HSRP and preempt command

Question: What does "preempt" command do in HSRP configuration. I thought also without this command the router with highest priority will become active.

Answer: To get the role of this command suppose that the active HSRP router fails, in this case the standby router will become active. If no preempt command is used in HSRP setting after the previously active router, the one with the highest priority, comes up again the status of HSRP will not change. This means that the previous standby router, current active one, will stay active even though it's priority is lower.
When preempt is used, whenever a router with higher priority comes up it will become active.

Checking the integrity of a copied file.

Question: I have to upload IOS images to tens of Cisco routers and switches for my company these days; sometimes after I upgrade an IOS and reboot the router (switch) the router (switch) boots in ROMMON. I think images are not copied to flash properly in these cases. Is there a way to see if the image is copied correctly to the flash?

Answer: To check the integrity of the file you copied use the MD5 signature.
Issue the following command to get the MD5 key of the copied file and compare it with the MD5 key provided by Cisco.

verify /md5 flash:filename

After you get the MD5 key go to Cisco.com and try to download the same image in Software Downloads part, just before you download the software you will be provided with a table containing some details about the image and in that table you will find the MD5 key of the file.
Compare it with the one you get as a result of verify command, if they are the same your file is copied properly.

tftp file size limit

Question: I tried to upgrade the IOS on a Catalyst 6509-Sup720 with tftp but I get this error in my tftp server log: "File is too large for tftp protocol". Is this a bug in my tftp server software? If not, how can I upgrade the IOS? Oh I forgot to say that the file size is almost 80MB.

Answer:
Originally tftp protocol supports maximum file size of 32MB, however some changes have been made so tftp can support up to 4GB. But to be practical you need to upgrade your IOS using FTP or RCopy.
To do this with FTP all you need is replacing "tftp" with "ftp" in the commands you used before:

enable
copy ftp: sup-bootdisk:

Of course you need an FTP server instead of a tftp server as well.

If your ftp server requires username and password you have too define them in your switch:

conf t
ip ftp username xxxx
ip ftp password yyyy

Ping all subnets

Question: When I telnet to a switch I can only ping within the subnet of the management VLAN of my switch, how can I ping the other subnets IP addresses?

Answer:

There is an easy way, connect to the switch via it’s console port (con 0) then you can ping every subnet!

SFP Negotiation

Question: I have an SFP gbic port in a Cisco 6509 switch with supervisor 720 which does not come up. i checked the cable (fiber) and changed the gbic but it did not help. What should I do next?

Answer:

When a port with a SFP module does not come up and you are sure about the cable and connections always check the speed negotiation and even disable it by:

speed nonegotiate

interface command.

Inactive status in CatOS switches

Question: I connect a server to my old Cisco 5000 (CatOS) switch, when I enable the port it goes to "inactive" mode. I checked the cable and changed the port but it did not help. Is there something wrong with the server NIC?

Answer:

When you see a port in ‘Inactive’ status in a switch running CatOS it is very likely that you have VLAN problems. For instance, if you put a port in VLAN X and that VLAN X is not defined in the switch then the port will go to inactive status.

In order to resolve the issue, first make sure that the VLAN to which the inactive port is assigned to exists or not (mostly not); do this by issuing the following command:

sh vlan

You probably do not see the VLAN in the list, if you want to create the VLAN:

First make sure that the switch is in one of the VTP transparent or server modes,

show vtp domain

Look at the phrase under Local Mode to see in which mode the switch operates.

(For IOS:

show vtp status

Look at the VTP Operating Mode in the out put of the above command to see the VTP mode.)

If your switch is in VTP client mode you should create the VLAN in the VTP domain server switch.

To create the vlan use the following command:

set vlan vlan-number name vlan-name

Monitoring and troubleshooting an interface in CATOS

For CatOS, the show mac command displays the MAC counters for traffic passing through each port such as, received frames, transmit frames, out-lost, in-lost, etc. (This command does not list the MAC addresses learned on a port by the bridging software. Use the command show cam dynamic for that information.)

For Cisco IOS, the show interfaces card-type x/y counters command is similar to show mac for CatOS platforms. show counters mod/port is also useful.

Power Supply Monitoring

The most useful command to see if power supply(s) in a Cisco switch is/are working properly is:

show environment power

However this command will fail in some CATOS versions, in this case you can use:
show systemto see if PSU(s) is/are in a proper working condition

Introduction

I am Babak Dorrani, I am a Cisco network engineer working in an International company in the Netherlands.
While I was living in my home country, Iran, I was also involved in Cisco networking and mostly in teaching some Cisco courses. Fortunately my old students have not forgotten me yet and they still send me their problems and questions and I am more than happy to help them out (if I can!).

After about two years being away, I decided to put these questions and answers, which are now quite a lot, here in this blog mainly to share them with others and discuss about them.

In most of the cases the questions are in Farsi language so I do not mention the exact question here to make it a bit more useful for my international friends. From now on I will try to translate the questions but for the previous ones No Way!!

And one last thing, please correct me and let me know in case you find any mistakes in my writings.
Good Luck!
Babak