Category Archives: 1.5 Configure and verify EtherChannels

SWITCH 300-115 1.5 Configure and verify EtherChannels

1.5.d EtherChannel misconfiguration guard
Etherchannel misconfiguration guard is enabled by default and does what it says it does; helps prevent misconfiguration of etherchannels.
We know that the interfaces we bundle into a channel need to have matching configurations or they will not be suitable, but often enough they are mistakenly put together in a hurry without verifying both sides interfaces first. Etherchannel misconfiguration guard will place the channel in errdissable state and issues an error message if it detects a possible misconfiguration.
To verify that etherchannel guard misconfig is in place as the default use:
sh spann summ | i Ether
If you do break the etherchannel by purposely misconfiguring, or not, you can reenable the channel with shut/no shut or by adjusting the errdisable recovery time interval.
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1.5 Configure and verify EtherChannels

 

1.5.c Load balancing

by default load balancing is:

SW3#sh etherchannel load
EtherChannel Load-Balancing Configuration:
src-mac    

EtherChannel Load-Balancing Addresses Used Per-Protocol:
Non-IP: Source MAC address                                                      
  IPv4: Source MAC address                                                      
  IPv6: Source MAC address    

from Catalyst 3750 Switch Software Configuration Guide 1-8

EtherChannel balances the traffic load across the links in a channel by reducing part of the binary pattern formed from the addresses in the frame to a numerical value that selects one of the links in the channel.
EtherChannel load balancing can use MAC addresses or IP addresses, source or destination addresses, or both source and destination addresses. The selected mode applies to all EtherChannels configured on the switch. You configure the load balancing and forwarding method by using the port-channel load-balance global configuration command.

With source-MAC address forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the channel based on the source-MAC address of the incoming packet. Therefore, to provide load balancing, packets from different hosts use different ports in the channel, but packets from the same host use the same port in the channel.

With destination-MAC address forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the channel based on the destination host’s MAC address of the incoming packet. Therefore, packets to the same destination are forwarded over the same port, and packets to a different destination are sent on a different port in the channel.
With source-and-destination MAC address forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the channel based on both the source and destination MAC addresses.

This forwarding method, a combination source-MAC and destination-MAC address forwarding methods of load distribution, can be used if it is not clear whether source-MAC or destination-MAC address forwarding is better suited on a particular switch.

With source-and-destination MAC-address forwarding, packets sent from host A to host B, host A to host C, and host C to host B could all use different ports in the channel.
With source-IP address-based forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the EtherChannel based on the source-IP address of the incoming packet.
Therefore, to provide load-balancing, packets from different IP addresses use different ports in the channel, but packets from the same IP address use the same port in the channel.
With destination-IP address-based forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the EtherChannel based on the destination-IP address of the incoming packet. Therefore, to provide load-balancing, packets from the same IP source address sent to different IP destination addresses could be sent on different ports in the channel. But packets sent from different source IP addresses to the same destination IP address are always sent on the same port in the channel.
With source-and-destination IP address-based forwarding, packets are sent to an EtherChannel and distributed across the EtherChannel ports, based on both the source and destination IP addresses of the incoming packet. This forwarding method, a combination of source-IP and destination-IP address-based forwarding, can be used if it is not clear whether source-IP or destination-IP address-based forwarding is better suited on a particular switch. In this method, packets sent from the IP address A to IP address B, from IP address A to IP address C, and from IP address C to IP address B could all use different ports in the channel.

Different load-balancing methods have different advantages, and the choice of a particular
load-balancing method should be based on the position of the switch in the network and the kind of traffic that needs to be load-distributed.

SWITCH 300-115 1.5 Configure and verify EtherChannels

1.5.b Layer 2, Layer 3
Surprisingly, the Switch blueprint does not mention routed ports or SVI’s (switched virtual interfaces) specifically. However, in this section we get the first hint of Layer 3 on a switch so now is a good time to expound a bit. Naturally, the Certification Guides and  Foundation Learning Guides are indispensible throughout the certification process, and while the blueprint is a great road map, it may not mention everything. So you need a bit of everything and for that there is a whole internet out there to be had. So go forth and be all up inside the internet.
Multilayer switching is what it says it is; switches can support more layers than layer 2. Simply put, the switch is a router.
Multiple access ports can be assembled into one Vlan and share a common Gateway address to other layer 3 capable devices. This describes a Switched Virtual interface.
The three hallmarks of SVI creation are:
make a vlan
make an interface vlan, assign it an ip address and turn it up with no shut
and make sure to assign at least 1 port to the vlan.
By default an SVI is created for you, Vlan 1, you just need to give it an address, and you know by default all ports belong to Vlan 1 out of the box.
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SWITCH 300-115 1.5 Configure and verify EtherChannels

1.5.a LACP, PAgP, manual
Etherchannel comes in three flavors, on, my preference often called static or manual,  whereby there is no negotiation of the channel and thus, no extra protocol traffic, PAGP or port aggregation protocol which is Cisco proprietary, and LACP or link aggregation protocol which is the open standard 802.3ad. It is interesting to note that PAGP is not supported on Cisco’s Nexus OS line and that LACP is the preferred method of aggregation from the data center side of the house.
They all have one thing in common, however, and that is to bundle together a group of ports on a switch with the net effect of increasing bandwidth between two connected switches. For instance switches will support 2 to  8 active members on each side of a connection; so considering they are 100 Meg each, a total of 1600 Meg bandwidth can be achieved between them.

Another 8 ports may be used as backup but only 8 may be active at one time. The aggregated ports of a channel must be setup on the individual switch but the ports do not have to be contiguous, and they can cross modules in the event of a chassis type switch or a stack. Also the channel numbers on either side do not have to match, but I advise making it a practice of matching the sides as it makes for easier documentation, and more intuitive troubleshooting.

Another inherent benefit is lessening the impact of Spanning-tree on the network. The etherchannel or port-channel is treated as a single link by STP, therefore there can be no blocking of individual links within the channel, although multiple redundant channels between switches would still be governed by spanning-tree.
A caveat in the creation of a channel is that both sides port configurations need to be exactly the same or the channel will not form. This is one reason why care should be taken when using on or manual mode because without negotiation, there will be no warning in the event of misconfiguration. Another thing to keep in mind is that there is no mixing of channel protocols. For instance, PAGP can use desirable/auto, desirable/desirable, but not auto/auto, similar to DTP. LACP can be act/passive, active/active but not passive/passive. On mode is simply on for both sides.
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