Category Archives: CCIE Data Center

CCIE DC Written – 1.1.a Link Aggregation – LACP

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NX-OS

Here are some Nexus facts to keep in mind:

  • With LACP, you can bundle up to 16 interfaces in a channel group. If the channel group has more than 8 interfaces, the remaining interfaces are in hot standby for the port channel associated with this channel group on the M-series modules.
  • From Cisco NX-OS Release 5.1, you can bundle up to 16 active links into a port channel on the F-series module.
  • When you delete the port channel, the software automatically deletes the associated channel group. All member interfaces revert to their original configuration.
  • You cannot disable LACP while any LACP configurations are present.
  • When you run static port channels with no aggregation protocol, the channel mode is always set to on.

Of course, you must globally enable LACP before you can use it on the Nexus device. There are two modes:

  • Passive – responds to negotiations, but does not initiate them – sounds like me at the High School dance
  • Active – initiates negotiations

Starting at 4.2(3) – Cisco introduced some LACP compatibility enhancements as follows:

  •  When a Cisco Nexus device is connected to a non-Nexus peer, its graceful failover defaults may delay the time taken for a disabled port to be brought down or cause traffic from the peer to be lost. To address these conditions, the lacp graceful-convergence command was added.
  • By default, LACP sets a port to the suspended state if it does not receive an LACP PDU from the peer. In some cases, although this feature helps in preventing loops created due to misconfigurations, it can cause servers to fail to boot up because they require LACP to logically bring up the port. You can put a port into an individual state by using the lacp suspend-individual command.

Starting with Release 5.1 Cisco introduced the Minimum Links feature as well as MaxBundle. The Minimum Links feature allows you to:

  • Configure the min number of links that must be in the bundle
  • Prevent low bandwidth LACP bundles from becoming available
  • Causes the port channel to go inactive if the required min bandwidth is not available

MaxBundle allows:

  • Upper limit on ports that are bundled
  • Allows the designation of ports as hot standby

Basic Configuration

  • Use feature lacp to enable the feature
  • Create the port channel interface with interface port-channel 10, use the switchport command in the interface
  • Add a Layer 2 interface to the port channel with switchport followed by channel-group 10 mode passive

 

CCIE DC Written – 1.1.a Link Aggregation – Load Balancing

CCIE Data Center, ,

Port Channels

The Cisco NX-OS software load balances traffic across all operational interfaces in a port channel by hashing the addresses in the frame to a numerical value that selects one of the links in the channel. Port channels provide load balancing by default. Port-channel load balancing uses MAC addresses, IP addresses, or Layer 4 port numbers to select the link. Port-channel load balancing uses either source or destination addresses or ports, or both source and destination addresses or ports.

You can configure the load-balancing mode to apply to all port channels that are configured on the entire device or on specified modules. The per-module configuration takes precedence over the load-balancing configuration for the entire device. You can configure one load-balancing mode for the entire device, a different mode for specified modules, and another mode for the other specified modules. You cannot configure the load-balancing method per port channel.

The default load-balancing mode for Layer 3 interfaces is the source and destination IP address (src-dst-ip), and the default load-balancing mode for non-IP traffic is the source and destination MAC address (src-dst-mac). Use the port-channel load-balance  command to set the load-balancing method among the interfaces in the channel-group bundle.

NOTE: F1-series modules do not support load balancing of non-IP traffic based on a MAC address. If ports on an F1-series module are used in a port channel and non-IP traffic is sent over the port channel, Layer 2 traffic might get out of order.

Here are the additional load balancing methods you can choose from:

  •  Destination MAC address
  • Source MAC address
  • Source and destination MAC address
  • Destination IP address
  • Source IP address
  • Source and destination IP address
  • Source TCP/UDP port number
  • Destination TCP/UDP port number
  • Source and destination TCP/UDP port number

Symmetric Hashing and Random Load Balancing

Here are a couple of other related topics you should be aware of:

Symmetric Hashing – To effectively monitor traffic on a port channel, it is essential that each interface connected to a port channel receives both forward and reverse traffic flows.

Normally, there is no guarantee that the forward and reverse traffic flows will use the same physical interface. However, when you enable symmetric hashing on the port channel, bidirectional traffic is forced to use the same physical interface and each physical interface in the port channel is effectively mapped to a set of flows. When symmetric hashing is enabled, the parameters used for hashing, such as the source and destination IP address, are normalized before they are entered into the hashing algorithm. This process ensures that when the parameters are reversed (the source on the forward traffic becomes the destination on the reverse traffic), the hash output is the same. Therefore, the same interface is chosen.

Random Load Balancing – Random load balancing on port channels is a software solution that enables better port-link bandwidth utilization for GPRS Tunneling Protocol (GTP) over IP-UDP packets. The existing M1, M2, F1, F2 and F2e line card hardware does not have the capability to perform random load balancing and hence, this software solution helps in load balancing and optimizing the port channels bandwidth. Random load balancing is supported only on F3 series line cards. Random load balancing is applicable on all types of traffic and is effective on egress ports of Layer 3 traffic. The Cisco NX-OS software does random load balancing of all traffic across all interfaces in a port channel by using polynomial scheme.