Etherchannel Load Balancing

By | March 12, 2020

Ether-channel is used to bundle multiple links to increase the bandwidth as well redundancy. All bundled links are used as an single virtual link with total bandwidth of all physical links.

Traffic distributes on individual links according the load balancing method. If a link fails then traffic will be move on adjacent link or if a down link came up again then traffic will be automatically distributed among the active links. STP does’t block the bundled links and considers a single logical link.

We can bundle up to 8 links in a ether-channel. When we are configuring the ether-channel following parameters should be same on all the ports: Media type , speed , duplex.

Ether-channel can be used for a trunk , access port or Layer 3 interface.

If we are configuring the ether-channel for trunk then all the bundled ports should be in trunking mode as well as native VLAN should be same.

NOTE:- We can also create a etherchannel using the ports of different modules to increase the hardware base redundancy. But it is not supported by all the Cisco devices.

If we want to increase the more hardware redundancy then we should move to MCE (Multi Chassis Etherchannel) or VPC.

We are having three options for etherchannel :

  1. Static (Without using any negotiation Protocol)
  2. PAgP (Port Aggregation Protocol)
  3. LACP (Link Aggregation Control Protocol)

PAgP (Port Aggregation Protocol)

PAgP is a cisco proprietary protocol and it used for negotiation between bundled links. It can also dynamically modify the parameters of bundled links. If one of bundled links modified like : VLAN , Speed , Duplex Then PAgP modifies these parameters for all the links of etherchannel.

It uses three types of negotiation modes : On , Auto , Desirable

It also uses silent or non silent sub mode.

On – With this mode links can be bundle unconditionally But PAgP packets will not send or receive. Means , negotiation is off in this mode.

Auto – In this mode , Port tries to listen PAgP messages and waits to be asked.

Desirable – In This mode , port asks to neighboring port to bring up the channel.

PAgP mode combination

On + On = LACP messages will not synchronize and channel will bring up unconditionally.

Auto + Auto = Channel will not form

Auto + Desirable = Channel will be form

Desirable + Desirable = Channel will be form

LACP (Link Aggregation Control Protocol) / IEEE 802.3ad

LACP is open standard protocol and it is an alternative for PAgP. With LACP , we can assign upto 16 potential links for a etherchannel and upto 8 links from them can be bundle in etherchannel and rest of links will be use as standby.

A switch with lowest system priority elects as decision maker and takes the decision link , which link will participate in ether channel and which one will stay in standby state. Links selection is based on the port priority. first 8 links with lower port priority will be bundle in ether-channel and other 8 links will be use as standby.

System Priority – Default system priority is 32768 and it can be set from 1 to 65535. If priority is same then lowest system MAC will be use as tie breaker.

Port Priority – Default system priority is 32768 and it can be set from 1 to 65535. If priority is same then MAC will be use as tie breaker.

Channel group Number – Channel group number can be set using any number from 1 to 64.

LACP offers three negotiation modes : On , Active and Passive

On Mode – With this mode links can be bundle unconditionally But LACP packets will not send or receive. Means , negotiation is off in this mode.

Passive – In this mode , Port tries to listen LACP messages and waits to be asked.

Active – In This mode , port asks to neighboring port to bring up the channel.

LACP mode combination

On + On = LACP messages will not synchronize and channel will bring up unconditionally.

Passive + Passive = Channel will not form

Passive + Active = Channel will be form

Active + Active = Channel will be form

Etherchannel Load Balancing Method

Etherchannel load balancing methods simply distributes the traffic on individual links. So equal load balancing of traffic is not possible. If we have bundled 8 Fast Ethernet links in a etherchannel and total bandwidth is 800 MBPS. But still we can face the high utilization issue. because few links can be fully utilized and others can be with low bandwidth utilization. this can be happen due to load balancing method. so we should use the load balancing method very care fully.

Below are the available etherchannel load balancing methods

MethodHash InputHash Operation
src-ipSource IP AddressBits
dst-ipDestination IP Address Bits
src-dst-ipSource & Destination IP Address X-OR
src-macSource MAC Address Bits
dst-macDestination MAC Address Bits
src-dst-mac Source & Destination MAC Address X-OR
src-portSource Port Bits
src-dst-portSource & Destination Port X-OR
dst-portDestination Port Bits

Load Balancing link calculation Example:

Suppose , We are using src-ip load balancing method. In this case , Device will convert the source IP address into binary and after converted into binary. device will check the last octet of IP address. We are are assuming that below is the last octet of IP address after converted in to binary:

00000001 = 1 link

Last bit of last octet is ON so here only one link will be use for transfer the traffic. If last 2 bits are on then two links will be use and so on.

Now suppose we are using src-dst-ip method of load balancing and we have converted last octet into binary of both source IP address and destination IP address. below is the last octet of source and destination:

00000001

+

00000110

= 111

We can see output is having 3 bits ON so traffic will be distribute on three links.

below are the rules of this method:

1+1=0

1+0=1

0+1=1

Configuration Example

Etherchannel port-channel lacp pagp

LACP configuration using above topology

SwitchA(config)# port-channel load-balance src-ip
SwitchA(config)# interface range e0/0 - 3
SwitchA(config-if)# channel-protocol lacp
SwitchA(config-if)#channel-group 1 mode active
SwitchA(config-if)#lacp port-priority 100
SwitchB(config)# port-channel load-balance src-ip
SwitchB(config)# interface range e0/0 - 3
SwitchB(config-if)# channel-protocol lacp
SwitchB(config-if)#channel-group 1 mode passive
SwitchB(config-if)#lacp port-priority 100

PAgP configuration using above topology

SwitchA(config)# port-channel load-balance src-ip
SwitchA(config)# interface range e0/0 - 3
SwitchA(config-if)# channel-protocol pagp
SwitchA(config-if)#channel-group 1 mode desirable
SwitchB(config)# port-channel load-balance src-ip
SwitchB(config)# interface range e0/0 - 3
SwitchB(config-if)# channel-protocol pagp
SwitchB(config-if)#channel-group 1 mode desirable

Show Commands

Switch# Show etherchannel port-channel
Switch# Show etherchannel summary
Switch# Show etherchannel port
Switch# Show interface e0/0 etherchannel
Switch# Show etherchannel load-balance
Switch# Show pagp neighbor
Switch# Show lacp neighbor
Switch# Show lacp sys-id
Switch# Show etherchannel detail

Thanks For Reading ………………….

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