Load Balacing
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It is possible to use nginx as a very efficient HTTP load balancer todistribute traffic to several application servers and to improveperformance, scalability and reliability of web applications with nginx.
In the example above, there are 3 instances of the same applicationrunning on srv1-srv3.When the load balancing method is not specifically configured,it defaults to round-robin.All requests areproxied to the server group myapp1, and nginx applies HTTP loadbalancing to distribute the requests.
In addition, there are more directives and parameters that control serverload balancing in nginx, e.g.proxy_next_upstream,backup,down, andkeepalive.For more information please check ourreference documentation.
Fast EtherChannel allows multiple physical Fast Ethernet links to combine into one logical channel. This allows load sharing of traffic among the links in the channel as well as redundancy in the event that one or more links in the channel fail. Fast EtherChannel can be used to interconnect LAN switches, routers, servers, and clients via unshielded twisted pair (UTP) wiring or single-mode and multimode fiber. This document refers to Fast EtherChannel, Gigabit EtherChannel, port channel, channel and port group with a single term, EtherChannel. The information in the document applies to all of these EtherChannels.
This document presents the concept of load balancing and redundancy on Cisco Catalyst switches with the use of the EtherChannel. This document also covers the Port Aggregation Protocol (PAgP) and trunking support over EtherChannel. This document does not cover how to configure EtherChannel on Catalyst switches. Refer to the documents in the \"Related Information\" section for details on how to configure EtherChannel on Catalyst switches.
In Catalyst 6500/6000 switches that run Catalyst OS (CatOS), EtherChannel aggregates the bandwidth of up to eight compatibly configured ports into a single logical link. With software releases 6.2(1) and earlier, the six- and nine-slot Catalyst 6500 series switches support a maximum of 128 EtherChannels. In software release 6.2(2) and later releases, the spanning tree feature handles the port ID. Therefore, the maximum number of EtherChannels with support is 126 for a six- or nine-slot chassis and 63 for a 13-slot chassis. All Ethernet ports on all modules, which include those on a standby Supervisor Engine, support EtherChannel with no requirement that ports be contiguous or on the same module. All ports in each EtherChannel must be the same speed. You can base the load-balance policy (frame distribution) on a MAC address (Layer 2 [L2]), an IP address (Layer 3 [L3]), or a port number (Layer 4 [L4]). You can activate these policies, respectively, if you issue the set port channel all distribution {ip mac session ip-vlan-session} [source destination both] command. The session keyword is supported on the Supervisor Engine 2 and Supervisor Engine 720. The ip-vlan-session keyword is only supported on the the Supervisor Engine 720. Use this keyword in order to specify the frame distribution method, with the IP address, VLAN, and Layer 4 traffic.
Issue the show etherchannel load-balance command in order to check the frame distribution policy. You can determine which interface in the EtherChannel forwards traffic, with the frame distribution policy as a basis. Issue the remote login switch command to log in remotely to the Switch Processor (SP) console in order to make this determination. Then, issue the test etherchannel load-balance interface port-channelnumber{ip l4port mac} [source_ip_addsource_mac_addsource_l4_port] [dest_ip_adddest_mac_adddest_l4_port] command.
A Catalyst 5500/5000 series switch allows from two to four links to be present per Fast EtherChannel. A connection across a Fast EtherChannel is determined by source/destination address pairs. An XOR mathematical operation is performed on the last two bits of the source MAC address and the destination MAC address. This operation yields one of four results: (0 0), (0 1), (1 0), or (1 1). Each of these values points to a link in the Fast EtherChannel bundle. In the case of a two-port Fast EtherChannel, only a single bit is used in the XOR operation. This scenario yields two possible results, and each points to a link in the bundle. Circumstances can occur where one address in the source/destination pair is a constant. For example, the destination can be a server or, even more likely, a router. In that case, you still see statistical load balancing because the source address is always different. In Cisco IOS Software Release 3.1.1 and later, spanning tree is supported. As far as spanning tree is concerned, a Fast EtherChannel looks like a single bridge port, and bridge protocol data units (BPDUs) are sent down on only one of the links. A Fast EtherChannel that is in blocking mode blocks all ports on that EtherChannel connection.
Catalyst OS for Catalyst 4500/4000 uses MAC address based load balancing. EtherChannel distributes frames across the links in a channel based on the low-order bits of the source and destination MAC addresses of each frame. The frame distribution method is not configurable.
EtherChannel reduces part of the binary pattern that is formed from the addresses in the frame to a numerical value that selects one of the links in the channel in order to balance the traffic load across the links in a channel. EtherChannel load balancing can use MAC addresses, IP addresses, or Layer 4 port numbers and either source mode, destination mode, or both. Use the option that provides the greatest variety in your configuration. For example, if the traffic on a channel only goes to a single MAC address, use of the destination MAC address results in the choice of the same link in the channel each time. Use of source or IP addresses can result in a better load balance. Issue the port-channel load-balance {src-mac dst-mac src-dst-mac src-ip dst-ip src-dst-ip src-port dst-port src-dst-port} global configuration command in order to configure load balancing. Load Balance must be configured globally and the load balancing option cannot be changed on a per port basis.
EtherChannel balances the traffic load across the links in a channel through the reduction of part of the binary pattern that the addresses in the frame form 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 mode applies to all EtherChannels that are configured on the switch. You configure the load balancing and forwarding method with use of the port-channel load-balance {dst-ip dst-mac src-dst-ip src-dst-mac src-ip src-mac} global configuration command.
You can find out which interface is used in the EtherChannel to forward traffic based on the load balancing method. The command for this determination is test etherchannel load-balance interface port-channel numbe {ip mac} [source_ip_addsource_mac_add] [dest_ip_adddest_mac_add] .
For the 2950/2955/3550 series switch, EtherChannel balances the traffic load across the links in a channel by random association with a newly learned MAC address with one of the links in the channel. EtherChannel load balancing can use either source-MAC or destination-MAC address forwarding.
With source-MAC address forwarding, when packets are forwarded to an EtherChannel, the packets 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, the packets are distributed across the ports in the channel based on the destination host 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.
For the 3550 series switch, when source-MAC address forwarding is used, load distribution based on the source and destination IP address is also enabled for routed IP traffic. All routed IP traffic chooses a port based on the source and destination IP address. Packets between two IP hosts always use the same port in the channel, and traffic between any other pair of hosts can use a different port in the channel.
With the enablement of PAgP, the two possible methods of link determination are preserve order and maximize load balancing between the links on the Fast EtherChannel. The What Is PAgP and Where Do You Use It section of this document describes PAgP. The default is to maximize load balancing. PAgP is used to negotiate the configured method with the device at the other side of the channel. If preserve order is configured, the device at the other side is instructed in order to use source-based transmissions so that the Catalyst 1900/2820 always receives packets with the same source MAC address on the same link in the channel. This is the link that the Catalyst 1900/2820 always uses to send traffic to this MAC address. If maximize load balancing is configured, PAgP tells the other side that it can distribute traffic arbitrarily, and unicast traffic is transmitted by the Catalyst 1900/2820 on the link where the source address was last seen. This provides the maximum possible load-balancing configuration. When Fast EtherChannel is configured with PAgP disabled, the switch cannot negotiate with the partner about the switch learning capability. Whether the switch preserves frame order depends on whether the Fast EtherChannel partner performs source-based distribution.
1 For the 3550 series switch, when source-MAC address forwarding is used, load distribution based on the source and destination IP address is also enabled for routed IP traffic. All routed IP traffic chooses a port based on the source and destination IP address. 59ce067264
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