Dell Networking 7048, Reference Manual

Page: 1146 / 1788

1146
OSPF Commands
Route Preferences
Normally, OSPF select routes in the following order:
• Local
• Static
• Intra-area
• Inter-area
• External
• RIP
PowerConnect OSPF allows the administrator to change the preference for
selecting intra, inter, and external routes according to the following rules:
a
External route preferences apply to all ospf external routes like type1,
type2, nssa-type1, nssa-type2 equally.
b
Multiple route types may be configured with equal preference values.
c
Configuring a route preference of 255 makes the route ineligible to be
selected as the best route to its destination. That is, a route type with
a preference of 255 shall never be used for forwarding.
The RIP preference is not used in IPv6 routing.
OSPF Equal Cost Multipath (ECMP)
A device running the IP routing protocol OSPF maintains multiple equal-cost
routes to all destinations. The multiple routes are of the same type (intra-
area, inter-area, type 1 external or type 2 external), cost, and have the same
associated area. However, each route is defined by a separate advertising
router and next hop.
With ECMP, a device forwards traffic to a specified destination through
multiple paths thereby taking advantage of the bandwidth of both links.
ECMP routes are configured statically or learned dynamically as follows:
• Configured Statically: If an operator configures multiple static routes to
the exact same destination but with different next hops, those routes are
treated as a single route with two next hops.
2CSPC4.X7000-SWUM204.book Page 1146 Friday, March 15, 2013 8:22 AM