Cisco - Aggregating Switch Links

Switch Port Aggregation with Ether Channels

• Under normal circumstances STP will only allow a single link to forward traffic
• Etherchannels bundle multiple parallel links to at a single link
• Between 2 - 8 Fast, Gigabit or 10Gb ethernet interfaces can be bundled to give full-duplex bandwidths of:
  • FEC - Up to 1600Mbps
  • GEC - Up to 16Gbps
  • 10GEC - Upto 160Gbps
• The bundled link acts as a single access or trunk link
• When created on Cisco devices the interface type is known as a “Port-Channel”
• Traffic is distributed based on the defined load balancing algorithm
• Traffic will be distributed across the links but not necessarily equally
• Muliple links increase redudancy with the STP port cost adjusted according
• STP reconvergence is only needed as a last resort as long as it least one link in the EtherChannel is available
• If the platform supports switch stacking, the EtherChannel can be distributed across multiple stack members
• Higher end switches that support Virtual Switching System (VSS) can form the EtherChannel across the switch chassis’s into a Multi-Chassis EtherChannel (MEC)
• All interfaces that are combined into a single EtherChannel must have identical settings
• Etherchannel can be used on both Switched (Layer 2) and Routed (Layer 3) ports

Distributing Traffic In An EtherChannel

• Traffic is distributed over all the links in the bundle in a deterministic way
• Traffic is not ballanced equally depending on the trafffic flow
• Hashing Algorithm used takes into account
  • Source/Destination IP addresses
  • Source/Destination MAC Addresses
  • Source/Destination TCP/UDP Port Numbers
• Output of the algorithm determines the link to be used
• An XOR operation on one or more lower order bits is used depending on the number of links in the bundle
  • 2 Links - 1-bit XOR operation
  • 4 Links - 2-bit XOR operation
  • 8 Links - 3-bit XOR operation
• Host-To-Host communication always goes over sam3 link as long as addresses remain constant
• Load balancing is globally defined, not per port
• All models can load balance using MAC and IP addresses
• Higher End switches (4500, 6500) can also use TCP/UDP port numbers
• When using a router-to-router (layer 3) connection, consider using IP instead of MAC addresses for better distribution
• If non-IP traffic is seen, load balancing based on MAC addresses is used automatically

EtherChannel Negotiation Protocols

• EtherChannels can be negotiated for use when needed
• Two protocols are available
  • Port Aggregation Protocol (PAgP) - Cisco Proprietary
  • Link Aggregation Control Protocol (LCAP) - Standards Based
• When negotiation is used, an additinal 15 second delay may occur
• Configure the negotion mode to “on” to not use negotiation

Port Aggregation Protocol (PAgP)

• Cisco Proprietary protocol
• EtherChannel can only be formed when identical parameters configued
• Member port config is modified if EtherChannel is changed
• Port modes
  • Desirable - Actively try to establsh an EtherChannel
  • Auto - Form an EtherChannel but only if requested to do so

Link Aggregation Control Protocol (LACP)

• Standards Based on IEEE 802.3ad
• Up to 16 links can be configured in the EtherChannel but only 8 active at any one time
• Switch which has the lowest system priority (2-bytes) chooses which ports are active in the EtherChannel
• Links selected are based on port priority
• Unused ports put in standby state
• Port modes
  • Active - Try to establish EtherChannel with neighbour switch
  • Passive - Wait for neighbour switch to request negotiation of EtherChannel

Avoiding Misconfiguration with EtherChannel Guard

• Enabled by default
• Issues unlikely to occur when PAgp/LACP are used
• Likely to be an if configured to EtherChannel unconditionally
• Ports are port inn errdisable state if misconiguration (i.e. cabling) issue detected

Troubleshooting An EtherChannel

• Ensure both switches are configured for unconditional etherchannel or one should actively to negotiate
• Ensure configuration is consistent across all ports to be bundled on both switches
• Check cabling is correct

EtherChannel Configuration

Global Configuration

Set the Load Balancing Method

port-channel load-balance <method>

Verify Load Balancing Method

show etherchannel load-balance

Disable/Enable EtherChannel Guard

[no] spanning-tree etherchannel guard misconfig

Check if ports are disabled due to EtherChannel Guard

show interface status err-disabled

PAgP EtherChannel Configuration

interface <name>
  channel-protcol pagp
  channel-group <id> mode {on | {{auto | desirable} [non-silent]}}

LACP EtherChannel Configuration

lacp system-priority <priority>

interface <name>
  channel-protocol lacp
  channel-group <id> mode {on | passive | active}
  lacp port-priority <priority>

Troubleshooting EtherChannels

Enable interface following misconfiguration

interface port-channel<id>
  shutdown
  no shutdown

Status/Validation Commands

show etherchannel summary
show etherchannel {port | port-channel | detail}
show running-config interface <name>
show inteface <name> etherchannel
show {pagp|lacp} neigbor
show lacp sys-id