EtherChannel

Intro

  • ASW1 (Access layer switch) is a switch that end hosts connect to.

  • DSW1 (Distribution layer switch) is a switch that access layer switches connect to.

  • If there are 40 hosts connected to ASW1, the connection is congested. The admin should add another link to increase the bandwidth, so it can support all of the end hosts.

The network is still congested, and users are reporting problems. SO, the admin adds another link.

When the bandwidth of the interfaces connected to end hosts is greater than the bandwidth of the connection to the distribution switch(es), this is called oversubscription. Some oversubscription is acceptable, but too much will cause congestion.

Even with three links, the congestion does not seem any better. SO it adds again another link.

If you connect two switches together with multiple links, all except one will be disabled by STP.

  • If all of ASW1's interfaces were forwarding, Layer 2 loops would form between ASW1 and DSW1, leading to broadcast storms.

  • Other links will be unused unless the active link fails. IN that case, one of the inactive links will start forwarding.

  • It's a waste of bandwidth to have these three interfaces disabled, not forwarding any traffic.

  • However, by forming these four physical interfaces into one logical interface, EtherChannel can solve this problem, giving us redundancy and increased bandwidth.

An EtherChannel is represented in network diagrams by drawing a circle around the interfaces.

  • EtherChannel groups multiple interfaces together to act as a single interface.

  • STP will treat this group as a single interface.

  • Traffic using the EtherChannel will be load balanced among the physical interfaces in the group. An algoritm is used to determine which traffic will use which physical interface.

  • Other names for EtherChannel are:

    • Port Channel

    • LAG (Link Aggregation Group)

Load Balancing

  • EtherChannel load balances bases on 'flows'.

  • A flow is a communication between two nodes in the network.

  • Frames in the same flow will be forwarded using the same physical interface.

  • If frames in the same flow were forwarded using different physical interfaces, some frames may arrive at the destination out of order, which can cause problems.

ASW1#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  
ASW1#conf t  
Enter configuration commands, one per line.  End with CNTL/Z.  

**ASW1(config)#port-channel load-balance src-dst-mac**  

ASW1(config)#do show etherchannel load-balance  
EtherChannel Load-Balancing Configuration:  
src-dst-mac  

**EtherChannel Load-Balancing Addresses Used Per-Protocol:**  
Non-IP: Source XOR Destination MAC address  
IPv4: Source XOR Destination MAC address  
IPv6: Source XOR Destination MAC address  

ASW1(config)#
ASW1(config)#port-channel load-balance ?
  dst-ip      Dst IP Addr
  dst-mac     Dst Mac Addr
  src-dst-ip  Src XOR Dst IP Addr
  src-dst-mac Src XOR Dst Mac Addr
  src-ip      Src IP Addr
  src-mac     Src Mac Addr
ASW1(config)#port-channel load-balan

EtherChannel Configuration

PAgP (Port Aggregation Protocol)

  • PAgP is a Cisco proprietary protocol.

  • It dynamically negotiates the creation/maintenance of EtherChannel.

ASW1(config)#interface range g0/0 - 3
ASW1(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

ASW1(config-if-range)#channel-group 1 mode desirable
Creating a port-channel interface Port-channel 1
////////////////////////////////////////////////////////////////////////////////
auto + auto = no EtherChannel
desirable + auto = EtherChannel
desirable + desirable = EtherChannel
SW(config-if)# channel-group <number> mode <mode>

The channel-group number has to match the member interfaces on the same switch. However, it does not have to match the channel-group number on the other switch. (channel-group 1 on ASW1 can form an EtherChannel with channel-group 2 on DSW1)

LACP (Link Aggregation Control Protocol)

  • Industry standard protocol (IEEE 802.3ad)

  • It dynamically negotiates the creation/maintenance of EtherChannel.

ASM1(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

ASM1(config-if-range)#channel-group 1 mode active
Creating a port-channel interface Port-channel 1
////////////////////////////////////////////////////////////////////////////////
passive + passive = no EtherChannel
passive + active = EtherChannel
active + active = EtherChannel

Static EtherChannel

  • A protocol is not used to determine if an EtherChannel should be formed.

  • Interfaces are statically configured to form an EtherChannel.

Up to 8 interfaces can be formed into a single EtherChannel (LACP allows up to 16, but only 8 will be active, the other 8 will be in standby mode, waiting for an active interface to fail).

  • There aren't two separate modes, just one, 'ON' which manually tells these interfaces to form an EtherChannel.

  • On mode only works with on mode (on + desirable or on + active will not work).

Configure port-channel interface

ASM1(config)#interface port-channel 1
ASM1(config-if)#switchport trunk encapsulation dot1q
ASM1(config-if)#switchport mode trunk
ASM1(config-if)#do 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

Port        Vlans in spanning tree forwarding state and not pruned
Po1         1

  • Member interfaces must have matching configurations.

    • Same duplex (full/half)

    • Same speed

    • Same switchport mode (access/trunk)

    • Same allowed VLANs/native VLAN (for trunk interfaces)

  • If an interface's configurations does not match the others, it will be excluded from the EtherChannel.

ASW1# show etherchannel summary
ASW1# show etherchannel port-channel

Manually configure the negotiation protocol

  • Not a very useful command, because there is no need to configure it.

ASM1(config-if-range)#channel-protocol ?
lacp  Prepare interface for LACP protocol
pagp  Prepare interface for PAgP protocol

ASM1(config-if-range)#channel-protocol lacp
ASM1(config-if-range)#channel-group 1 mode desirable
Command rejected (Channel protocol mismatch for interface Gi0/0 in group 1): the interface can not be added to the channel group
% Range command terminated because it failed on GigabitEthernet0/0
ASM1(config-if-range)#channel-group 1 mode on
Command rejected (Channel protocol mismatch for interface Gi0/0 in group 1): the interface can not be added to the channel group
% Range command terminated because it failed on GigabitEthernet0/0
ASM1(config-if-range)#channel-group 1 mode active
Creating a port-channel interface Port-channel 1
ASM1(config-if-range)#

Layer 3 EtherChannels

ASM1(config)#int range g0/0 - 3
ASM1(config-if-range)#no switchport
ASM1(config-if-range)#channel-group 1 mode active
Creating a port-channel interface Port-channel 1
ASM1(config-if-range)#int po1
ASM1(config-if)#ip address 10.0.0.1 255.255.255.252
ASM1(config-if)#

Commands

SW(config) port-channel load-balance mode
# configures the EtherChannel load-balancing method on the switch

SW# show etherchannel load-balance
# displays information about the load-balancing settings

SW(config-if)# channel-group number mode {desirable|auto|active|passive|on}
# configures an interface to be part of an EtherChannel

SW# show etherchannel summary
# displays a summary of EtherChannels on the switch

SW# show etherchannel port-channel
# displays information about the virtual port-channel interfaces on the switch
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