EtherChannel Tutorial

Page 1

Notes:

The Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP) facilitate the automatic creation of EtherChannels by exchanging packets between Ethernet interfaces. The Port Aggregation Protocol (PAgP) is a Cisco-proprietary solution, and the Link Aggregation Control Protocol (LACP) is standards based.

LACP modes:

+ on: the link aggregation is forced to be formed without any LACP negotiation. A port-channel is formed only if the peer port is also in “on” mode.

+ off: disable LACP and prevent ports to form a port-channel

+ passive: the switch does not initiate the channel, but does understand incoming LACP packets

+ active: send LACP packets and willing to form a port-channel

The table below lists if an EtherChannel will be formed or not for LACP:

LACP Active Passive

Active Yes Yes

Passive Yes NoPAgP modes:

+ on: The link aggregation is forced to be formed without any PAgP negotiation. A port-channel is formed only if the peer port is also in “on” mode.

+ off: disable PAgP and prevent ports to form a port-channel

+ desirable: send PAgP packets and willing to form a port-channel

+ auto: does not start PAgP packet negotiation but responds to PAgP packets it receives

The table below lists if an EtherChannel will be formed or not for PAgP:

PAgP Desirable Auto

Desirable Yes Yes

Auto Yes NoAn EtherChannel in Cisco can be defined as a Layer 2 EtherChannel or a Layer 3 EtherChannel.

+ For Layer 2 EtherChannel, physical ports are placed into an EtherChannel group. A logical port-channel interface will be created automatically. An example of configuring Layer 2 EtherChannel can be found in Question 1 in this article.

+ For Layer 3 EtherChannel, a Layer 3 Switch Virtual Interface (SVI) is created and then the physical ports are bound into this Layer 3 SVI.

For more information about EtherChannel, please read our EtherChannel tutorial.

Page 2

EtherChannel is the technology which is used to combine several physical links between switches or routers into one logical connection and treat them as a single link. Let’s take an example to see the benefits of this technology: Suppose your company has two switches connecting with each other via a FastEthernet link (100Mbps):

Your company is growing and you need to transfer more than 100 Mbps between these switches. If you only connect other links between the two switches it will not work because Spanning-tree protocol (STP) will block redundant links to prevent a loop:

To extend the capacity of the link you have two ways:

+ Buy two 1000Mbps (1Gbps) interfaces

+ Use EtherChannel technology to bundle them into a bigger link The first solution is expensive with the new hardware installed on the two switches. By using EtherChannel you only need some more unused ports on your switches:

EtherChannel bundles the physical links into one logical link with the combined bandwidth and it is awesome! STP sees this link as a single link so STP will not block any links! EtherChannel also does load balancing among the links in the channel automatically. If a link within the EtherChannel bundle fails, traffic previously carried over the failed link is carried over the remaining links within the EtherChannel. If one of the links in the channel fails but at least one of the links is up, the logical link (EtherChannel link) remains up. EtherChannel also works well for router connections:

When an EtherChannel is created, a logical interface will be created on the switches or routers representing for that EtherChannel. You can configure this logical interface in the way you want. For example, assign access/trunk mode on switches or assign IP address for the logical interface on routers…

Note: A maximum of 8 Fast Ethernet or 8 Gigabit Ethernet ports can be grouped together when forming an EtherChannel. There are three mechanisms you can choose to configure EtherChannel:

+ Port Aggregation Protocol (PAgP)

+ Link Aggregation Control Protocol (LACP)

+ Static (“On”)

LACP is the IEEE Standard (IEEE 802.3ad) and is the most common dynamic ether-channel protocol, whereas PAgP is a Cisco proprietary protocol and works only between supported vendors and Cisco devices. All ports in an EtherChannel must use the same protocol; you cannot run two protocols on two ends. In other words, PAgP and LACP are not compatible so both ends of a channel must use the same protocol.The Static Persistence (or “on” mode) bundles the links unconditionally and no negotiation protocol is used. In this mode, neither PAgP nor LACP packets are sent or received. (Reference: http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a0080094714.shtml)

Next we will learn more about the three EtherChannel mechanisms above.

Port Aggregation Protocol (PAgP)

PAgP dynamically negotiates the formation of a channel. There are two PAgP modes:

Auto Responds to PAgP messages but does not aggressively negotiate a PAgP EtherChannel. A channel is formed only if the port on the other end is set to Desirable. This is the default mode.

Desirable Port actively negotiates channeling status with the interface on the other end of the link. A channel is formed if the other side is Auto or Desirable.The table below lists if an EtherChannel will be formed or not for PAgP:

PAgP Desirable Auto

Desirable Yes Yes

Auto Yes No

Link Aggregation Protocol (LACP)

LACP also dynamically negotiates the formation of a channel. There are two LACP modes:

Passive Responds to LACP messages but does not aggressively negotiate a LACP EtherChannel. A channel is forms only if the other end is set to Active

Active Port actively negotiates channeling with the interface on the other end of the link. A channel is formed if the other side is Passive or ActiveThe table below lists if an EtherChannel will be formed or not for LACP:

LACP Active Passive

Active Yes Yes

Passive Yes NoIn general, Auto mode in PAgP is the same as Passive mode in LACP and Desirable mode is same as Active mode. Auto = Passive Desirable = Active

Static (“On”)

In this mode, no negotiation is needed. The interfaces become members of the EtherChannel immediately. When using this mode make sure the other end must use this mode too because they will not check if port parameters match. Otherwise the EtherChannel would not come up and may cause some troubles (like loop…). Note: All interfaces in an EtherChannel must be configured identically to form an EtherChannel. Specific settings that must be identical include:

+ Speed settings

+ Duplex settings

+ STP settings

+ VLAN membership (for access ports)

+ Native VLAN (for trunk ports)

+ Allowed VLANs (for trunk ports)

+ Trunking Encapsulation (ISL or 802.1Q, for trunk ports)

Note: EtherChannels will not form if either dynamic VLANs or port security are enabled on the participating EtherChannel interfaces. In the next part we will learn how to configure EtherChannel on switch/router interfaces.

Page 3

EtherChannel Configuration

To assign and configure an EtherChannel interface to an EtherChannel group, use the channel-group command in interface mode: channel-group number mode { active | on | {auto [non-silent]} | {desirable [non-silent]} | passive}

For example we will create channel-group number 1:

Switch(config-if)#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 detectedIf a port-channel interface has not been created before using this command, it will be created automatically and you will see this line: “Creating a port-channel interface Port-channel 1”. In this example, we will create an EtherChannel via LACP between SwA & SwB with the topology shown below:

SwA Configuration

//Assign EtherChannel group 1 to fa0/0 and fa0/1 and set Active mode on them

SwA(config)#interface range fa0/0 – 1

SwA(config-if-range)#channel-group 1 mode active

Creating a port-channel interface Port-channel 1

//Next configure the representing port-channel interface as trunk

SwA(config)#interface port-channel 1

SwA(config-if)#switchport trunk encapsulation dot1q

SwA(config-if)#switchport mode trunk

SwB Configuration

//Assign EtherChannel group 2 to fa0/5 and fa0/6 and set Passive mode on them

SwB(config)#interface range fa0/5 – 6

SwB(config-if-range)#channel-group 2 mode passive

Creating a port-channel interface Port-channel 2

//Next configure the representing port-channel interface as trunk

SwB(config)#interface port-channel 2

SwB(config-if)#switchport trunk encapsulation dot1q

SwB(config-if)#switchport mode trunkThat is all the configuration for the EtherChannel to work well on both switches. We can verify with the “show etherchannel <port-channel number> port-channel” or “show etherchannel summary” command.

That is all the configuration for the EtherChannel to work well on both switches. We can verify with the “show etherchannel <port-channel number> port-channel” or “show etherchannel summary” command.

SwA# show etherchannel 1 port-channel

Port-channels in the group:

----------------------------

Port-channel: Po1

Age of the Port -channel = 0d:00h:02m:37s

Logical slot/port = 2/1 Number of ports = 2

GC = 0x00010001 HotStandBy port = null

Port state = Port-channel Ag -Inuse

Protocol = LACP

Port security = Disabled

Ports in the Port-channel:

Index Load Port EC state No of bits

---------+--------+--------+------------------ +-----------

0 00 Fa0/0 Active 0

0 00 Fa0/1 Active 0

Time since last port bundled: 0d:00h:02m:27s Fa0/1

The “show etherchannel number port-channel” command can be used to display information about a specific port channel (in this case port-channel 1). From the command above we can see Port-channel 1 consists of Fa0/0 & Fa0/1 and they are in Active state.

SwA# 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 Fa0/0(P) Fa0/1(P)

The “show etherchannel summary” can be used to simply display one line of information per port-channel. In this case we learn from the last line that Group 1 uses LACP. This is a Layer 2 EtherChannel (symbolized by “SU”, in which “S” means “Layer2” & “U” means this port-channel is up.

EtherChannel Load-Balancing

EtherChannel load-balances traffic among port members of the same channel. Load balancing between member interface is based on:

+ Source MAC address

+ Destination MAC address

+ Source IP address

+ Destination IP address

+ Combination of Source and Destination MAC address

+ Combination of Source and Destination IP address

Note: Some old switch/router flatforms do not support all the load-balancing methods above. To configure load-distribution method, use the command port-channel load-balance under global configuration mode. For example to load-balance based on destination MAC use the command:

Router(config)#port-channel load-balance dst-mac

How the router/switch load-balances traffic among member interface is out of the scope of this article. For more information about EtherChannel load-balancing please visithttp://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a0080094714.shtml#topic1.