Ecmp Vs. Ucmp Load Balancing; Ucmp Minimum Integer Ratio - Cisco NCS 5500 Series Configuration Manuals

ECMP vs. UCMP Load Balancing

IGP selects the lowest path links, i.e E-B-A and E-D-A. The path E-C-A is not considered for load balancing
because of higher cost. The lowest path link E-D (5) is not a tie breaker, as the end to end cost to the Network
X is considered.
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ECMP vs. UCMP Load Balancing
Load balancing is a forwarding mechanism that distributes traffic over multiple links based on certain
parameters. Equal Cost Multi Path (ECMP) is a forwarding mechanism for routing packets along multiple
paths of equal cost with the goal to achieve almost equally distributed link load sharing. This significantly
impacts a router's next-hop (path) decision.
In ECMP, it is assumed that all links available are of similar speed which inherently means that the hash
values that are computed are equally shared over the multiple paths available.
For instance, if we have two paths available, the buckets (which in the end identify the links to be chosen)
will be assigned in a 50% / 50% loadsharing. This can be problematic when one path is say a 10G link and
the other link is a 1G link. In this case, you probably want to assign a (near) 90/10 type deviation, but
considering that BGP is not bandwidth aware, the 10G path is still chosen 50% of the time as much as the 1G
link. In this scenario, not all paths are of equal cost path.
What UCMP does in this case is apply a weight to a path which means that we are giving more hash buckets
to one path that has a higher weight. The weight applied is static in the sense that it is derived by the DMZ
bandwidth extended community either assigned to a peer or as configured via the Route Policy Language
(RPL) route manipulation functionality.
In general, a routing protocol decides a best path to a destination based on a metric. This metric is generally
driven by the bandwidth of the circuit. When we have 3 paths available, say 1G/10G/100G, routing protocols
generally discard the 1G/10G paths available. In defined cases, one may want to spread the load over the
circuits based on the load they can carry. In this example, one may want to distribute traffic in a 1%/10%/89%
fashion over the 1G/10G/100G paths available.

UCMP Minimum Integer Ratio

The UCMP Minimum Integer Ratio feature saves hardware resources when programming UCMP, by using
optimized number of buckets.
To calculate the UCMP minimum integer ratio, find the greatest common divisor (GCD) and divide all the
calculated normalized weights.
In the following Figure, we have three configured weights 40, 50, and 40, with GCD as 10. To calculate the
normalized weight, divide the configured weight by GCD. In this example, we need to divide 40 by 10, 50
by 10, and 40 by 10, which is 4, 5, and 4 respectively. Therefore 4, 5, and 4 are the new normalized weights.
Routing Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.3.x
230
ECMP vs. UCMP Load Balancing, on page 230
UCMP Minimum Integer Ratio, on page 230
Configuring IS-IS With Weight, on page 231
Configuring IS-IS With Metric, on page 232
Configuring BGP With Weights, on page 233
Configuring TE Tunnel With Weights, on page 234
Policy-Based Tunnel Selection, on page 235
Implementing UCMP