- Preface
- Product Overview
- Command-Line Interfaces
- Configuring the Switch for the First Time
- Administering the Switch
- Configuring Virtual Switching Systems
- Programmability
- Configuring the Cisco IOS In-Service Software Upgrade Process
- Configuring the Cisco IOS XE In Service Software Upgrade Process
- Configuring Interfaces
- Checking Port Status and Connectivity
- Configuring Supervisor Engine Redundancy Using RPR and SSO on Supervisor Engine 6-E and Supervisor Engine 6L-E
- Configuring Supervisor Engine Redundancy Using RPR and SSO on Supervisor Engine 7-E, Supervisor Engine 7L-E, and Supervisor Engine 8-E
- Configuring Cisco NSF with SSO Supervisor Engine Redundancy
- Environmental Monitoring and Power Management
- Configuring Power over Ethernet
- Configuring Cisco Network Assistant
- Configuring VLANs, VTP, and VMPS
- Configuring IP Unnumbered Interface
- Configuring Layer 2 Ethernet Interfaces
- Configuring EVC-Lite
- Configuring SmartPort Macros
- Configuring Cisco IOS Auto Smartport Macros
- Configuring STP and MST
- Configuring Flex Links and MAC Address-Table Move Update
- Configuring Resilient Ethernet Protocol
- Configuring Optional STP Features
- Configuring EtherChannel and Link State Tracking
- Configuring IGMP Snooping and Filtering, and MVR
- Configuring IPv6 Multicast Listener Discovery Snooping
- Configuring 802.1Q Tunneling, VLAN Mapping, and Layer 2 Protocol Tunneling
- Configuring Cisco Discovery Protocol
- Configuring LLDP, LLDP-MED, and Location Service
- Configuring UDLD
- Configuring Unidirectional Ethernet
- Configuring Layer 3 Interfaces
- Configuring Cisco Express Forwarding
- Configuring Unicast Reverse Path Forwarding
- Configuring IP Multicast
- Configuring ANCP Client
- Configuring Bidirectional Forwarding Detection
- Configuring Campus Fabric
- Configuring Policy-Based Routing
- Configuring VRF-lite
- Configuring Quality of Service
- Configuring AVC with DNS-AS
- Configuring Voice Interfaces
- Configuring Private VLANs
- Configuring MACsec Encryption
- Configuring 802.1X Port-Based Authentication
- X.509v3 Certificates for SSH Authentication
- Configuring the PPPoE Intermediate Agent
- Configuring Web-Based Authentication
- Configuring Wired Guest Access
- Configuring Auto Identity
- Configuring Port Security
- Configuring Auto Security
- Configuring Control Plane Policing and Layer 2 Control Packet QoS
- Configuring Dynamic ARP Inspection
- Configuring the Cisco IOS DHCP Server
- Configuring DHCP Snooping, IP Source Guard, and IPSG for Static Hosts
- DHCPv6 Options Support
- Configuring Network Security with ACLs
- Support for IPv6
- Port Unicast and Multicast Flood Blocking
- Configuring Storm Control
- Configuring SPAN and RSPAN
- Configuring ERSPAN
- Configuring Wireshark
- Configuring Enhanced Object Tracking
- Configuring System Message Logging
- Onboard Failure Logging (OBFL)
- Configuring SNMP
- Configuring NetFlow-lite
- Configuring Flexible NetFlow
- Configuring Ethernet OAM and CFM
- Configuring Y.1731 (AIS and RDI)
- Configuring Call Home
- Configuring Cisco IOS IP SLA Operations
- Configuring RMON
- Performing Diagnostics
- Configuring WCCP Version 2 Services
- Configuring MIB Support
- Configuring Easy Virtual Networks
- ROM Monitor
- Acronyms and Abbreviations
- Index
Configuring Flex Links and MAC Address-Table Move Update
Flex Links provide a fast and simplified Layer 2 Link redundancy mechanism. This chapter describes how to configure Flex Links on the Catalyst 4500 series switch. It also describes how to configure the MAC address-table move update (MMU) feature, also referred to as the Flex Links bidirectional fast convergence feature.
The chapter consists of these sections:
Note
For complete syntax and usage information for the switch commands used in this chapter, see the
Cisco IOS Command Reference Guides for the Catalyst 4500 Series Switch.
If a command is not in the Cisco Catalyst 4500 Series Switch Command Reference , you can locate it in the Cisco IOS Master Command List, All Releases.
About Flex Links
This section describes this information:
Flex Links
Flex Links are a pair of Layer 2 interfaces (switch ports or port channels) where one interface is configured to act as a backup to the other. Users can disable STP and still retain basic link redundancy. Flex Links are typically configured in service provider or enterprise networks where customers do not want to run STP on some interfaces.
Note The Catalyst 4500 series switch supports a maximum of 16 Flex Links.
You configure Flex Links on one Layer 2 interface (the active link) by assigning another Layer 2 interface as the Flex Link or backup link. When one of the links is up and forwarding traffic, the other link is in standby mode, ready to begin forwarding traffic if the other link fails. At any given time, only one of the interfaces is in the forwarding state and forwarding traffic. If the primary link fails, the standby link starts forwarding traffic. When the active link reactivates, it enters standby mode and does not forward traffic. STP is disabled on Flex Links interfaces.
In Figure 24-1, ports 1 and 2 on switch A are connected to uplink switches B and C. Because they are configured as Flex Links, only one of the interfaces is forwarding traffic; the other is in standby mode. If port 1 is the active link, it begins forwarding traffic between port 1 and switch B; the link between port 2 (the backup link) and switch C is not forwarding traffic. If port 1 shuts down, port 2 activates and starts forwarding traffic to switch C. When port 1 reactivates, it enters standby mode and does not forward traffic; port 2 continues forwarding traffic.
You can also choose to configure a preemption mechanism, specifying the preferred port for forwarding traffic. In Figure 24-1, for example, you can configure the Flex Links pair with preemption mode so that after port 1 reactivates in the scenario, and it has greater bandwidth than port 2, port 1 begins forwarding after a duration equal to the preemption delay; and port 2 becomes the standby. You do this by entering the interface configuration switchport backup interface preemption mode bandwidth and switchport backup interface preemption delay commands.
Figure 24-1 Flex Links Configuration Example
If a primary (forwarding) link shuts down, a trap notifies the network management stations. If the standby link shuts down, a trap notifies the users.
Flex Links are supported only on Layer 2 ports and port channels. Flex Links are compatible with promiscuous trunks. It is not supported on isolated PVLAN trunks.
VLAN Flex Links Load Balancing and Support
VLAN Flex Links load balancing allows you to configure a Flex Links pair so that both ports simultaneously forward the traffic for mutually exclusive VLANs. For example, if Flex Links ports are configured for 1 to100 VLANs, the traffic of the first 50 VLANs can be forwarded on one port and the rest on the other port. If one of the ports fail, the other active port forwards all the traffic. When the failed port reactivates, it resumes forwarding traffic in the preferred VLANs. In addition to providing the redundancy, this Flex Links pair can be used for load balancing. Also, Flex Links VLAN load balancing also does not impose any restrictions on uplink switches (Figure 24-2).
Figure 24-2 VLAN Flex Links Load Balancing Configuration Example
Note A static MAC address must point to a Flex Links interface that is forwarding for given VLAN. For example, if a backup interface is forwarding VLAN X, then a static MAC address in VLAN X must point to the backup interface. Misconfiguration might cause unexpected results.
Flex Links Failover Actions
When a Flex Links primary fails, the following important actions are taken:
- Detects link failure.
- Moves static unicast MAC addresses that are configured on the primary link to the standby link.
- Moves dynamic unicast MAC addresses that are learned on the primary link to the standby link.
- Moves the standby link to a forwarding state.
- Transmits MAC address-table move updates over a new active link, if you enter the
mac move update transmit command. - Transmits dummy multicast packets over a new active interface.
Note Local administrative shut down or a link that starts forwarding again due to preemption is not considered a link failure. In those cases, flush the dynamic hosts and not move them.
Static MAC addresses configured on a Flex Links member interface are moved over to the backup, if it fails. Static MAC addresses configured on a Flex Links member interface are restored when it starts forwarding again.
Note The show mac address-table command always shows static MAC addresses as associated with the interface on which it was configured even if it may have been moved to the standby link because of a Flex Links failover.
MAC Address-Table Move Update
In Figure 24-3, ports 1 and 2 on switch A are connected to uplink switches B and D through a Flex Links pair. Port 1 is forwarding traffic, and port 2 is in the blocking state. Traffic from the PC to the server is forwarded from port 1 to port 3. The MAC address of the PC was learned on port 3 of switch C. Traffic from the server to the PC is forwarded from port 3 to port 1.
If port 1 shuts down, port 2 starts forwarding traffic. If there is no traffic from PC to the server after failover to port 2, switch C does not learn MAC address of the PC on port 4. As a result, switch C keeps forwarding traffic from the server to the PC out of port 3. There is traffic loss from server to PC because port 1 is down. This problem is alleviated by sending out a dummy multicast packet with source MAC address of the PC over port 2. Switch C learns the PC MAC address on port 4 and start forwarding traffic from server to the PC out of port 4. One dummy multicast packet is sent out for every MAC address. which is the default Flex Links behavior. The MAC address-table move update (MMU) feature may be enabled to further expedite downstream convergence. MMUs are special packets that carry multiple MAC addresses. Switch A is configured to transmit these packets and switches B, C, and D are configured to receive such packets. If MMU transmit is enabled on Switch A, MAC move updates are transmitted before dummy multicast packets over port 2. Switch D processes and floods MMUs over to Switch C. Switch C processes these packets, and moves the MAC addresses contained within the packets from port 3 to port 4. Because one packet carries multiple MAC addresses, downstream convergence is faster.
Figure 24-3 MAC Address-Table Move Update Example
Configuring Flex Links
These sections contain this configuration information:
- Default Configuration
- Configuration Guidelines
- Configuring Flex Links
- Configuring VLAN Load Balancing on Flex Links
Default Configuration
Configuration Guidelines
Follow these guidelines to configure Flex Links and associated features:
- You can configure only one Flex Link backup link for any active link, and it must be a different interface from the active interface.
- An interface can belong to only one Flex Links pair. An interface can be a backup link for only one active link, but an active link cannot belong to another Flex Links pair.
- Neither of the links can be a port that belongs to an EtherChannel. However, you can configure two port channels (EtherChannel logical interfaces) as Flex Links. Moreover, you can configure a port channel and a physical interface as Flex Links, with either the port channel or the physical interface as the active link.
- The types (Fast Ethernet, Gigabit Ethernet, or port channel) of the backup link and the active link can differ. However, you should configure both Flex Links with similar characteristics so that no loops exist or changes occur in behavior if the standby link begins to forward traffic.
- STP is disabled on Flex Links ports. A Flex Links port does not participate in STP, even if the VLANs present on the port are configured for STP. When STP is not enabled, ensure that no loops exist in the configured topology.
- Configure any static MAC addresses on a Flex Links member interface after enabling Flex Links.
Follow these guidelines to configure VLAN load balancing on the Flex Links feature. For Flex Links VLAN load balancing, you must choose the preferred VLANs on the backup interface.
Set switchport backup interface interface-id preemption mode to forced. A default value of 35 seconds is used as the delay timeout. You cannot configure switchport backup interface interface-id preemption mode on the interface. So, mode bandwidth and off cannot be configured.
Adjust the delay time with the switchport backup interface interface-id preemption delay delay-time command.
Configuring Flex Links
To configure a pair of Flex Links, perform this task:
To disable a Flex Links backup interface, enter the no switchport backup interface interface-id interface configuration command.
This example shows how to configure an interface with a backup interface and to verify the configuration:
To configure a preemption scheme for a pair of Flex Links, perform this task:
To remove a preemption scheme, enter the no switchport backup interface interface-id preemption mode interface configuration command. To reset the delay time to the default, enter the
no switchport backup interface interface-id preemption delay interface configuration command.
This example shows how to configure preemption mode as bandwidth for a backup interface pair and to verify the configuration:
Configuring VLAN Load Balancing on Flex Links
To configure VLAN load balancing on Flex Links, perform this task:
To disable the VLAN load balancing feature, enter the no switchport backup interface prefer vlan interface configuration command.
In this example, VLANs 1 to 50, 60, and 100 to 120 are configured on the switch:
When both interfaces are up, Fast Ethernet port 1/0/8 forwards traffic for VLANs 60 and 100 to 120 and Fast Ethernet port 1/0/6 forwards traffic for VLANs 1 to 50.
When a Flex Links interface shuts down, VLANs preferred on this interface are moved to the peer interface of the Flex Links pair. In this example, if interface 1/6 shuts down, interface 1/8 carries all VLANs of the Flex Links pair.
When a Flex Links interface becomes active, VLANs preferred on this interface are blocked on the peer interface and moved to the forwarding state on the interface that has just come up. In this example, if interface Fast Ethernet port 1/6 becomes active, VLANs preferred on this interface are blocked on the peer interface Fast Ethernet port 1/8 and forwarded on Fast Ethernet port 1/6.
Configuring MAC Address-Table Move Update
These sections contain this configuration information:
Default Configuration
By default, the MAC address-table move update feature is disabled.
Configuration Guidelines
Follow these guidelines to configure the MAC address-table move update feature:
Configuring the MAC Address-Table Move Update Feature
Configuring a Switch to Send MAC Address-Table Move Updates
To configure an access switch to send MAC address-table move updates, perform this task:
To disable the MAC address-table move update feature on the access switch, enter the
no mac address-table move update transmit interface configuration command. To display the MAC address-table move update information, enter the show mac address-table move update command.
This example shows how to configure an access switch to send MAC address-table move update messages and to verify the configuration:
Configuring a Switch to Receive MAC Address-Table Move Updates
To configure a switch to receive and process MAC address-table move update messages, perform this task:
To disable the MAC address-table move update feature on the access switch, enter the
no mac address-table move update receive configuration command. To display the MAC address-table move update information, enter the show mac address-table move update command.
This example shows how to configure a switch to receive and process MAC address-table move update messages:
Monitoring Flex Links and the MAC Address-Table Move Update
Table 24-1 shows the commands for monitoring the Flex Links configuration and the MAC address-table move update information.
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