CCIE Notes: Etherchannels

Whether we are talking about EtherChannels or PortChannels, we are talking about the same technology. That technology is a way to allow you to treat multiple physical links as one link to Spanning-Tree, and you can also do some load-balancing on them.

There are two types of port channels.

FEC – Fast EtherChannel
GEC – Gigabit EtherChannel

They can be a layer 2 EtherChannel, or they can be a layer 3 EtherChannel

When a switch decides to load-balance, it will choose from an algorithm (or you can set it manually) on how it should load-balance, and the default varies by switch model. Load-balancing can happen based on layer 2, 3, and/or 4 headers. If load-balancing is based on one header, a bitmap of the low-order bits is used to accomplish the task, but if multiple headers are used, an XOR of the low-order bits is used to determine how to load-balance.

I have a 3550 and 2950, and their default load-balancing is based on source-mac address

3550 – show etherchannel load-balance

SW1#show etherchannel load-balance
EtherChannel Load-Balancing Configuration:
        src-dst-ip
 
EtherChannel Load-Balancing Addresses Used Per-Protocol:
Non-IP: Source XOR Destination MAC address
  IPv4: Source XOR Destination IP address
  IPv6: Source XOR Destination IP address

2950 – show etherchannel load-balance

SW1#show etherchannel load-balance
EtherChannel Load-Balancing Configuration:
Source MAC Address

You can explicitly set a port to join an EtherChannel or use a dynamic protocol to create the EtherChannel. The dynamic protocols are;

LACP – Link Aggregation Control Protocol (802.1AD, Standard)
PAgP – Port Aggregation Protocol (Cisco Proprietary)

PAgP and LACP Configuration Settings and Recommendations

PAgP Setting	LACP Setting	Action
On	On	Disables PAgP/LACP; forces interfaces into joining a port-channel
Off	Off	Disables PAgP/LACP; stops ports from joining a port-channel
Auto	Passive	Enables LACP; waits for another side to send the first message
Desirable	Active	Enables PAgP; initiates the port-channel connection

Using Auto/Auto or Passive/Passive will make a port-channel, not form. It is like both sides of a trunk being in dynamic auto mode.

Cisco recommends that you use Desirable/Desirable or Active/Active to form a port-channel dynamically.

Before a dynamic port-channel is created, several items must be identical on links;

Speed and duplex
If not a trunk, the same access VLAN
If a trunk, same trunk type, allowed VLANS and Native VLAN
Ports on either side must be the same: trunk, access, routed.
No SPAN ports can be a part of a port-channel. If you try to use a SPAN port as a source or destination SPAN port and it is a part of a port-channel, it will give you a warning that it will be removed
On a single switch, the STP port costs on all ports in the port channel must be identical
For a layer 3 port-channel, the IP is configured on the port-channel interface, not on the physical interfaces

Sample layer 2 port-channel configuration.

The far side is set to LACP Active; in channel-group 6, the command is channel-group 6 mode active.

We will configure our site to be in channel-group 1, using LACP passive; let’s see if it forms.

The configuration is done on ports Ethernet 0/1 and 0/3

SW1(config)#int range e0/1, e0/3
SW1(config-if-range)#channel-group 1 mode passive
Creating a port-channel interface Port-channel 1

Verification 1 – show interface trunk

SW1#show interfaces trunk
Port        Mode             Encapsulation  Status        Native vlan
Po1         on               802.1q         trunking      1
Port        Vlans allowed on trunk
Po1         1-4094
Port        Vlans allowed and active in management domain
Po1         1,10,20,30,40,100-101
Port        Vlans in spanning tree forwarding state and not pruned
Po1         1,10,20,30,40,100-101
SW1#

OK, so far, it looks good. We see only one trunking interface, and it is the newly created port-channel; let’s do a second verification.

Verification 2 – 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      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      Et0/0(P)    Et0/3(P)
SW1#

The ports are bundled in the port-channel as shown by the (P), and the port-channel Po1 is a Layer 2 port-channel and in-use as shown by (SU)

So that’s it. We have verified a successful port-channel configuration, and just in case anyone wanted to see what the configuration looks like on the port channel, it is

SW1#show run interface port-channel1
Building configuration...
Current configuration : 104 bytes
!
interface Port-channel1
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk

I did not add any of those commands; the switch auto-populated based on what was configured on the physical ports. Remember, it can do that because since this is a layer 2 trunk, the allowed VLAN list, trunk type, and native VLAN have to match the port-channel to be created.

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