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
bandwidth command in interface configuration mode.
UCMP ratio can be adjusted by any of the following ways:
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.
More than 32 ECMP and UCMP paths are not supported for these features:
-
LI
-
GRE
-
BVI
-
NetFlow
-
Satellite
-
MCAST
-
SPAN
-
PWHE
-
ABF
-
P2MP
-
MVPN
-
VPLS
-
L2TPv3
-
LISP
-
VIDMON
-
PBR
During a route processor failover (RPFO), IS-IS does not maintain maximum paths for a certain minimum amount of time. During
this time, there is a minor amount of traffic flowing through a certain number of paths beyond the configured number of maximum
paths.
To avoid such a scenario in Cisco access points, the interfaces are configured to have a maximum number of maximum paths so
that traffic is forced to choose one among the configured ECMP maximum paths only.