B Credit is 16
Receive B2B Credit is 16
16 receive B2B credit remaining
16 transmit B2B credit remaining
When I run "switchport fcrxbbcredit 63 mode E" I get an error "fc1/13: (error) Invalid rxb2b credit for desired port-mode and rate-mode". I've tried 30 instead of 63, but I'm getting the same error.
How do I change the b2b credits for this E port to 63 ?
How do I config the port to run at 2Gb (if possible) ?
ConnectrixHelpe
259 Posts
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June 24th, 2008 08:00
I strongly recommend against changing buffer to buffer credits unless directed to do so by EMC or Cisco support. If there are connection issues, a case should be opened with EMC support so that the issue can be looked into and all other work arounds tried before changing buffer to buffer credits. I make this recommendation after Cisco Engineering STRONGLY recommended against any buffer to buffer changes. The switches intelligently deal with the buffers and should only be adjusted if directed by EMC or Cisco.
Thank you.
RRR
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June 25th, 2008 04:00
The outcome was that the B2B credits need to be changed whenever the default 16 aren't enough. We calculated the distance / speed and in out case we needed 22 B2B credits on a 22km 4Gb link and 61 B2B credits on a 61km 2Gb link.
ConnectrixHelpe
259 Posts
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June 26th, 2008 09:00
Buffer to buffer credits and how to change them are covered in the current Cisco CLI Configuration guide under pages 13-32 through 13-37. I have to repeat again that I don't recommend they are changed unless directed by EMC or Cisco.
Thank you.
Siaotong
19 Posts
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April 22nd, 2010 00:00
On a 9124, you have a maximum of 64 buffer credits on every 4-port (1-4, 5-8, 9-12,13-16, etc.). Thus, in order for you to set 63 b2b credits on 1 port you need to set the b2b credits to 1 on the other 3 ports.
You need to set the following on ports 1/14-16 (this is assuming that you don't and WILL NOT use them):
mode FX
speed 40000
rcvb2bcredit 1
config t
interface fc 1/14-16
switchport mode FX
switchport speed 4000
switchport fcrxbbcredit 1
interface fc 1/13
switchport fcrxbbcredit 63 mode E
But consider consulting EMC/Cisco if you really need to change your b2b credits.
RRR
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April 26th, 2010 07:00
If a port group has 64 buffers and you limit 3 ports to 1 buffer, you are left with 61 for 1 port, not 63.
And I think the 9124 can go to 61 max on 1 port (and leave 3 ports with 1 buffer, no matter if you diable these 3 ports or not) and the 9134 all the way to 64, but for that you really need to disable 3 ports.
RRR
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April 26th, 2010 08:00
For a 9222i I completely agree with you, but on a 91xx you need to change the b2b credits almost always in order to get reasonable performance.
The 92xx has a default of 250 buffers or something, so that's enough for most cases.
Siaotong
19 Posts
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May 4th, 2010 01:00
I stand corrected.... it should 61 and not 63
By the way, do you have any idea on how to increase the rcvb2bcredit for 9222i from 250 to say 350?
RRR
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May 4th, 2010 07:00
I haven't done this yet, but I think you first need to "steal" some B2B credits from other ports so you have 100 credits available and then you could set a new value on this 1 port.
Stealing buffers from other ports mean you have to disable these ports (or 1 port) en lower the buffer credits there and enable the port again.
The whole switch has 4000 buffers or something and AFAIK all of them have already been assigned evenly to all ports, so 16 x 250 = 4000. I think ports 17 and 18 cannot be used to long range ISL's.
ARSNMJ
33 Posts
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July 16th, 2012 17:00
Hi,
i read from the blog that you are saying this
The outcome was that the B2B credits need to be changed whenever the default 16 aren't enough. We calculated the distance / speed and in out case we needed 22 B2B credits on a 22km 4Gb link and 61 B2B credits on a 61km 2Gb link.
how did you calculated it ?
i have to do the same exercise to get the best result ..
i have 300km between sites and using DEDM dark fiber between 9509 switches, link speed is 1g.. atpresent i am using befault b2b that is 250.. is this enough ?
how do i knw that if i running out of brb credit ?
RRR
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July 17th, 2012 01:00
I see I made a miscalculation back then. For the 22km link we needed 44 + 10% B2B credits.
And on the 63km link we could only use 61 B2B credits, since the port group has 64 B2B credits and on that switch each port needs at least 1 credit, so that leaves 61 for 1 port if the other 3 are brought back to 1. Newer 9134 and 9148 switches can assign the maximum amount for the port group to a single port.
And on the 92xx and 95xx you can assign even more (up to 4096 I think it was).
@2Gbps on a 2km link you'll need 2 B2B credits + 10%, so on 1Gbps you'll need half of that.
If your link speed is 1Gb and the distance is 300km, you'll need 150 + 10% = 165 B2B credits, so your 250 should be ok. But please consider the dampening factor: will the light reach the other end at all? 300km is long way!