1.1.d Etherchannel

EtherChannel is a technology that lets you bundle multiple physical links into a single logical link.

Etherchannel consists of two parts

• Port-channel Interface: Logical interface representing the link bundle
• Member interfaces: Physical links that are part of the link bundle

Goal is to hide the member interfaces to the upper layer Protocol so that STP sees one 2Gbps link and not 2 x 1 Gbps link. This results in an active/active forwarding instead of active/standby with STP

1.1.d i LACP, PAgP, static

Although only LACP and static are listed in the blueprint, it makes sense to look at the third option (PAgP) as well.

The protocols of interest are the Link Aggregation Control Protocol (LACP) and the Port Aggregation Protocol (PAgP).  PAgP is Cisco proprietary and LACP is open standard and vendor neutral.  All participating interfaces in an EtherChannel bundle must be using the same protocol, they are not compatible with each other. 

PAgP 

PAgP advertises to the multicast address 01:00:0c:cc:cc:cc with the protocol code: 0x0104 and operates in two modes: 

Auto	Effectively a listening mode.  Interfaces in Auto mode will not actively attempt to form an EtherChannel bundle.  If a PAgP packet is received from the remote switch, the receiving interface will respond and will establish a PAgP adjacency.
Desirable	In Desirable mode, the interface will actively attempt to form an EtherChannel.

To summaries, if both ends of a proposed PAgP Etherchannel bundle are operating in Auto mode, they will not form an EtherChannel.  Interfaces in Desirable mode, will form Etherchannels with remote interfaces that are either in Auto mode or are also in Desirable mode. 

Additionally, PAgP by default will use ‘silent’ mode, this allows a port to establish an EtherChannel with a device which is not PAgP capable and will rarely send packets.  Using the optional ‘non-silent’ command, requires participating interfaces to first receive PAgP packets before forming an EtherChannel. 

LACP 

LACP advertises to the multicast address 01:80:c2:00:00:02.  LACP also operates in two modes: 

Passive	Effectively a listening mode and comparable to PAgP’s ‘auto’ mode.
Active	In Active mode, the interface will actively attempt to form an EtherChannel.

To summaries the protocols, similar to PAgP, if both ends are configured as Passive, an EtherChannel will not form. Active interfaces will form Etherchannels if the remote ends are either in Active or Passive mode. 

Static

A switch can successfully create an EtherChannel bundle by statically configuring member interfaces to ‘on’ state. By configuring interfaces statically with ‘on’ mode, you will lose the ability for the devices to perform line heath integrity checks – e.g. If the physical medium degrades but keeps the line protocol in the ‘up’ state, the port channel will still keep attempting to forward traffic even if it couldn’t reach the other end of the EtherChannel. To that end, dynamic link aggregation protocols are definitely preferred.

Configuration

The configuration depends on the protocol (LACP, PAgP, static) you want to use:

SW1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
SW1(config)#int rang gi1/0 - 1
SW1(config-if-range)#channel-group 1 mode ?
  active     Enable LACP unconditionally
  auto       Enable PAgP only if a PAgP device is detected
  desirable  Enable PAgP unconditionally
  on         Enable Etherchannel only
  passive    Enable LACP only if a LACP device is detected

Verifying EtherChannel Status 

Once an EtherChannel has been configured, it is essential to verify that it has established properly.  To do this, use command 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      N - not in use, no aggregation
        f - failed to allocate aggregator

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

        A - formed by Auto LAG


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

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi1/0(s)    Gi1/1(P)

When viewing the output of the show etherchannel summary command, it is important to check the status, listed under the “Port-Channel” column and denoted by a variety of case-sensitive letters, each with a different meaning. 

1.1.d ii Layer 2, Layer 3

For layer 2 EtherChannels, ensure that all member interfaces are layer 2 (‘switchport’ may be required) and are operating in the same port mode. 

For layer 3 EtherChannels, ensure that all member interfaces are layer 3 (‘no switchport’ may be required).  IP address and other Layer 3 configuration should be applied to the logical interface. 

1.1.d iii Load balancing

Traffic on an EtherChannel link is not transmitted on a round-robin basis.  A hash is calculated and packets are consistently forwarded across a link based on that hash which runs on the various packet header fields.  The load-balancing hash is a global command and is not set on a per EtherChannel basis. 

Use command port-channel load-balance <hash option>. 
The following are hash options available (depending on the platform not all are available) : 

Dst-ip	Destination IP address
Dst-mac	Destination MAC address
Dst-mixed-ip-port	Destination IP addresses and destination TCP/UDP port
Dst-port	Destination TCP/UDP port
Src-dst-ip	Source and destination IP addresses
Src-dest-ip-only	Source and destination IP addresses only
Src-dst-mac	Source and destination MAC addresses
Src-dst-mixed-ip-port	Source and destination IP addresses and source and destination TCP/UDP ports
Src-dst-port	Source and destination TCP/UDP ports
Src-ip	Source IP address
Src-mac	Source MAC address
Src-mixed-ip-port	Source IP address and source TCP/UDP port
Src-port	Source TCP/UDP port

1.1.d iv EtherChannel Misconfiguration Guard

EtherChannel misconfiguration guard, as the name may allude to, stops misconfigured EtherChannels going into service.  It will check that all member interfaces have the same configuration.  If there are any misconfigurations discovered, it will place all member interfaces of that EtherChannel into ‘errdisable’ state. 

To configure, use command spanning-tree etherchannel guard misconfig.