Learn Computer Networking PNETLab - Link Aggregation with EtherChannel (LACP & PAgP)

#Introduction

In episode 7 you made sure your Layer 2 network was loop-free with STP. Now we add another layer of reliability: link aggregation. Instead of letting two switches connect with a single cable, we combine several cables into one logical channel — that is EtherChannel.

Episode 8 discusses the EtherChannel concept, the LACP and PAgP negotiation protocols, and Layer 2 and Layer 3 EtherChannel configuration in PNETLab. This concept becomes the backbone of the core design in episode 20 later.

#The EtherChannel / Link Aggregation Concept

#Combining Physical Cables into a Single Logical Channel

EtherChannel combines up to 8 physical links into a single logical channel called a Port-Channel. The benefit is twofold: the total bandwidth becomes the sum of all links, and if one link fails, traffic is automatically rerouted to the other links without downtime.

It must be emphasized: EtherChannel is not just plugging in many cables. All links in one channel must have identical configuration (speed, duplex, and VLANs) and must connect to the same device. STP treats the port-channel as a single interface, so no loop is formed.

#EtherChannel Negotiation Protocols

#LACP: Open Standard (IEEE 802.3ad)

LACP (Link Aggregation Control Protocol) is an open standard used across vendors. Two modes:

• active: the switch actively initiates negotiation.
• passive: the switch waits for negotiation from the other side.

Valid pairs: active with active, or active with passive.

#PAgP: Cisco Proprietary

PAgP (Port Aggregation Protocol) is a Cisco-owned protocol:

• desirable: actively offers aggregation.
• auto: waits for an offer.

Besides those two protocols there is the on mode, which forms a channel statically without negotiation. As best practice, use LACP because it is an open standard — your PNETLab lab can be connected to non-Cisco devices.

#EtherChannel Configuration on a Cisco Switch

#Layer 2 EtherChannel

In PNETLab, build two switches connected by two cables. Configure the channel on SW1:

configure terminal
interface range g0/1 - 2
 channel-group 1 mode active
exit
interface port-channel 1
 switchport mode trunk
 switchport trunk allowed vlan 10,20
exit

channel-group 1 mode active adds both interfaces to port-channel 1 with active LACP negotiation. The trunk configuration is then applied to interface port-channel 1, not to each physical link.

#Layer 3 EtherChannel

For links between routers or between Layer 3 switches, build the EtherChannel as a routed interface:

configure terminal
interface range g0/1 - 2
 no switchport
 channel-group 1 mode active
exit
interface port-channel 1
 ip address 10.1.1.1 255.255.255.252
exit

no switchport changes the interfaces to Layer 3, then the IP address is applied to the port-channel. The opposite side uses 10.1.1.2 in the same /30 subnet.

#Verifying EtherChannel

#show etherchannel summary

Check whether the channel formed correctly:

SW1# show etherchannel summary
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)       LACP        Gi0/1(P)  Gi0/2(P)

show etherchannel summary displays the Port-Channel status: SU means Layer 2 is up, and the (P) mark indicates the ports are bundled. If you see (D) or (S), check the configuration on both sides of the channel.

#Common EtherChannel Mistakes

Three mistakes most often prevent a channel from ever coming up: incompatible configuration between the two sides (one LACP, one PAgP), different speeds or duplex between the links, and using different trunk modes at both ends. Before checking cables, always run show etherchannel summary and make sure the status is SU on both switches.

#Application in Enterprise Design

In a common core-access design, two access switches are aggregated to the core with dual EtherChannels. Combining EtherChannel with STP from episode 7 and HSRP redundancy from episode 14 produces an office network resilient to single link failures. You will assemble this scenario in full in episode 20.

#Closing

Key takeaways:

• EtherChannel combines up to 8 physical links into a single port-channel.
• LACP open standard has active and passive modes; Cisco PAgP has desirable and auto.
• All links in one channel must have identical configuration.
• Layer 2 EtherChannel is used for trunks; Layer 3 for routed links.
• Trunk and IP configuration goes on the port-channel, not the physical links.
• show etherchannel summary is the main verification of bundling status.

In the next episode, episode 9, we move into full Layer 3: routing concepts and static routing — how the routing table works, the administrative distance and metric components, the longest prefix match principle, configuring static routes between three routers, a default route toward an ISP, and a floating static route as an automatic backup path.