Load balancing terms in EIGRP

Unanswered Question
Dec 3rd, 2008

What does terms "procces-switced" and "fast-switched" mean?

What is it "per-packet" and "per-destination"?

When I can use this terms on practice?

Please , explain by own words.

Thank you

I have this problem too.
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Giuseppe Larosa Thu, 12/04/2008 - 00:24

Hello Alexandr,

these terms are general and not specific of EIGRP.

First routers were workstations with special software: each packet had to be "process switched" by the main cpu.

In developing router platforms Cisco and other vendors had introduced different generations of forwarding optimizasition tecniques: the first one to be developed was Fast Switching and the more modern and more used now is CEF (Cisco Express Forwarding).

Let's consider the communication between two hosts in two different subnets:

with process switching each single packet is processed in the same way using software and cpu resources.

However, all the packets after the first one could reuse the activity made for the first packet:

the router has found out what is the L3 next-hop for the packet (either another router or the destination end device) and it has found the way to build the frame (it has made ARP for mapping the next-hop ip address to the correct MAC address or the correct serial encapsulation)

This means that packets after the next one need only to be forwarded : the routing lookup has already be done.

This type of optimization traffic driven is called Fast switching.

CEF is topology driven: it builds all the information to forward packets before traffic is seen using routing table, ARP table and so on: with CEF also the first packet of a flow is just forwarded (switched) and doesn't require a routing lookup.

However, not all packets can be processed by CEF: some type of packets still need to be process switched: for examples routing protocol messages, CDP messages, STP BPDus and packets with some options in the IP header.

per-destination load-sharing means flow based load balancing: when multiple paths are available a flow with a specific source and a specific destination is fowarded in the same way: all packets use the same path.

per-packet load-sharing: the parallel links are used in round-robin a state information about what link has been used for packet N is used to decide out what interface to send packet N+1.

Fast switching uses normally destination based load balancing and it is not able to perform per packet sharing.

CEF uses both source and destination in flow based load balancing or it can perform per packet load balancing with an explicit configuration.

To be noted that usually flow based load balancing is to be preferred because it avoids out of order arrival of packets classified in the same flow

EIGRP specifics are that it can allow load-sharing over paths not of equal cost

Hope to help

Giuseppe

thotsaphon Thu, 12/04/2008 - 00:29

Hi,

There are 2 things here.

- How to install mutiple routes going to the same destination in the routing table?

A: It's routing protocols or static routes.

- How does the router/switch forward packets?

A: Packet-Switching,Fast-Switching or CEF

Eigrp is a routing protocol. You can have mutiple paths to the same destination installed in the routing table. Eigrp also supports un-equal cost load-balance.

Per-packet or Per-destination is a mechanishm that the router usess to forward packets out. CEF supports per-packet and per-destination.

Per-packet can extremely utilize links.But the other end may receive out-of-order packets. It has to re-order those packets. It's not good for voice traffic.

Per-destination is as the name imples. If you have lots of destinations to go. It's good for you. It's enabled by default when using fast-switching or CEF.

How to change the mode?

You can change this in the interface configuration mode.

Int f0/0

Router(config-if)#ip load-sharing [per-packet|per-destination]

Toshi

lejoe.thomas Thu, 12/04/2008 - 00:34

Hi Alexandr A,

What does terms "procces-switced" and "fast-switched" mean?

Process-switching means the router will perform route table look up (nexthop,int), along with layer 2 information, for every packet.

Fast switching means, for the first packet to particular destination a route table lookup is performed and the information retrieved is stored into a cache (next hope, int, ARP info). Subsequent packets to same destination are forwarded based on the information in this cache rather performing a route table lookup, ARP information etc

What is it "per-packet" and "per-destination"?

Fast switching should be per-destination and process-switching should be per-packet

When I can use this terms on practice?

Didnt quite understand what you meant here. But the preferred switching would be Fast switching (because processor utilisation is decreased) over process-switching.

HTH

Lejoe

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