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PowerConnect B-Series FCX Configuration Guide 831
53-1002266-01
Configuring IP parameters – Layer 3 Switches
26
The source of a path cost value depends on the source of the path:
• IP static route – The value you assign to the metric parameter when you configure the route.
The default metric is 1. Refer to “Configuring load balancing and redundancy using multiple
static routes to the same destination” on page 824.
• RIP – The number of next-hop routers to the destination.
• OSPF – The Path Cost associated with the path. The paths can come from any combination of
inter-area, intra-area, and external Link State Advertisements (LSAs).
• BGP4 – The path Multi-Exit Discriminator (MED) value.
NOTE
If the path is redistributed between two or more of the above sources before entering the IP route
table, the cost can increase during the redistribution due to settings in redistribution filters.
Static route, OSPF, and BGP4 load sharing
IP load sharing and load sharing for static routes, OSPF routes, and BGP4 routes are individually
configured. Multiple equal-cost paths for a destination can enter the IP route table only if the
source of the paths is configured to support multiple equal-cost paths. For example, if BGP4 allows
only one path with a given cost for a given destination, the BGP4 route table cannot contain
equal-cost paths to the destination. Consequently, the IP route table will not receive multiple
equal-cost paths from BGP4.
Table 149 lists the default and configurable maximum numbers of paths for each IP route source
that can provide equal-cost paths to the IP route table. The table also lists where to find
configuration information for the route source load sharing parameters.
The load sharing state for all the route sources is based on the state of IP load sharing. Since IP
load sharing is enabled by default on all Layer 3 Switches, load sharing for static IP routes, RIP
routes, OSPF routes, and BGP4 routes also is enabled by default.
How IP load sharing works
When the Layer 3 Switch receives traffic for a destination and the IP route table contains multiple,
equal-cost paths to that destination, the device checks the IP forwarding cache for a forwarding
entry for the destination. The IP forwarding cache provides a fast path for forwarding IP traffic,
including load-balanced traffic. The cache contains entries that associate a destination host or
network with a path (next-hop router).
TABLE 149 Default load sharing parameters for route sources
Route source
Default maximum number of paths
Maximum
number of
paths
See...
PowerConne
ct B-Series
FCX
Static IP route 4
1
1. This value depends on the value for IP load sharing, and is not separately configurable.
8
1
page 832
RIP 4
1
8
1
page 832
OSPF 4 8 page 832
BGP4 1 4 page 1006