This document provides a sample configuration for the configuration of cross-stack EtherChannel on a Cisco Catalyst 3750 Switch that runs Cisco IOS® system software Release 12.2(25)SEC. EtherChannel can be called Fast EtherChannel or Gigabit EtherChannel. This depends on the speed of the interfaces or ports that are used to form the cross-stack EtherChannel.
The Cisco StackWise interconnect technology is designed with two counter-rotating paths of 16 Gb each. In order to efficiently load balance traffic, packets are allocated between these two logical counter-rotating paths, which creates the 32-Gb interconnection. There are dual paths from any port to any other port within the Catalyst 3750 stack. Therefore, maximum uptime is ensured because there is always an alternate path available if a failure occurs in either path. The Catalyst 3750 supports:
EtherChannels have automatic configuration with either Port Aggregation Protocol (PAgP) or Link Aggregation Control Protocol (LACP). PAgP is a Cisco-proprietary protocol that you can only run on Cisco switches and on those switches that licensed vendors license to support PAgP. IEEE 802.3ad defines LACP. LACP allows Cisco switches to manage Ethernet channels between switches that conform to the 802.3ad protocol.
PAgP cannot be enabled on cross-stack EtherChannels while LACP is supported on cross-stack EtherChannels from Cisco IOS Software Release 12.2(25)SEC and later. Switch interfaces exchange LACP packets only with partner interfaces with the active or passive mode configuration. You can configure up to 16 ports to form a channel. Eight of the ports are in active mode, and the other eight are in standby mode. When any one of the active ports fails, a standby port becomes active.
Interfaces with the on mode configuration do not exchange PAgP or LACP packets.
These EtherChannel modes are supported on cross-stack EtherChannel:
If a stack member that has ports participating in an EtherChannel fails or leaves the stack, the stack master removes the failed stack member switch ports from the EtherChannel. The remaining ports of the EtherChannel, if any, continue to provide connectivity.
When a switch is added to an existing stack, the new switch receives the running configuration from the stack master and updates itself with the EtherChannel-related stack configuration. The stack member also receives the operational information (the list of ports that are up and are members of a channel).
When two stacks merge that have EtherChannels configured between them, self-looped ports result. Spanning tree detects this condition and acts accordingly. Any PAgP or LACP configuration on a winning switch stack is not affected, but the PAgP or LACP configuration on the losing switch stack is lost after the stack reboots.
With PAgP, if the stack master fails or leaves the stack, a new stack master is elected. A spanning-tree reconvergence is not triggered unless there is a change in the EtherChannel bandwidth. The new stack master synchronizes the configuration of the stack members to that of the stack master. The PAgP configuration is not affected after a stack master change unless the EtherChannel has ports residing on the old stack master.
With LACP, the system-id uses the stack MAC address from the stack master, and if the stack master changes, the LACP system-id can change. If the LACP system-id changes, the entire EtherChannel will flap, and there will be an STP reconvergence. Use the stack-mac persistent timer command to control whether or not the stack MAC address changes during a master failover.
The network setup is used to configure the ports as trunk ports.
This document uses these configurations:
This configuration example provides the cross-stack EtherChannel configuration if you turn off PAgP or LACP:
This example shows the configuration of the EtherChannel when you enable LACP. The minimum version of IOS that supports LACP in cross-stack Etherchannel is Cisco IOS Software Release12.2(25)SEC. This example uses the active-active mode LACP configuration:
Background Theory
In this document, these interfaces are bundled for the cross-stack EtherChannel:- Two Gigabit Ethernet interfaces of one of the Catalyst 3750 Switches
- One Gigabit Ethernet interface of another Catalyst 3750 Switch of the same stack
- Three Gigabit Ethernet interfaces on a Catalyst 3750 Switch of a different stack
The Cisco StackWise interconnect technology is designed with two counter-rotating paths of 16 Gb each. In order to efficiently load balance traffic, packets are allocated between these two logical counter-rotating paths, which creates the 32-Gb interconnection. There are dual paths from any port to any other port within the Catalyst 3750 stack. Therefore, maximum uptime is ensured because there is always an alternate path available if a failure occurs in either path. The Catalyst 3750 supports:
- Cross-stack EtherChannel
- Cross-stack UplinkFast (with subsecond failover)
- Cross-stack equal cost routes across different switches in the stack
Link Aggregation Control Protocol (LACP) and Port Aggregation Protocol (PAgP)
EtherChannels have automatic configuration with either Port Aggregation Protocol (PAgP) or Link Aggregation Control Protocol (LACP). PAgP is a Cisco-proprietary protocol that you can only run on Cisco switches and on those switches that licensed vendors license to support PAgP. IEEE 802.3ad defines LACP. LACP allows Cisco switches to manage Ethernet channels between switches that conform to the 802.3ad protocol.
PAgP cannot be enabled on cross-stack EtherChannels while LACP is supported on cross-stack EtherChannels from Cisco IOS Software Release 12.2(25)SEC and later. Switch interfaces exchange LACP packets only with partner interfaces with the active or passive mode configuration. You can configure up to 16 ports to form a channel. Eight of the ports are in active mode, and the other eight are in standby mode. When any one of the active ports fails, a standby port becomes active.
Interfaces with the on mode configuration do not exchange PAgP or LACP packets.
These EtherChannel modes are supported on cross-stack EtherChannel:
- active—Places an interface into an active negotiation state, in which the interface starts negotiations with other interfaces by sending LACP packets.
- passive—Places an interface into a passive negotiation state, in which the interface responds to LACP packets that the interface receives, but does not start LACP packet negotiation. This setting minimizes the transmission of LACP packets.
- on—Forces the interface into an EtherChannel without PAgP or LACP. With the on mode, a usable EtherChannel exists only when an interface group in the on mode has a connection to another interface group in the on mode.
EtherChannel and Switch Stacks
If a stack member that has ports participating in an EtherChannel fails or leaves the stack, the stack master removes the failed stack member switch ports from the EtherChannel. The remaining ports of the EtherChannel, if any, continue to provide connectivity.
When a switch is added to an existing stack, the new switch receives the running configuration from the stack master and updates itself with the EtherChannel-related stack configuration. The stack member also receives the operational information (the list of ports that are up and are members of a channel).
When two stacks merge that have EtherChannels configured between them, self-looped ports result. Spanning tree detects this condition and acts accordingly. Any PAgP or LACP configuration on a winning switch stack is not affected, but the PAgP or LACP configuration on the losing switch stack is lost after the stack reboots.
With PAgP, if the stack master fails or leaves the stack, a new stack master is elected. A spanning-tree reconvergence is not triggered unless there is a change in the EtherChannel bandwidth. The new stack master synchronizes the configuration of the stack members to that of the stack master. The PAgP configuration is not affected after a stack master change unless the EtherChannel has ports residing on the old stack master.
With LACP, the system-id uses the stack MAC address from the stack master, and if the stack master changes, the LACP system-id can change. If the LACP system-id changes, the entire EtherChannel will flap, and there will be an STP reconvergence. Use the stack-mac persistent timer command to control whether or not the stack MAC address changes during a master failover.
Configure
In the attached network diagram, there are two Catalyst 3750 Switch stacks, Stack A and Stack B. Stack A has three switch members, and Stack B has only one switch member. The EtherChannel is formed with two ports on Switch 1 and one port on Switch 3 of Stack A. These ports connect to the three ports in Stack B.The network setup is used to configure the ports as trunk ports.
Configurations
This document uses these configurations:
- [#config1 Configure Cross-Stack EtherChannel Without PAgP or LACP]
- [#config2 Configure Cross-Stack EtherChannel with LACP]
Configure Cross-Stack EtherChannel Without PAgP or LACP
This configuration example provides the cross-stack EtherChannel configuration if you turn off PAgP or LACP:
| Catalyst 3750 Switch Stack A |
|---|
3750switchstackA(config)#interface range gigabitethernet 1/0/4 - 5 3750switchstackA(config-if-range)#channel-group 1 mode on <font>!--- This command creates the port channel 1 interface. Because the mode !--- is configured ON, both the PAgP and LACP are disabled on these ports. !--- Issue the channel-group command first, before you enter any other commands on these !--- interfaces. Any commands that you issue on these interfaces after you issue the !--- channel-group command are added to the port channel interface automatically. !--- If you configure the port with all the commands and you issue the channel-group !--- command last, the port channel interface is created but does not have any !--- configurations. You must then add the other commands to the port channel interface !--- manually.</font> 3750switchstackA(config-if-range)#switchport trunk encapsulation dot1q 3750switchstackA(config-if-range)#switchport mode trunk 3750switchstackA(config)#interface gigabitethernet 3/0/3 3750switchstackA(config-if)#channel-group 1 mode on 3750switchstackA(config-if)#switchport trunk encapsulation dot1q 3750switchstackA(config-if)#switchport mode trunk |
| Catalyst 3750 Switch Stack B |
|---|
3750switchstackB(config)#interface range gigabitethernet 1/0/2 - 4 3750switchst(config-if-range)#channel-group 1 mode on 3750switchst(config-if-range)#switchport 3750switchst(config-if-range)#switchport trunk encapsulation dot1q 3750switchst(config-if-range)#switchport mode trunk |
Configure Cross-Stack EtherChannel with LACP
This example shows the configuration of the EtherChannel when you enable LACP. The minimum version of IOS that supports LACP in cross-stack Etherchannel is Cisco IOS Software Release12.2(25)SEC. This example uses the active-active mode LACP configuration:
| Catalyst 3750 Switch Stack A |
|---|
3750switchstackA(config)#interface range gigabitethernet 1/0/4 - 5 3750switchstackA(config-if-range)#channel-group 1 mode active <font>!--- This creates port channel 1 and configures it with LACP.</font> 3750switchstackA(config-if-range)#switchport trunk encapsulation dot1q 3750switchstackA(config-if-range)#switchport mode trunk 3750switchstackA(config)#interface gigabitethernet 3/0/3 3750switchstackA(config-if)#channel-group 1 mode active 3750switchstackA(config-if)#switchport trunk encapsulation dot1q 3750switchstackA(config-if)#switchport mode trunk |
| Catalyst 3750 Switch Stack B |
|---|
3750switchstackB(config)#interface range gigabitethernet 1/0/2 - 4 3750switchst(config-if-range)#channel-group 1 mode active 3750switchst(config-if-range)#switchport trunk encapsulation dot1q 3750switchst(config-if-range)#switchport mode trunk |
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