VLAN Trunks and EtherChannels
VLAN Trunking Protocol (VTP) allows for a server client approach at provisioning VLANs in a topology. There are 4 modes Server, Client, Transparent, and Off(with Off a switch will not forward VTP traffic)
VTP mode 1 and 2 are limited to VLANs 1 – 1005 VTP mode 3 can use VLANs 1 – 4094
There can be multiple VTP servers in an environment. If using version 3 the primary VTP server must use the following command.
vtp primary
Sample configuration VTP Primary server config
vtp primary
!
conf t
vtp version 3
vtp domain DOMAIN_NAME
vtp mode server
vtp password password
Sample configuration VTP client config
conf t
vtp version 3
vtp domain DOMAIN_NAME
vtp mode client
vtp password password
Dynamic Trunking Protocol (DTP) allows 2 ports to dynamically set switch ports to a trunk port. requires that the VTP domain matches between switches. The following table outlines possible states depending on port configuration.
DTP Negotiated Interface Modes
Dynamic Auto Dynamic Desirable Trunk Access
Dynamic Auto Access Trunk Trunk Access
Dynamic Desirable Trunk Trunk Trunk Access
Trunk Trunk Trunk Trunk Limited connectivity
Access Access Access Limited connectivity Access
Configuration
conf t
int gi1/0/1
switchport mode dynamic auto
conf t
int gi1/0/1
switchport mode dynamic desirable
To disable use the following negotiation of trunking
conf t
int gi1/0/1
switchport nonegotiate
Best practice is to configure both sides of a link the same to remove any question about the ports function.
EtherChannels
802.3AD or link aggregation specification. There are 2 common link aggregation protocols; Link Aggregation Control Protocol(LACP) and Por Aggregation Protocol(PAgP) LACP is an open standard and PAgP is a Cisco proprietary protocol. PAgP Mode; uses multicast MAC 0100:0CCCC:CCCC Auto: the interface does not initiate the etherchannel Desirable: the interface does try initiate the etherchannel
LACP Mode; uses multicast MAC 0180:C200:0002 Passive: the interface does not initiate the etherchannel Active: the interface does try initiate the etherchannel
Static Mode Static mode is configured with the use of the “mode on” keyword.
The following command shows detailed information about the port-channel
Show etherchannel port
For information useful to LCAP
Show lacp neighbor
For information useful to PAgP
Show pagp neighbor
LACP Tuning
LACP fast: is an amendment to the LACP standard to send LACP packets every second instead of every 30 seconds. This way a link can be removed with in 3 seconds instead of 90 seconds. All ports must be configured the same. Example config below
Conf t
Int ran gi1/0/1 – 2
lcap rate fast
You can configure a minimum numbers of links needed to consider the port channel up
Conf t
Int port-channel 1
Port-channel min-links 2
You can also set the max number of links allowed in a port channel
Conf t
Int port-channel 1
lacp max-bundle 1
LACP system priority identifies which switch is the master for a port channel. This can be defined as follows
Conf t
Lacp system-priority 1
Show lacp sys-id #to verify change
Load Balancing
Traffic is not forwarded via a round robin per packet basis. Packets are consistently sent down the same link based on a hash that is calculated. This can be configured via the following commands
Port-channel load-balance {dst-ip | dst-mac | src-dst-ip | src-dst-mac | src-ip | src-mac}
!
Show etherchannel load-balance