Ecmp Considerations; Handling Of Control Packets - Alcatel-Lucent 7450 Manual

RSVP-TE LSP Shortcut for IGP Resolution
The relative-metric option is mutually exclusive with the lfa-protect or the lfa-only options. In
other words, an LSP with the relative-metric option enabled cannot be included in the LFA SPF
and vice-versa when the rsvp-shortcut option is enabled in the IGP.
Finally, it should be noted that the relative-metric option is ignored when forwarding adjacency is
enabled in IS-IS or OSPF by configuring the advertise-tunnel-link option. In this case, IGP
advertises the LSP as a point-to-point unnumbered link along with the LSP operational metric
capped to the maximum link metric allowed in that IGP.
The resolution and forwarding of IPv6 prefixes to IPv4 forwarding adjacency LSP is not
supported.

ECMP Considerations

When ECMP is enabled on the system and multiple equal-cost paths exist for a prefix, the
following selection criteria are used to pick up the set of next-hops to program in the data path:
• for a destination = tunnel-endpoint (including external prefixes with tunnel-endpoint as the
next-hop):
• for a destination != tunnel-endpoint:
• Note though no ECMP is performed across both the IP and tunnel next-hops the tunnel end-
point lies in one of the shortest IGP paths for that prefix. In that case, the tunnel next-hop is
always selected as long as the prefix cost using the tunnel is equal or lower than the IGP cost.
The ingress IOM will spray the packets for a prefix over the set of tunnel next-hops and IP next-
hops based on the hashing routine currently supported for IPv4 packets.

Handling of Control Packets

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→ select tunnel with lowest tunnel-index (ip next-hop is never used in this case)
→ exclude LSPs with metric higher than underlying IGP cost between the endpoint of
the LSP
→ prefer tunnel next-hop over ip next-hop
→ within tunnel next-hops:
i. select lowest endpoint to destination cost
ii. if same endpoint to destination cost, select lowest endpoint node router-id
iii. if same router-id, select lowest tunnel-index
→ within ip next-hops:
i. select lowest downstream router-id
ii. if same downstream router-id, select lowest interface-index
7450 ESS MPLS Guide