Cisco IOS 17 - more switches as Stack - StackWise technology

What is a Stack

Cisco switches can be divided according to several parameters. I would now allow myself to divide them into three groups. Basic fixed switches (fixed), which operate at the 2nd layer according to the OSI model, represented by Catalyst 2960. More advanced fixed switches, which can also operate at higher layers, mainly at the 3rd layer and thus perform routing, represented by Catalyst 3750. And modular switches, which consist of a chassis into which various modules are inserted (the first must be a supervisor), represented by Catalyst 6509.

For switches of the middle category (C3750), Cisco has a technology called StackWise and StackWise Plus (in newer C3750E), which allows switches to be connected into a stack. The basic features (one could say advantages) of a stack are:

• the entire stack is configured (and actually behaves) as one switch with the sum of the ports
• the configuration is shared and stored by all members (each has a copy)
• it has one common IP address and management interface (telnet, web, ssh)
• members of the stack can be easily added and removed
• in case of damage to one member of the stack, it can be easily replaced with a new one (usually without the need for configuration)
• stacks thus serve to increase reliability

Creating a Stack

A stack is created by connecting switches with a special stacking cable (usually included with the switch). The connectors are located on the back of the switch and there are two because the stack is connected in a closed loop. This loop always passes through the circuits inside the switch, if a cable is missing, a loopback is applied at the unconnected port. This bus (interconnect) has a speed of 16 Gbps or 32 Gbps with StackWise Plus, but data travels bidirectionally with intelligent route calculation, so the speed is stated as double, i.e., 32 Gbps or 64 Gbps.

Master Switch

Individual switches are referred to as members of the stack. One switch is always the master, which manages the entire stack (we connect to it on the CLI, it manages the exchange of routing information, forwards forwarding tables to other members, etc.). The master switch is selected by a certain process and theoretically, any switch can become it. Up to 9 switches can be connected together and they are assigned sequential numbers - identifiers. From the factory, each switch is set to number 1 and after connecting to the stack, it is assigned the lowest available number.

Master Switch Selection

The process that selects the master switch goes through certain criteria, which are hierarchically listed here:

• user selection - we can configure who should be the master (using the highest priority)
• priority by HW and SW - the newest switch, the highest HW version with the highest SW version (features - feature set) has priority
• existing configuration - a switch with a configuration has priority over a new switch
• uptime - the longest-running switch
• MAC address - the lowest MAC address wins

Adding a New Member

After connecting a new member, the master should check its IOS and possibly upload the correct version. All members of the stack must have the same IOS version with the same features. The master then configures the switch, copies the configuration and other data to it. So if everything goes as it should, there should be no need for any intervention after connecting the connectors and starting the switch.

Connecting a new member can be done while running. We disconnect one place in the stack (so that it does not fall apart) and connect the new switch (this is important). Of course, the new connection must again be in a loop. But beware, in practice, it happened to me that after connecting a new member and upgrading its IOS, the stack software fell apart. Although it then automatically reconfigured, the individual switches restarted.

A few more information about IOS upgrade/downgrade performed by the master switch. After connecting a new member, information about the HW is obtained, and if it is the same as the master, its IOS is uploaded. The archive command is used for installations. If the HW version is different, the master looks for the necessary IOS in its flash memory or on a TFTP server (if set). If the correct version is not found, the connected switch is set to Version mismatch and regularly reboots (at startup it always looks for the correct version).

Removing a Stack Member

When removing a switch or damaging a cable, the stack finds out and routes communication outside the removed switch or damaged path. The port configuration, however, remains, so after connecting a new switch, it gets the original configuration (depends on the stack member number).

If the connection is interrupted in one place, the loop is interrupted, reducing the speed by half, but everything continues to work. After restoring the loop, functionality automatically returns.

StackWise and StackWise Plus

The StackWise technology has a number of features that serve for efficient and powerful operation. StackWise Plus adds a few more improvements, doubling the bus speed and supporting 10 Gigabit Ethernet ports.

Switches with StackWise and StackWise Plus technology can be combined together, but the global StackWise mode is negotiated. The main thing is that the interconnect speed is degraded to a theoretical 32 Gbps.

Combination of HW Versions of Switches

An interesting solution for practice can be the combination of switches by using one or two (depending on the number of links to the core) Catalyst 3750E, which have 10 Gbps uplink ports, and the rest Catalyst 3750G, which are cheaper. Although the interconnect speed is degraded by half, this may not be a limiting factor.

In such a solution, I encountered a problem where, after connecting a new member with a different IOS version, its upgrade started, but somehow the flash memory on the master switch got locked. Therefore, it is necessary to perform the upgrade manually and restart the master to release its memory. The simplest way is to upgrade the IOS before connecting the switch to the stack.

Mode Button - Stack

If we are physically at the switch and need to find out basic information about the stack, the number of members, and the current switch ID in the stack, we can use the MODE button. By repeatedly pressing it, we light up the LED next to the STACK label. At this moment, the LEDs above the first ports light up. The number of lit ports determines the number of stack members, and the blinking port is the current position.

Further Reading

• Cisco StackWise Technology White Paper
• 3750 - Managing Switch Stacks
• Catalyst 3750 Hardware Installation Guide

Configuration in CLI

Stack Commands

The basic operations we can perform with the stack using the CLI are finding information about members, setting the switch identification number, and setting the priority (essentially setting the master switch).

SWITCH#show switch                  // information about stack members (including id and priority)
SWITCH#show switch detail           // additional information about the stack connection

SWITCH(config)#switch 1 renumber 3  // changes the identification number from 1 to 3
SWITCH(config)#switch 3 priority 15 // sets the priority of switch 3 to 15 (options 1 to 15), the higher the value, the greater the chance of being selected as master

SWITCH(config)#no switch 3 provision // removes a stack member

Provisioned Switch

There is also the option to configure a switch before connecting it to the stack. We create a provisioned switch (backup), thus we can configure ports and other switch properties even if it is not physically connected (ports cannot be switched to operational state - no shutdown). After connecting the new switch, it gets this configuration.

Note: The global configuration is the same for all stack members, but we can configure individual ports.

SWITCH(config)#switch 4 provision ws-c3750e-48pd  // creates a switch with the given id and model

Port Addressing

Access to individual ports within the stack now uses the stack member number. This is entered as the first in the trio of numbers identifying the port, for example, Gi2/0/1 is the first port on the second switch in the stack.

Only One Stack Member

Switch numbers can also be used in several other places. In some informational messages, a specific switch is also displayed using the name-id, for example, SWITCH-2.

SWITCH#reload slot 2 // restart only the second switch
SWITCH#session 2     // switch directly to switch 2, but in practice only for show and debug (return using exit)

IOS Upgrade in the Stack

Briefly, I will mention the options we can use (not only) within the stack for manual IOS upgrade. The main command used is archive, which is described in more detail in Cisco IOS 15 - backups and restores of configuration and images.

Note: Upgrading the IOS on the stack upgrades all members, but be careful with different HW versions. In other words, when we upgrade the master switch, it upgrades the other members after the restart.

Within the stack, the archive command has the copy-sw parameter, which copies the IOS between stack members. The following example copies the IOS from switch 2 to 3 and overwrites (deletes) the original version. The /reload parameter could also be used, which ensures the automatic restart of the switch after the upgrade, but if we upgrade multiple switches, they need to be restarted after all have the same version.

SWITCH#archive copy-sw /overwrite /destination-system 3 2
SWITCH#reload slot 3   // restart only switch 3
SWITCH#reload in 10:30 // schedule the restart of the entire stack (not just the switch) in 10 hours and 30 minutes

Upgrading a Combined Stack

If we want to upgrade the stack to a new version and have members with different versions of StackWise (StackWise is marked as version 0 and StackWise Plus as 1), we can copy the tar for 3750G (of course the same version) into the directory with the bin for 3750E. Then we upgrade the 3750E (master) like this:

SWITCH#archive download-sw /no-version-check /destination-system 1 ftp://user:password@10.0.0.10/ c3750e-universalk9-tar.122-44.SE2.tar

Stack members should upgrade automatically from the stored image, but if there is a problem, we can do it manually. We then upgrade another stack member (3750G) from the master:

SWITCH#archive download-sw /no-version-check /destination-system 2 flash1:/c3750e-universalk9-mz.122-40.SE/c3750-ipbasek9-tar.122-44.SE2.tar

After we have the new IOS on all stack members, we can restart it (to load the new version) using reload.

Note: The /no-version-check parameter is sometimes necessary. For example, in some newer IOS versions, there is a newer version of the stacking protocol, and since the other switches are still using the older version, the upgrade can only be performed without checking. Subsequently, the other members must be upgraded.

Other Upgrade Commands

In case of any problems, the following commands may be useful:

SWITCH#copy flash2:/c3750-ipbasek9-mz.122-40.SE/c3750-ipbasek9-mz.122-40.SE.bin flash3: // manual image copy
SWITCH#show boot                // displays info on where switches boot from
SWITCH(config)#boot system switch 3 flash:/c3750-ipbasek9-mz.122-40.SE.bin // sets which image switch 3 should boot from

Note: Booting can also be set for all stack members at once. Using the boot command, we can also set the auto-upgrade properties of stack members.