IP Router Configuration
Router Configuration Guide 175
When ECMP is enabled and multiple equal-cost next-hops exit for the IGP route, the ingress IOM,
IMMM, or XMA will spray the packets for this route based on hashing routine currently supported for
IPv4 packets. When the preferred RTM entry corresponds to an LDP shortcut route, spraying will be
performed across the multiple next-hops for the LDP FEC. The FEC next-hops can either be direct link
LDP neighbors or T-LDP neighbors reachable over RSVP LSPs in the case of LDP-over-RSVP but
not both.
When the preferred RTM entry corresponds to a regular IP route, spraying will be performed across
regular IP next-hops for the prefix.
The no form of this command disables the resolution of IGP routes using LDP shortcuts.
Default no ldp-shortcut
ldp-sync-timer
Syntax ldp-sync-timer seconds [end-of-lib]
no ldp-sync-timer
Context config>router>interface
Description This command enables synchronization of an IGP and LDP. When a link is restored after a failure, the
IGP sets the link cost to infinity and advertises it. The actual value advertised in OSPF is 0xFFFF
(65535). The actual value advertised in IS-IS regular metric is 0x3F (63) and in IS-IS wide-metric is
0xFFFFFE (16777214). This feature is not supported on RIP interfaces.
If an interface belongs to both IS-IS and OSPF, a physical failure will cause both IGPs to advertise an
infinite metric and to follow the IGP-LDP synchronization procedures. If only one IGP bounces on this
interface or on the system, then only the affected IGP advertises the infinite metric and follows the
IGP-LDP synchronization procedures.
Next, an LDP Hello adjacency is brought up with the neighbor. The LDP synchronization timer is
started by the IGP when the LDP session to the neighbor is up over the interface. This is to allow time
for the label-FEC bindings to be exchanged.
When the LDP synchronization timer expires, the link cost is restored and is readvertised. The IGP will
announce a new best next hop and LDP will use it if the label binding for the neighbor’s FEC is
available.
If the user changes the cost of an interface, the new value is advertised at the next flooding of link
attributes by the IGP. However, if the LDP synchronization timer is still running, the new cost value
will only be advertised after the timer expires. The new cost value will also be advertised after the user
executes any of the following commands:
• tools>perform>router>isis>ldp-sync-exit
• tools>perform>router>ospf>ldp-sync-exit
• config>router>interface>no ldp-sync-timer
• config>router>ospf>disable-ldp-sync
• router>isis>disable-ldp-sync
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