EtherChannel with LACP

What it is

EtherChannel bundles several physical links into one logical link, so no link is wasted.

Suppose two switches are joined by two cables. Spanning Tree sees a loop and blocks one cable, so you pay for two links and use only one. With EtherChannel, Spanning Tree sees a single link, so nothing is blocked, and traffic uses all the physical links at once.

The benefits are more bandwidth between devices, and redundancy: if one physical link fails, the bundle keeps working on the others. It is common between access and distribution switches, and between switches and servers.

A bundle is called a port-channel. On Cisco devices you meet it as an interface called Port-channel1, and so on.

How it works

Links join a bundle through LACP, PAgP or a static setting, and members must match.

Negotiation protocols

A bundle can be formed in three ways:

• LACP (Link Aggregation Control Protocol): the open standard, IEEE 802.3ad (later 802.1AX). It works with other vendors.
• PAgP (Port Aggregation Protocol): Cisco proprietary.
• Static (on): no negotiation. The ports are simply forced into the bundle.

The negotiation protocols check that both sides agree before links join. That protects you against a misconfiguration at one end. The modes are:

Two ends form a bundle if at least one side is actively asking: active with active, active with passive, desirable with desirable, or desirable with auto.

Passive with passive and auto with auto both wait, so no bundle forms. on forms a bundle only with another on. You cannot mix protocols: LACP cannot negotiate with PAgP.

Rules for member ports

All ports in a bundle must match. They need the same speed, the same duplex, the same VLAN or trunk settings, and the same native VLAN.

If the settings are different, the port is left out of the bundle or suspended. A bundle can have up to eight active member ports.

Configuring a Layer 2 EtherChannel

interface range GigabitEthernet0/1 - 2
 channel-group 1 mode active

The channel-group command creates Port-channel1 automatically. Further settings, such as trunking, are best applied to the port-channel interface so that they apply to all members.

interface Port-channel1
 switchport mode trunk
 switchport trunk allowed vlan 10,20

Configuring a Layer 3 EtherChannel

A routed bundle gives the port-channel an IP address. The member ports become routed ports.

interface range GigabitEthernet0/1 - 2
 no switchport
 channel-group 2 mode active
interface Port-channel2
 no switchport
 ip address 10.1.1.1 255.255.255.252

Load balancing

The device does not send each packet over a different link. It runs a hash on addresses, and sometimes port numbers, and uses the result to pick a link.

A given conversation always uses the same link, which keeps packets in order. A bundle may not divide traffic evenly if few conversations exist. The hash input can be changed:

port-channel load-balance src-dst-ip

Verifying

SW1# show etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator

        M - not in use, minimum links not met
        u - unsuitable for bundling
        w - waiting to be aggregated
        d - default port


Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi0/1(P)    Gi0/2(P)

show etherchannel summary
show interfaces Port-channel1
show etherchannel port-channel

show etherchannel summary is the main command. Look at the flags beside each port-channel and member:

• SU on the port-channel: Layer 2 and in use.
• RU: Layer 3 (routed) and in use.
• P on a member: bundled in the port-channel.
• D: down. I: stand-alone. s: suspended.

A healthy Layer 2 bundle shows SU and its members with P.