Thursday, November 23, 2017

VSS Related Configuration of a Cisco Catalyst 6500 Series Switches

Here are the essential steps to configure Virtual Switching System (VSS) of a Cisco Catalyst 6509 switch. Here in my setup I have 2 6509 switches with 720 Supervisor Engines, one per each chassis. In this setup, only 1 Supervisor engine will be active and the other will stay standby.

STEP1: Assigning Virtual Switch Domain and Switch Numbers

Domain name must be same on both the switches.

ADM-6509-VSS(config)#switch virtual domain 100
ADM-6509-VSS(config-vs-domain)#switch 1
ADM-6509-VSS(config-vs-domain)#switch 1 priority 110
ADM-6509-VSS(config-vs-domain)#switch 2 priority 100

ADM-6509-VSS-2(config)#switch virtual domain 100
ADM-6509-VSS-2(config-vs-domain)#switch 2
ADM-6509-VSS-2(config-vs-domain)#switch 1 priority 110
ADM-6509-VSS-2(config-vs-domain)#switch 2 priority 100

If the priority is not configured, the lowest switch number will be the active switch..

STEP2: Configuring VSL Port Channel

Virtual Switch Link (VSL) is the port channel which the VSS parameters will be exchanged to sync data & management planes. They are configured as L3 port-channels.

ADM-6509-VSS(config)#int port-channel 11
ADM-6509-VSS(config-if)#no switchport
ADM-6509-VSS(config-if)#switch virtual link 1

ADM-6509-VSS-2(config)#int port-channel 12
ADM-6509-VSS-2(config)#no switchport
ADM-6509-VSS-2(config-if)#switch virtual link 2


STEP3: configure the VSL ports

ADM-6509-VSS(config)#int range te1/5/4-5
ADM-6509-VSS(config-if-range)#switchport mode trunk
ADM-6509-VSS(config-if-range)#channel-group 11 mode on

ADM-6509-VSS-2(config)#int range te2/5/4-5
ADM-6509-VSS-2(config-if-range)#switchport mode trunk
ADM-6509-VSS-2(config-if-range)#channel-group 12 mode on


STEP4: Converting the Switch to Virtual Switch Mode:

ADM-6509-VSS#switch convert mode virtual 

ADM-6509-VSS-2#switch convert mode virtual

If there are no mismatching parameters, switches will reboot after entering this command..

Verification Commands

Following outputs are from a currently working VSS..

show switch virtual







show switch virtual role








show switch virtual link







show switch virtual link port-channel

















show switch virtual redundancy


















show redundancy


















You can also find the mismatch parameters in VSS by issuing the following command
show switch virtual redundancy mismatch

Saturday, November 18, 2017

Stackwise Technology of Cisco IOS Switches

Stackwise technology is used to combine several physical switches into a one logical switch.
Layer 3 switches like Cisco 3750, 3750-X, 3850 and layer 2 switches like 2960-X support this technology. 3750 supports maximum of 9 physical switches per stack..

Special cable type called stack cables are used to create a daisy-chain (loop) between switches. They are connected to the stack ports at the back of the switch. Each supported switch has 2 stack ports.
Following picture shows how it is to be done when it has 3 or more switches.

Either way it works..

Anyhow what required is that each switch is connected to the one below it and the bottom switch should be connected to the one on top.






I am using 2 Cisco 3750 series switches to experience how it works.. IOS version must be equal.
Before stacking, lets see some the stack details of both switches.. I reset both the switches before starting this lab, but remember even resetting does not change the pre-configured stack priorities..
Default priority is 1 and the highest is 15. Switch with the highest priority becomes the master.

Issuing show switch command;









As you can see, both the switches think they are the Master. Let's stack them with a one cable and see what happens..

SW1 went to an auto reboot, and it became just a member. Now I'm consoling to the SW1 but I can see its name as SW2. This means all the configuration of SW1 is overwritten from the configuration of SW2. Issuing show switch command;


Let's see why it became a member.




Stack Master election process is like the following..

1. Configured priority
we can configure a priority to decide which switch becomes the master.

2. Hardware / software priority
The switch with the most extensive feature set has a higher priority than another switch (for example: IP Services vs IP base).

3. Default Configuration 
A switch that already has a configuration will take precedence over switches with no configuration.

4. Uptime
The switch with the longest uptime.

5. MAC address
The switch with the lowest MAC address.

It looks like the MAC address is the tie breaker here.

Anyhow only one stack cable was needed to bring the stack up for 2 switches..

But why we always see 2 stack cables are used to bring a 2 switch stack in production?

Let's issue another command; show switch stack-ring speed


Now let's connect another stack cable and see what happens..
You can see the ring speed has become 32G and the ring configuration is full. This is the reason we always prefer to add 2 cables even for 2 switches..



My real physical connection is like this..



















You can see which switch is connected to which switch by issuing show switch neighbors








Best practice is to change the priority of the switches to be master.  Let's say I want to make the switch 1 to be master,

SW2(config)#switch 1 priority 15


As you can see, it changed the priority, but it did not took over the master role. It will only be the master if the current master is down.




Things to keep in mind:-

1. IOS version must be equal to begin with.
2. Even resetting does not change stack priority values configured.
3. Default priority is 1 and highest is 15 and the highest will be the master.
4. There is no preemption here.
5. Master will overwrite member configuration by its running config.
6. Even though you issued a reload command by consoling to a switch in the stack, all the switches will reboot.

Friday, November 17, 2017

Correct Order of L3 Etherchannel Configuration

If you want to know how to configure L2 etherchannels, please visit here











1. Go to the physical member interfaces with blank configuration  & shut them down
SW1(config )#int range e0/0-1
SW1(config-if-range)#shutdown

2. Configure the physical member interfaces to be routed
SW1(config-if-range)#no switchport

3. Assign the channel group to the physical member interfaces
SW1(config-if-range)#channel-group 1 mode on

Configure SW2 in same manner (above 3 steps) and finally;

4. Go to the portchannel interface and flap it from both switches
SW1(config )#int po1
SW1(config-if)#shutdown
SW1(config-if)#no shutdown

After the portchannel is operational, you must do all other configuration (ip address assignment) only inside the portchannel interface.

If everything went ok, following outputs will be present.
















If it says (RU) it's an L2 etherchannel and it is working in Routed mode. (P) after the member interfaces indicate they are correctly bundled in. Protocol will be a blank because I used unconditionally "on" (neither LACP or PAGP).

Here are the 2 protocols to negotiate etherchannels..

1. LACP (active & passive)
2. PAGP (desirable & auto)

Active and desirable modes start negotiation while passive and auto modes listen only..
As an example, if I used LACP, one side must be considered as active while the other end must be configured as active or passive..









Additional load balancing can be done in global configuration mode.
SW1(config)#port-channel load-balance src-ip