Unequal Cost Multipath Load-Balancing For Is-Is; Enabling Is-Is And Configuring Level 1 Or Level 2 Routing - Cisco NCS 6000 Series Configuration Manual

Unequal Cost Multipath Load-balancing for IS-IS

Unequal Cost Multipath Load-balancing for IS-IS
The unequal cost multipath (UCMP) load-balancing adds the capability with intermediate system-to-intermediate
system (IS-IS) to load-balance traffic proportionally across multiple paths, with different cost.
Generally, higher bandwidth paths have lower IGP metrics configured, so that they form the shortest IGP
paths. With the UCMP load-balancing enabled, IGP can use even lower bandwidth paths or higher cost paths
for traffic, and can install these paths to the forwarding information base (FIB). IS-IS IGP still installs multiple
paths to the same destination in FIB, but each path will have a 'load metric/weight' associated with it. FIB
uses this load metric/weight to decide the amount of traffic that needs to be sent on a higher bandwidth path
and the amount of traffic that needs to be sent on a lower bandwidth path.
The UCMP computation is provided under IS-IS per address family, enabling UCMP computation for a
particular address family. The UCMP configuration is also provided with a prefix-list option, which would
limit the UCMP computation only for the prefixes present in the prefix-list. If prefix-list option is not provided,
UCMP computation is done for the reachable prefixes in IS-IS. The number of UCMP nexthops to be considered
and installed is controlled using the variance configuration. Variance value identifies the range for the UCMP
path metric to be considered for installation into routing information base (RIB) and is defined in terms of a
percentage of the primary path metric. Total number of paths, including ECMP and UCMP paths together is
limited by the max-path configuration or by the max-path capability of the platform.
Enabling the UCMP configuration indicates that IS-IS should perform UCMP computation for the all the
reachable ISIS prefixes or all the prefixes in the prefix-list, if the prefix-list option is used. The UCMP
computation happens only after the primary SPF and route calculation is completed. There would be a delay
of ISIS_UCMP_INITIAL_DELAY (default delay is 100 ms) milliseconds from the time route calculation is
completed and UCMP computation is started. UCMP computation will be done before fast re-route computation.
Fast re-route backup paths will be calculated for both the primary equal cost multipath ( ECMP) paths and
the UCMP paths. Use the ucmp delay-interval command to configure the delay between primary SPF
completion and start of UCMP computation.
To manually change each path's bandwidth to adjust UCMP ratio, use the bandwidthcommand in interface
configuration mode.
UCMP ratio can be adjusted by any of the following ways:
• By using the bandwidth command in interface configuration mode to manually change the UCMP ratio.
• By adjusting the ISIS metric on the links.
There is an option to exclude an interface from being used for UCMP computation. If it is desired that a
particular interface should not be considered as a UCMP nexthop, for any prefix, then use the ucmp exclude
interface command to configure the interface to be excluded from UCMP computation.

Enabling IS-IS and Configuring Level 1 or Level 2 Routing

This task explains how to enable IS-IS and configure the routing level for an area.
Note
Configuring the routing level in Step 4 is optional, but is highly recommended to establish the proper level
of adjacencies.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
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Implementing IS-IS