Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 91)

Brocade Virtual ADX Switch and Router Guide 77
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Configuring IP parameters
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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
Virtual ADX 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 Virtual ADX uses IP load sharing to select a path to the destination.
1
IP load sharing is based on the destination address of the traffic.
You can enable a Virtual ADX 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 that are eligible for load
sharing can enter the table from IP static routes.
Administrative distance
The administrative distance is a unique value associated with each type (source) of IP route. Each
path has an administrative distance. The administrative distance is not used when performing IP
load sharing, but the administrative distance is used when evaluating multiple equal-cost paths to
the same destination from different sources.
The value of the administrative distance is determined by the source of the route. The Virtual ADX is
configured with a unique administrative distance value for each IP route source.
1. IP load sharing is also called “Equal-Cost Multi-Path (ECMP)” load sharing or just “ECMP”
Virtual ADX(config)# show ip route
Total number of IP routes: 2
Start index: 1 B:BGP D:Connected S:Static O:OSPF *:Candidate default
Destination NetMask Gateway Port Cost Type
1 10.157.20.0 255.255.255.0 0.0.0.0 lb1 1 D
2 10.157.22.0 255.255.255.0 0.0.0.0 4/11 1 *D