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XMODEM recovery speed

Configuring switches and routers is regular work for me. But if I would like to configure a switch or a router, I have to be able to boot the specific device…. Today I had to configure some new Cisco Catalyst 3650(E) en 3750 switches. In total I had 16 switches to configure, but three of them didn’t have any IOS image in flash and weren’t able to boot.

I have never seen this before. The switches aren’t refurbished, at least that is what the customer told me. At first I didn’t see any problem, because I wanted to upload an image from rommon through TFTP. After accessing rommon, I noticed that the Catalyst 3560 en 3750 don’t support TFTP upload in rommon. This leaves an XMODEM transfer as the only available option.

The image I wanted to upload was approximately 10 MB and upload with XMODEM at a baud rate of 9600 bps isn’t really fast. I had only one laptop to use, so it would take a whole day to upload the correct image into the three switches. Because I had only one COM port, I wasn’t able to configure anything.

I wanted to speed up my XMODEM transfer to buy some time and I found a way. At the switch prompt I set the baud rate to 115200:

switch: set BAUD 115200

Next I reconfigured my terminal (TeraTerm) to use the new baud rate of 115200. I started the XMODEM recovery procedure:

switch: copy xmodem: flash:c3560-ipbasek9-mz.122-50.SE.bin

I was satisfied while looking at the transfer rate. I had some time to invite myself to a cappuccino and  chat a little with the customer. The image was transferred in approximately 30 minutes. The last step in the recovery was setting back the baud rate to 9600, reconfigure my terminal and boot the image:

switch: set BAUD 9600

switch: boot flash:c3560-ipbasek9-mz.122-50.SE.bin

It only took two hours upload the correct IOS image to the three switches. Now I am set to start the configuration.

IEEE 802.3x FlowControl between Cat3750E and Cat2960

I have a network with two Catalyst 3750E switch stacks, which are connected with a 2 x 10Gbps Etherchannel. Every stack facilitates a ring topology of approximately 10 to 15 Catalyst 2960 switches. Two of the 2960 are connected with 1Gbps links to a switch stack to create the ring topology. So lets say that 7 24-ports Catalyst 2960 switch share a 1 Gbps link to the switch stack. With this customer, this won’t be any problem, because there are no heavy users and/or applications.

But let’s imagine that a link between a Catalyst 3750E and Catalyst 2960 switch or between two Catalyst 2960 switches is giving problems and the Catalyst 2960 cannot handle the receiving traffic. You need to find some way to slow done the traffic. I normally start thinking about the usage of IEEE 802.3x FlowControl.

Flow control enables connected Ethernet ports to control traffic rates during congestion by allowing congested nodes to pause link operation at the other end. If one port experiences congestion and cannot receive any more traffic, it notifies the other port by sending a pause frame to stop sending until the condition clears. Upon receipt of a pause frame, the sending device stops sending any data packets, which prevents any loss of data packets during the congestion period.

But after reading some documentation, FlowControl isn’t an option. When a link between both switches gets congested the Catalyst 2960 would have to send a pause frame to the Catalyst 3750E and that’s the problem.

Both, Catalyst 3750E and Catalyst 2960, can only receive, but not send, pause frames. So configuring FlowControl between Catalyst 3750E and Catalyst 2960 is useless, because no switch can inform the counterpart about the congested link.