Dell BigIron RX Series, Configuration Manual

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BigIron RX Series Configuration Guide 209
53-1002253-01
Configuring forwarding parameters
7
This example shows two routes. Both of the routes are directly attached, as indicated in the Type
column. However, one of the routes is shown as type “*D”, with an asterisk (*). The asterisk
indicates that this route is a candidate default network route.
Configuring IP load sharing
The IP route table can contain more than one path to a given destination. When this occurs, the
device selects the path with the lowest cost as the path for forwarding traffic to the destination. If
the IP route table contains more than one path to a destination and the paths each have the lowest
cost, then the device uses
IP load sharing to select a path to the destination.
1
IP load sharing is based on the destination address of the traffic. device supports load sharing
based on individual host addresses or on network addresses.
You can enable a device to load balance across up to eight equal-cost paths. The default maximum
number of equal-cost load sharing paths is four.
NOTE
IP load sharing is not based on source routing, only on next-hop routing.
NOTE
The term “path” refers to the next-hop router to a destination, not to the entire route to a destination.
Thus, when the software compares multiple equal-cost paths, the software is comparing paths that
use different next-hop routers, with equal costs, to the same destination.
In many contexts, the terms “route” and” path” mean the same thing. Most of the user
documentation uses the term “route” throughout. The term “path” is used in this section to refer to
an individual next-hop router to a destination, while the term “route” refers collectively to the
multiple paths to the destination. Load sharing applies when the IP route table contains multiple,
equal-cost paths to a destination.
How multiple equal-cost paths enter the IP Route table
IP load sharing applies to equal-cost paths in the IP route table. Routes eligible for load sharing
can enter the table from the following sources:
•
IP static routes
•
Routes learned through RIP, OSPF, and BGP4
1. IP load sharing is also called “Equal-Cost Multi-Path (ECMP)” load sharing or just “ECMP”
BigIron RX(config)# show ip route
Total number of IP routes: 2
Start index: 1 B:BGP D:Connected R:RIP S:Static O:OSPF *:Candidate default
Destination Gateway Port Cost Type
1 209.157.20.0 0.0.0.0 lb1 1 D
2 209.157.22.0 0.0.0.0 4/11 1 *D