EtherChannel: bundling links

Why bundle links?

An access switch has many users and a single 1 Gbps uplink is getting full. You add a second cable to the same distribution switch. But spanning tree sees two parallel links between the same two switches, which is a loop, and blocks one of them. You paid for two links and use one.

EtherChannel (also called link aggregation or a port-channel) bundles several physical links into one logical link. Spanning tree sees a single link, so nothing is blocked and every member carries traffic. If one cable fails, traffic moves to the remaining members within a moment, without any spanning tree recalculation.

Three ways to form a bundle

• LACP (Link Aggregation Control Protocol): the open standard, IEEE 802.3ad, now maintained as IEEE 802.1AX. Modes: active (sends LACP messages to start a bundle) and passive (answers but does not start).
• PAgP (Port Aggregation Protocol): Cisco proprietary. Modes: desirable (starts) and auto (answers only).
• Static on: no protocol at all. Both sides must be set to on. Simple, but nothing checks that the other side is set up the same way.

The pattern is the same as DTP: at least one side must actively ask. Prefer LACP with active on both ends: it is standard, works with other vendors and servers, and detects cabling mistakes that static on would miss.

Configuring an LACP EtherChannel

SW1(config)# interface range gigabitEthernet 0/1 - 2
SW1(config-if-range)# channel-group 1 mode active
SW1(config-if-range)# exit
SW1(config)# interface port-channel 1
SW1(config-if)# switchport mode trunk
SW1(config-if)# switchport trunk allowed vlan 10,20,30
SW1(config-if)# end
SW1# show etherchannel summary

Members must match

All member ports must have the same speed and duplex, the same switchport mode (all access or all trunk), the same access VLAN, or the same allowed VLANs and native VLAN on trunks. A port that does not match is suspended or left out of the bundle. The easiest way to stay consistent is to configure the port-channel interface, not the members one by one.

Load balancing: one flow, one link

An EtherChannel does not split a single frame across links. The switch calculates a hash from fields of each frame, such as source and destination IP addresses, and the result picks one member link. All frames of the same conversation (a flow) give the same hash, so they use the same link and arrive in order. Many different flows spread across all links.

Consequence: one big file copy between two servers never goes faster than one member link, even on a 4-link bundle. And if the hash fields are badly chosen (for example only the source MAC, when all traffic comes from one router), most traffic can end up on one link. The default method depends on the platform; check it and change it globally.

SW1(config)# port-channel load-balance src-dst-ip
SW1(config)# end
SW1# show etherchannel load-balance

Reading show etherchannel summary

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      ...

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

Healthy: SU for a layer 2 channel (RU for layer 3), and P on every member. Trouble: SD means the port-channel is down; (I) means a member stands alone because negotiation failed; (s) means suspended, usually a settings mismatch; (D) means the member itself is down.

Layer 3 EtherChannel, briefly

Between two layer 3 switches you can bundle routed ports: put no switchport on the members and the port-channel, and give the port-channel an IP address.

DSW1(config)# interface range gigabitEthernet 1/0/1 - 2
DSW1(config-if-range)# no switchport
DSW1(config-if-range)# channel-group 2 mode active
DSW1(config-if-range)# interface port-channel 2
DSW1(config-if)# no switchport
DSW1(config-if)# ip address 10.0.0.1 255.255.255.252