Convergence - Cisco Nexus 7000 Series Configuration Manual

Information About Layer 3 Unicast Routing
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .
The autonomous system number assignment for public and private networks is governed by the Internet
Note
Assigned Number Authority (IANA). For information about autonomous system numbers, including the
reserved number assignment, or to apply to register an autonomous system number, refer to the following
URL:
http://www.iana.org/

Convergence

A key aspect to measure for any routing algorithm is how much time a router takes to react to network
topology changes. When a part of the network changes for any reason, such as a link failure, the routing
information in different routers might not match. Some routers will have updated information about the
changed topology, other routers will still have the old information. The
time before all routers in the network have updated, matching routing information. The convergence time
varies depending on the routing algorithm. Fast convergence minimizes the chance of lost packets caused
by inaccurate routing information.
Load Balancing and Equal Cost Multipath
Routing protocols can use
paths.When a router learns multiple routes to a specific network, it installs the route with the lowest
administrative distance in the routing table. If the router receives and installs multiple paths with the
same administrative distance and cost to a destination, load balancing can occur. Load balancing
distributes the traffic across all the paths, sharing the load. The number of paths used is limited by the
number of entries that the routing protocol puts in the routing table. Cisco NX-OS supports up to 16
paths to a destination.
The Enhanced Interior Gateway Routing Protocol (EIGRP) also supports unequal cost load-balancing.
For more information, see
Route Redistribution
If you have multiple routing protocols configured in your network, you can configure these protocols to
share routing information by configuring route redistribution in each protocol. For example, you can
configure Open Shortest Path First (OSPF) to advertise routes learned from the Border Gateway Protocol
(BGP). You can also redistribute static routes into any dynamic routing protocol. The router that is
redistributing routes from another protocol sets a fixed route metric for those redistributed routes. This
avoids the problem of incompatible route metrics between the different routing protocols. For example,
routes redistributed from EIGRP into OSPF are assigned a fixed link cost metric that OSPF understands.
Route redistribution also uses an administrative distance (see the
page
routing protocol is given a lower administrative distance so that its routes are picked over routes from
another protocol with a higher administrative distance assigned.
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
1-6
load balancing
Chapter 8, "Configuring EIGRP."
1-7) to distinguish between routes learned from two different routing protocols. The preferred
or equal cost multipath (ECMP) to share traffic across multiple
"Administrative Distance" section on
Chapter 1 Overview
convergence is the amount of
OL-20002-02