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Catalyst 3750 Switch Stack

08/17/2012 By Andrew Roderos 2 Comments

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I like using Catalyst 3750 stack for user access switch. Don’t get me wrong, I’d rather use chassis based switches (Catalyst 4500 or 6500) but as we all know that money is usually a concern. 3750 stack is way less than a dual sup and fully populated 4500 or 6500 so if you’re in a budget then 3750 for user access is the way to go, in my opinion. Yes, you don’t have to get dual sup, but you don’t get the redundancy and it is still more money than a 3750 stack. With the 3750 stack, you get the redundancy since all other switches in the stack can take over as the master switch – equivalent to active switch (VSS) or active sup (dual sup chassis based switch).

3750’s StackWise/StackWise Plus/StackPower, 2960’s FlexStack, and VSS, which was mentioned here, are pretty much the same thing – device pooling/stackable switch technologies. With StackWise/StackWise Plus, you can stack up to nine switches. StackPower and FlexStack allows you to stack up to four switches. The last time I checked, the VSS (Virtual Switching System) allows you to stack only up two chassis based switches. If I remember correctly, VSS used to be a 6500 only technology but now the newer 4500s chassis can support VSS as long as it is equipped with the right supervisor – like Sup 7E.

Stacking Catalyst 3750 does not need any manual configuration. All you have to do is connect the StackWise cables (included when you buy it), boot it up and the stack will automatically form. When you rack and stack nine 3750s with 1U space in between, you’ll need one 3M (~9ft) StackWise cable. The one that comes with the switch is only 50CM (~1ft) which is enough to connect all switches except from the top to the bottom switch with 1U space between switches. If you don’t need the 3M then you can also buy the 1M (~3ft) which is good for 3 – 5 switch stack configuration with 1U space between switches, if I remember correctly. Make sure you plan accordingly when ordering your 3750 stack.

Once all the StackWise cables are connected and the switch powers up, it’ll go through its POST sequence, master election, and etc. What I am concern here is the master election. I like my switch stack to be predictable. As what Jeremy Cioara would say, auto = ought not to use it. Though in QoS video, he admitted liking the AutoQoS. Having said all that, I want to know exactly who should take over as a master when it boots up or when the master fails. As mentioned, the switch stack goes through a master election and is described here. In all the scenarios that I’ve experienced, there are only two things that I need to remember and they are the priority and MAC address.

MAC address is only important when you’re working on a new stack or a stack which has no config in it. The switch with the lowest MAC address wins the master election when the priority is set by default (value of 1). See the example below. The stack below is fresh out of the box.

Switch#show sw
Switch/Stack Mac Address : a493.4cd6.5b80
                                           H/W   Current
Switch#  Role   Mac Address     Priority Version  State
----------------------------------------------------------
 1       Member d867.d990.bb80     1      1       Ready
*2       Master a493.4cd6.5b80     1      1       Ready

As you can tell, I only have two switches in this stack. The switch 2 won the master election since the switch priority is a tie and has a lower MAC address than the switch 1. Switch priority is the first one to be checked when it boots up. That’s not what the Cisco’s documentation says, but again I only consider switch priority and MAC address in the election process. Higher switch priority wins the master election and when all the switches have the same priority then the one with the lowest MAC address wins, as already mentioned.

There maybe some network professionals out there that do not care about who the master is in the stack, but I am not one of them. The scenario in this post is physically laid out as switch 1 being the bottom switch and switch 2 as the top switch. I like my switch stack organized, so whenever I set one up I try to match the logical and physical view of the switch stack. In addition, I like my master to be on the top of the switch stack, physically and logically, so I have to alter my configuration to match the way I want it to look like. The configuration below shows you how to alter the switch stack to match the logical and physical view of the stack.

The first step I do is to configure one of the switches to have a higher priority so I know exactly who the master is on a switch stack. In this scenario, I picked switch 2 to have the highest priority. I don’t have to change the priority on this stack since I know this stack will never grow more than two. However, as best practice, I like to configure the priority value of the switches.

Switch#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)#switch 2 priority 15
Changing the Switch Priority of Switch Number 2 to 15
Do you want to continue?[confirm]
New Priority has been set successfully

If you recall, switch 2 is placed on the top of the stack (physically) and as mentioned earlier I want both physical and logical to match so I have to configure that as well to reflect it. Below is how to configure it to match physical and logical view of the stack.

Switch(config)#switch 2 renumber 1
WARNING: Changing the switch number may result in a configuration change for that switch.
The interface configuration associated with the old switch number will remain as a provisioned configuration.
Do you want to continue?[confirm]
Changing Switch Number 2 to Switch Number 1
New Switch Number will be effective after next reboot

As you may have guessed, you can’t have two switches with the same number so you have to change the remaining switches in the stack. If your stack contains nine switches, then knowing the switch number is important so you can renumber the right switch. To do this easily, you press the mode button of the switch until the stack LED lights up. You’ll now notice that the LED of the port numbers are blinking as well. Take note of those numbers so you can rename the proper switch with the right number. I normally use the top to bottom approach when I number my stack. In this scenario, I only have two switches so it’s easy to do but if you have you nine switches it’ll most likely be jumbled up. Your switch 3 (physically) may be numbered as 9, so make sure to write it down. Anyway, below is to finish what I’ve started.

Switch(config)#switch 1 renumber 2
WARNING: Changing the switch number may result in a configuration change for that switch.
The interface configuration associated with the old switch number will remain as a provisioned configuration.
Do you want to continue?[confirm]
Changing Switch Number 1 to Switch Number 2
New Switch Number will be effective after next reboot

Since I only have two switches and I know that this stack is not going to get another switch, I didn’t change the switch priority on the other one. I normally change the priorities of all the switches in the stack whenever I configure a stack from scratch except for the bottom switch. This way, I know which switch will be the next master switch when the original master fails. Obviously, this is a matter of preference so you don’t have to follow my method.

Once everything has been renumbered, it’s time to reboot the stack. When the stack is fully booted, verify the switch stack to make sure that everything is what you desire it to be.

Switch>sho switch
Switch/Stack Mac Address : a493.4cd6.5b80
                                           H/W   Current
Switch#  Role   Mac Address     Priority Version  State
----------------------------------------------------------
*1       Master a493.4cd6.5b80     15     1       Ready
 2       Member d867.d990.bb80     1      1       Ready

If you compare the previous show switch output earlier, the switch with a493.4cd6.5b80 is now showing up as switch 1 and has a priority of 15. With the priority set to 15, this helps determine who will be the master of the switch during election. Again, this does not guarantee that it’ll always be the master, however, since when there’s a tie in priority the next one to check is the MAC address.

Hope this has been helpful and thank you for reading!

Disclosure

NetworkJutsu.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.

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Switch port density

12/22/2011 By Andrew Roderos Leave a Comment

A Network Engineer was tasked to upgrade the existing switch infrastructure for a site since the current infrastructure does not support PoE for the new project – VoIP and IP enabled CCTV cameras. Upon investigating, he saw that the site has three switches and some ports were not lit up and now needs to know if those ports were lit up last week to help him decide how many switches he really needs to order. Ordering the same amount of switches will drive up the cost, which he is not willing to do since the company is tight with money. Now, the question is, how can he tell that the port is really unused or the people are just on vacation and their PCs are turned off?

There may be tools out there that I am unaware of, but Cisco IOS has a built in show command that you are already familiar with. This is the show interface command. Please look below for the example.

The show command output below was issued on a switch with an uptime of 6 months.

Switch#sh int g0/32
GigabitEthernet0/32 is down, line protocol is down (notconnect)
  Hardware is Gigabit Ethernet, address is f866.f2fd.c020 (bia f866.f2fd.c020)
  MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Auto-duplex, Auto-speed, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output never, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
! Remaining output were omitted for brevity

The output below is a port with activity.

Switch#sh int g0/24
GigabitEthernet0/24 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is f866.f2fd.c041 (bia f866.f2fd.c041)
  Internet address is 172.30.99.37/30
  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
! Remaining output were omitted for brevity

If you compare the line where it says “Last input”, you’ll see that the first show interface command output says “never” while the other one is saying 00:00:00. Since this switch has been up for six months, it is safe to assume that this port hasn’t been used for six months so time to move on to another port and collect the total port count.

Fortunately, the Network Engineer took his time to collect the information and only ordered two switches which saved the company over $6,000! IT, in most companies, do not generate revenue but can definitely help with the company’s bottom line by spending money efficiently and effectively.

I hope this has been helpful and I thank you for reading!

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Switch Port Template

12/13/2011 By Andrew Roderos Leave a Comment

This post will cover a switch port template that I consistently use for user switch ports. These commands also help in speeding up switch port initialization. Anything that speeds up the process is normally a good thing.

Without further ado, here are the commands that I use for user access switch ports, excluding QoS settings which will be covered in the future article:

Switch (config)# int range g0/1 - 48
 switchport host
 spanning-tree bpduguard enable
 switchport access vlan 10
 switchport voice vlan 20

The switchport host macro command was designed to facilitate the configuration of switch ports that connect to end stations. Entering this command sets the switch port mode to access, enables spanning tree PortFast, and disables channel grouping, all at the same time. You will also see a notification of what it did once the command took effect, as shown below. Additionally, you can also hard code the speed and duplex to prevent auto negotiation process to initiate. Other Network Engineers will tell you to disable anything auto, but I personally leave the auto negotiation for speed and duplex alone. I only mess with speed and duplex if the node does not negotiate properly and there’s no way to change the settings to auto.

Switch (config-if)#sw host
switchport mode will be set to access
spanning-tree portfast will be enabled
channel group will be disabled

For more information about the switch port initialization, please read this post.

I hope this has been helpful and I thank you for reading!

Reference

Switching Infrastructure

Disclosure

NetworkJutsu.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.

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