Monday, November 17, 2014

Cisco IOS Layer 3 EtherChannel and Jumbo Frames


In this post I'll be explaining how to create a Layer 3 EtherChannel on a Cisco IOS-based, multilayer switch.  The EtherChannel will provide Layer 3 load balancing for network attached storage (NAS) in a multi-segmented (VLAN's) network environment.  Additionally, I'll show how to enable Ethernet jumbo frames for the NAS.


Figure 1 below depicts an example LAN environment.  The LAN is segmented into multiple VLAN's and terminated into a multilayer switch.  An EtherChannel is created for a NAS to provide link redundancy and additional through-put for mixed traffic.  As a reminder, EtherChannel does not increase available bandwidth for a single host.

Figure 1


Create a Port Channel Interface

1:  interface Port-channel1  
2:   no switchport  
3:   ip address  

Line 1:  Creates a logical port-channel interface.
Line 2:  The port-channel needs to be specified as a non-switched interface (Layer 3).  Any physical interfaces in the port-channel also need to be non-switched.
Line 3:  Assigns an IP address to the interface.  That address cannot overlap with any existing network segments.

Assign Physical Interfaces to the Port-Channel

1:  interface GigabitEthernet0/17  
2:   no switchport  
3:   no ip address  
4:   channel-group 1 mode active  
5:  !  
6:  interface GigabitEthernet0/18  
7:   no switchport  
8:   no ip address  
9:   channel-group 1 mode active  

Lines 1 & 6:  Two gigabit Ethernet interfaces are selected for the EtherChannel.
Lines 2 & 7:  As mentioned previously, these need to be specified as non-switched interfaces.
Lines 3 & 8:  No Layer 3 address is assigned to these either.
Lines 4 & 9:  Assign each interface to the port-channel number (1 in this case).  The 'active' command indicates the interfaces will actively negotiate LACP.

Configure Load Balancing

IOS provides six different load balancing algorithms for EtherChannel.  All of them consist of hashing a source or destination MAC or IP address to one of the channel members.  The command below XOR's the source and destination IP addresses for that hash calculation.

 port-channel load-balance src-dst-ip  

Configure Routing

Now to be able to reach the NAS, you'll need to assign the port-channel IP address as the default gateway on the NAS.  Additionally, you'll need to configure a route for the NAS/port-channel segment to enable access outside of the LAN.

Configure Jumbo Frames

For 3500 series switches, MTU sizing is set globally.  Other models allow per-interface configuration.

1:  switch(config)#system mtu jumbo 9000  
2:  switch(config)#system mtu routing 9000  

Line 1:  Sets the jumbo frame size to 9000 bytes.  If you're using Layer 2 only for your EtherChannel, this setting is all that is required for jumbo frame support.
Line 2:  Sets the Layer 3 max MTU size to 9000 bytes.  This is necessary given Layer 3 is being used for this EtherChannel example.

A switch reboot will be required before these new MTU sizes take effect.

You can display the resulting MTU sizes with the command below:

 switch#show system mtu  
 System MTU size is 1500 bytes  
 System Jumbo MTU size is 9000 bytes  
 System Alternate MTU size is 1500 bytes  
 Routing MTU size is 9000 bytes  

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