Vrf-Lite Support For Ospfv2; Update; Unequal Cost Multipath Load-Balancing For Ospf - Cisco ASR 9000 Series Configuration Manual

Implementing OSPF

VRF-lite Support for OSPFv2

VRF-lite capability is enabled for OSPF version 2 (OSPFv2). VRF-lite is the virtual routing and forwarding
(VRF) deployment without the BGP/MPLS based backbone. In VRF-lite, individual provider edge (PE)
routers are directly connected using VRF interfaces. To enable VRF-lite in OSPFv2, configure the capability
vrf-lite command in VRF configuration mode. When VRF-lite is configured, the DN bit processing and the
automatic Area Border Router (ABR) status setting are disabled.
OSPFv3 Timers Link-state Advertisements and Shortest Path First Throttle
Default Values Update
The Open Shortest Path First version 3 (OSPFv3) timers link-state advertisements (LSAs) and shortest path
first (SPF) throttle default values are updated to:
• timers throttle lsa all—start-interval: 50 milliseconds and hold-interval: 200 milliseconds
• timers throttle spf —spf-start: 50 milliseconds, spf-hold: 200 milliseconds, spf-max-wait: 5000

Unequal Cost Multipath Load-balancing for OSPF

The unequal cost multipath (UCMP) load-balancing adds the capability with Open Shortest Path First (OSPF)
to load-balance traffic proportionally across multiple paths, with different cost. Without UCMP enabled, only
the best cost paths are discovered by OSPF (ECMP) and alternate higher cost paths are not computed.
Generally, higher bandwidth links 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 links or higher cost links
for traffic, and can install these paths to the forwarding information base (FIB). OSPF 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 OSPF VRF context, enabling UCMP computation for a particular
VRF. For default VRF the configuration is done under the OSPF global mode. 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 OSPF. The number of UCMP paths 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/FIB) 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.
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 the UCMP configuration indicates that OSPF should perform UCMP computation for the all the
reachable OSPF prefixes or all the prefixes permitted by 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 configurable delay (default delay is 100 ms) from the time primary route calculation is completed and UCMP
OL-30423-03
milliseconds
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
VRF-lite Support for OSPFv2
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