We have a network design with one segment of LAN connected to another segment using a switch, as shown in the figure.
The links connecting the two switches are of 100mbps bandwidth. Both LAN segments are communicating with each other and transmitting traffic, as given below-
- PC A sends 40Mbps traffic to PC D.
- PC B sends 80Mbps traffic to PC E.
- PC F sends 50Mbps traffic to PC C.
The link connecting both segments would not be able to transmit traffic since the total bandwidth of a link is 100Mbps, while the segments transmit traffic at 170Mbps. A simple solution to this problem is to add another link between both segments with higher bandwidth.
But wait, adding another link reminds me of the Spanning Tree Protocol, which will block a port to remove the loop, and adding another link with higher bandwidth is costly. Along with the cost, a single link cannot provide redundancy.
EtherChannel helps us deal with this situation.
What is Ether Channel?
Ether Channel, also called port channel, is a network technology providing a method of combining multiple physical Ethernet links into a single logical link.
It creates a virtual pipe that can handle more traffic and offers an alternate path in case of failure. Since multiple links get combined virtually as a single link, Spanning Tree Protocol will not block any port. Even if one link fails, other links will continue to exchange traffic, and hence Ether Channel provides redundancy in a network which is impossible if we use a single link with higher bandwidth.
A maximum of 16 physical interfaces get assigned to an Ether Channel, but eight remain active at a time. Other links are in standby mode and become active if any active link fails.
The following conditions should get fulfilled to create an Ether Channel:
- All the links should have the same speed.
- The same VLAN should get configured.
- Duplex mode enabled
- Same switch port mode on both sides (Access mode or Trunk mode)
What are the benefits of using an Ether Channel?
The benefits of using an Ethernet Channel are as follows-
Enhanced Bandwidth
Ether Channel enables the aggregation of multiple links into one by multiplying the total available bandwidth. This boost in bandwidth is very useful in a network where high data throughput is required, such as data centers or organizations with bandwidth-intensive applications.
Load Balancing
Ether Channel can help distribute traffic across the bundled links using load-balancing. This distribution prevents bottlenecks and optimizes resource utilization, resulting in improved network performance and reduced congestion.
Redundancy and Resilience
Ether Channel offers fault tolerance by providing link redundancy. If a link fails, traffic can automatically reroute through the remaining links, ensuring uninterrupted connectivity and minimizing the impact of link failures. This redundancy also extends to network devices, where multiple switches or routers can get interconnected via Ether Channel for increased reliability.
How to configure an EtherChannel?
Two protocols can help us to configure an EtherChannel.
Port Aggregation Protocol (PAgp)
It is a Cisco proprietary protocol and only runs on Cisco-licensed switches and cannot be used on other vendor switches. It enables the automatic creation of EtherChannel links by recognizing the link configuration on each side, ensuring that the links are compatible to create an EtherChannel.
Port Aggregation Protocol has three modes:
- On: – In this mode, EtherChannel is enabled, but there is no exchange of PAgp packets.
- Desirable: – Interface sends Port Aggregation Protocol packets and negotiates with each other to enable EtherChannel.
- Auto: – In auto mode, the interface only responds to PAgp packets but cannot negotiate.
The below table shows the combination of different modes that can enable the EtherChannel link-
S1 | S2 | EtherChannel Formation |
Auto | Auto | No |
Auto | Desirable | Yes |
Desirable | Desirable | Yes |
Link Aggregation Control Protocol
LACP is an open standard protocol. Since it is an open standard protocol, it can be enabled and used in multivendor devices and is not limited to Cisco switches.
It has three modes:
- On: – In this mode, EtherChannel is enabled, but there is no exchange of PAgP packets.
- Active: – Interface sends Link Aggregation Control packets and negotiates with each other to enable EtherChannel.
- Passive: – In auto mode, the interface only responds to LACP packets but cannot negotiate.
The below table shows the combination of different modes that can enable the EtherChannel link-
S1 | S2 | EtherChannel Formation |
Active | Active | Yes |
Active | Passive | Yes |
Passive | Passive | No |
Now let us take a look at this basic network topology and configure an Ether Channel-
Configuration
S1
Let us check the initial states of the port on S1
Switch#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.6354.77A9
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0001.6354.77A9
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio. Nbr Type
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
We can see that it is a root bridge, and hence both ports are in forwarding mode
Configuring Ether Channel: –
Switch#config terminal
Switch(config)#hostname S1
S1(config)#interface range fa0/1-2
S1(config-if-range)#switchport mode trunk
S1(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
{Above command shows the types of mode}
S1(config-if-range)#channel-group 1 mode active { Configured Active mode }
Creating a port-channel interface Port-channel 1
Let’s verify the Ether Channel
S1#show etherchannel summary
Flags: D – down P – 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
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 Fa0/1(P) Fa0/2(P)
Let us now check the status of the Spanning Tree protocol after configuring Ether Channel-
S1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.6354.77A9
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0001.6354.77A9
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio. Nbr Type
Po1 Desg FWD 9 128.27 Shr
After configuring Ether Channel, both the ports are looking like single port.
S2
Switch#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.6354.77A9
Cost 19
Port 1(FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0001.6428.2755
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio. Nbr Type
Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Altn BLK 19 128.2 P2p
As this is a non-root bridge, one port of this switch i.e., Fa0/2 is in a blocked state.
Switch(config)#hostname S2
S2(config)#interface range fa0/1-2
S2(config-if-range)#switchport mode trunk
S2(config-if-range)#channel-group 1
S2(config-if-range)#channel-group 1 mode active
Creating a port-channel interface Port-channel 1
Let’s verify the Ether Channel
S2#show etherchannel summary
Flags: D – down P – 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
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 Fa0/1(P) Fa0/2(P)
S2#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.6354.77A9
Cost 9
Port 27(Port-channel1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0001.6428.2755
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio. Nbr Type
Po1 Root LRN 9 128.27 Shr
Now, both the ports look like one single port, and hence no port is blocked.
