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29.6 Link Aggregation and Failover

Written by Andrew Thompson.

29.6.1 Introduction

The lagg(4) interface allows aggregation of multiple network interfaces as one virtual interface for the purpose of providing fault-tolerance and high-speed links.

Sends and receives traffic only through the master port. If the master port becomes unavailable, the next active port is used. The first interface added is the master port; any interfaces added after that are used as failover devices.

fec

Supports Cisco EtherChannel. This is a static setup and does not negotiate aggregation with the peer or exchange frames to monitor the link, if the switch supports LACP then that should be used instead.

Balances outgoing traffic across the active ports based on hashed protocol header information and accepts incoming traffic from any active port. The hash includes the Ethernet source and destination address, and, if available, the VLAN tag, and the IPv4/IPv6 source and destination address.

lacp

Supports the IEEE 802.3ad Link Aggregation Control Protocol (LACP) and the Marker Protocol. LACP will negotiate a set of aggregable links with the peer in to one or more Link Aggregated Groups. Each LAG is composed of ports of the same speed, set to full-duplex operation. The traffic will be balanced across the ports in the LAG with the greatest total speed, in most cases there will only be one LAG which contains all ports. In the event of changes in physical connectivity, Link Aggregation will quickly converge to a new configuration.

loadbalance

This is an alias of fec mode.

roundrobin

Distributes outgoing traffic using a round-robin scheduler through all active ports and accepts incoming traffic from any active port. This mode will violate Ethernet frame ordering and should be used with caution.

29.6.3 Examples

Example 29-1. LACP aggregation with a Cisco switch

This example connects two interfaces on a FreeBSD machine to the switch as a single load balanced and fault tolerant link. More interfaces can be added to increase throughput and fault tolerance. Since frame ordering is mandatory on Ethernet links then any traffic between two stations always flows over the same physical link limiting the maximum speed to that of one interface. The transmit algorithm attempts to use as much information as it can to distinguish different traffic flows and balance across the available interfaces.

On the Cisco switch add the interfaces to the channel group.

interface FastEthernet0/1
 channel-group 1 mode active
 channel-protocol lacp
!
interface FastEthernet0/2
 channel-group 1 mode active
 channel-protocol lacp
!

On the FreeBSD machine create the lagg interface.

# ifconfig lagg0 create
# ifconfig lagg0 up laggproto lacp laggport fxp0 laggport fxp1

View the interface status from ifconfig; ports marked as ACTIVE are part of the active aggregation group that has been negotiated with the remote switch and traffic will be transmitted and received. Use the verbose output of ifconfig(8) to view the LAG identifiers.

lagg0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=8<VLAN_MTU>
        ether 00:05:5d:71:8d:b8
        media: Ethernet autoselect
        status: active
        laggproto lacp
        laggport: fxp1 flags=1c<ACTIVE,COLLECTING,DISTRIBUTING>
        laggport: fxp0 flags=1c<ACTIVE,COLLECTING,DISTRIBUTING>

The switch will show which ports are active. For more detail use show lacp neighbor detail.

switch# show lacp neighbor 
Flags:  S - Device is requesting Slow LACPDUs 
        F - Device is requesting Fast LACPDUs
        A - Device is in Active mode       P - Device is in Passive mode     

Channel group 1 neighbors

Partner's information:

                  LACP port                        Oper    Port     Port
Port      Flags   Priority  Dev ID         Age     Key     Number   State
Fa0/1     SA      32768     0005.5d71.8db8  29s    0x146   0x3      0x3D  
Fa0/2     SA      32768     0005.5d71.8db8  29s    0x146   0x4      0x3D

Example 29-2. Failover mode

Failover mode can be used to switch over to another interface if the link is lost on the master.

# ifconfig lagg0 create
# ifconfig lagg0 up laggproto failover laggport fxp0 laggport fxp1

lagg0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=8<VLAN_MTU>
        ether 00:05:5d:71:8d:b8
        media: Ethernet autoselect
        status: active
        laggproto failover
        laggport: fxp1 flags=0<>
        laggport: fxp0 flags=5<MASTER,ACTIVE>

Traffic will be transmitted and received on fxp0. If the link is lost on fxp0 then fxp1 will become the active link. If the link is restored on the master interface then it will once again become the active link.

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29.6 Link Aggregation and Failover

29.6.1 Introduction

29.6.2 Operating Modes

29.6.3 Examples