Journey to CCIE LAB Exam – Part 20 – EtherChannels

It has been a long winding path so far.Everyday the road does get steeper and harder but again so is my determination.
Am now working on EtherChannels.I decided to go through switching from the basics as i go up to more advanced stuff.Reason why i chose to start with the Khawar butt LAB book was for me to have a complete over-view of the whole CCIE LAB expectations before i start drilling to the main stuff..More of feeling the marathon track before embarking on how to win the race.

Here is what i have been reading on :(Ref Cisco Press)

EtherChannel provides fault-tolerant high-speed links between switches, routers,
and servers. You can use it to increase the bandwidth between the wiring closets and the data center, and
you can deploy it anywhere in the network where bottlenecks are likely to occur. EtherChannel provides
automatic recovery for the loss of a link by redistributing the load across the remaining links. If a link
fails, EtherChannel redirects traffic from the failed link to the remaining links in the channel without

An EtherChannel consists of individual Fast Ethernet or Gigabit Ethernet links bundled into a single
logical link. The EtherChannel provides full-duplex bandwidth up to 800 Mbps
(Fast EtherChannel) or 8 Gbps (Gigabit EtherChannel) between your switch and another switch or host.

Each EtherChannel can consist of up to eight compatibly configured Ethernet interfaces. All interfaces
in each EtherChannel must be the same speed, and all must be configured as either Layer 2 or Layer 3
Note:The network device to which your switch is connected can impose its own limits on the number of
interfaces in the EtherChannel. For Catalyst 3550 switches, the number of EtherChannels is limited to
the number of ports of the same type.

If a link within an EtherChannel fails, traffic previously carried over that failed link changes to the
remaining links within the EtherChannel. A trap is sent for a failure, identifying the switch, the
EtherChannel, and the failed link. Inbound broadcast and multicast packets on one link in an
EtherChannel are blocked from returning on any other link of the EtherChannel.

Understanding Port-Channel Interfaces
You create an EtherChannel for Layer 2 interfaces differently from Layer 3 interfaces. Both
configurations involve logical interfaces.
• With Layer 3 interfaces, you manually create the logical interface by using the interface
port-channel global configuration command.
• With Layer 2 interfaces, the logical interface is dynamically created.
• With both Layer 3 and 2 interfaces, you manually assign an interface to the EtherChannel by using
the channel-group interface configuration command. This command binds the physical and logical
ports together.

When a port joins an EtherChannel, the physical interface for that port is shut down. When the port
leaves the port-channel, its physical interface is brought up, and it has the same configuration as it had
before joining the EtherChannel.

Exchanging LACP Packets
Both the active and passive LACP modes allow interfaces to negotiate with partner interfaces to
determine if they can form an EtherChannel based on criteria such as interface speed and, for Layer 2
EtherChannels, trunking state and VLAN numbers.
Interfaces can form an EtherChannel when they are in different LACP modes as long as the modes are
compatible. For example:
• An interface in the active mode can form an EtherChannel with another interface that is in the active
or passive mode.
• An interface in the active mode can form an EtherChannel with another interface in the passive
An interface in the passive mode cannot form an EtherChannel with another interface that is also in the
passive mode because neither interface starts LACP negotiation.
An interface in the on mode that is added to a port channel is forced to have the same characteristics as
the already existing on mode interfaces in the channel.

PAgP and LACP Interaction with Other Features
The Dynamic Trunking Protocol (DTP) and Cisco Discovery Protocol (CDP) send and receive packets
over the physical interfaces in the EtherChannel. Trunk ports send and receive PAgP and LACP protocol
data units (PDUs) on the lowest numbered VLAN.
Spanning tree sends packets over the first interface in the EtherChannel.
The MAC address of a Layer 3 EtherChannel is the MAC address of the first interface in the
PAgP sends and receives PAgP PDUs only from interfaces that have PAgP enabled for the auto or
desirable mode. LACP sends and receives LACP PDUs only from interfaces that have LACP enabled for
the active or passive mode.

Understanding Load Balancing and Forwarding Methods

EtherChannel balances the traffic load across the links in a channel by randomly associating a
newly-learned MAC address with one of the links in the channel.
With source-MAC address forwarding, when packets are forwarded to an EtherChannel, they are
distributed across the ports in the channel based on the source-MAC address of the incoming packet.
Therefore, to provide load balancing, packets from different hosts use different ports in the channel, but
packets from the same host use the same port in the channel (and the MAC address learned by the switch
does not change).
When source-MAC address forwarding is used, load distribution based on the source and destination IP
address is also enabled for routed IP traffic. All routed IP traffic chooses a port based on the source and
destination IP address. Packets between two IP hosts always use the same port in the channel, and traffic
between any other pair of hosts can use a different port in the channel.
With destination-MAC address forwarding, when packets are forwarded to an EtherChannel, they are
distributed across the ports in the channel based on the destination host’s MAC address of the incoming
packet. Therefore, packets to the same destination are forwarded over the same port, and packets to a
different destination are sent on a different port in the channel. You configure the load balancing and
forwarding method by using the port-channel load-balance global configuration command.
In Figure 29-3, multiple workstations are connected to a switch, and an EtherChannel connects the
switch to the router. Source-based load balancing is used on the switch end of the EtherChannel to ensure
that the switch efficiently uses the bandwidth of the router by distributing traffic from the workstation
across the physical links. Since the router is a single MAC address device, it uses destination-based load
balancing to efficiently spread the traffic to the workstations across the physical links in the
Use the option that provides the greatest variety in your configuration. For example, if the traffic on a
channel is going only to a single MAC address, using the destination-MAC address always chooses the
same link in the channel; using source addresses or IP addresses might result in better load balancing.

EtherChannel Configuration Guidelines
If improperly configured, some EtherChannel interfaces are automatically disabled to avoid network
loops and other problems. Follow these guidelines to avoid configuration problems:
• Configure an EtherChannel with up to eight Ethernet interfaces of the same type.
• Configure all interfaces in an EtherChannel to operate at the same speeds and duplex modes.
• Enable all interfaces in an EtherChannel. An interface in an EtherChannel that is disabled by using
the shutdown interface configuration command is treated as a link failure, and its traffic is
transferred to one of the remaining interfaces in the EtherChannel.
• When a group is first created, all ports follow the parameters set for the first port to be added to the
group. If you change the configuration of one of these parameters, you must also make the changes
to all ports in the group:
– Allowed-VLAN list
– Spanning-tree path cost for each VLAN
– Spanning-tree port priority for each VLAN
– Spanning-tree Port Fast setting
• An EtherChannel interface that is configured as a Switched Port Analyzer (SPAN) destination port
does not join the group until it is deconfigured as a SPAN destination port.Do not configure a port
that belongs to an EtherChannel port group as a secure port
• For Layer 2 EtherChannels:
– Assign all interfaces in the EtherChannel to the same VLAN, or configure them as trunks.
Interfaces with different native VLANs cannot form an EtherChannel.
– If you configure an EtherChannel from trunk interfaces, verify that the trunking mode (ISL or
802.1Q) is the same on all the trunks. Inconsistent trunk modes on EtherChannel interfaces can
have unexpected results.
– An EtherChannel supports the same allowed range of VLANs on all the interfaces in a trunking
Layer 2 EtherChannel. If the allowed range of VLANs is not the same, the interfaces do not
form an EtherChannel even when PAgP is set to the auto or desirable mode.
– Interfaces with different spanning-tree path costs can form an EtherChannel if they are
otherwise compatibly configured. Setting different spanning-tree path costs does not, by itself,
make interfaces incompatible for the formation of an EtherChannel.
• For Layer 3 EtherChannels, assign the Layer 3 address to the port-channel logical interface, not to
the physical interfaces in the channel.

Next post shall be on the logical configurations :).For now am a bit sleepy.. 🙂

About CCIE R&S :- IP/MPLS Networks Consultant

>>>A full CCIE (R&S) from March 18th 2013 >>>Passionate about IP networks. >>>>A consultant plying his trade across the world. >>> Currently digging deeper and deeper into Network Automation with Python Language
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