Interface Configuration Guide 7705 SAR Interfaces
Edition: 01 3HE 11011 AAAC TQZZA 123
3.2.14.3 LSR Hashing
LSR hashing operates on the label stack and can also include hashing on the IP
header if the packet is an IPv4 packet. The label-IP hashing algorithm can also
include the Layer 4 header and the TEID field. The default hash is on the label stack
only. IPv4 is the only IP hashing supported on a 7705 SAR LSR.
When a 7705 SAR is acting as an LSR, it considers a packet to be IP if the first nibble
following the bottom of the label stack is 4 (IPv4). This allows the user to include an
IP header in the hashing routine at an LSR in order to spray labeled IP packets over
multiple equal-cost paths in ECMP in an LDP LSP and/or over multiple links of a LAG
group in all types of LSPs.
Other LSR hashing options include label stack profile options on the significance of
the bottom-of-stack label (VC label), the inclusion or exclusion of the ingress port,
and the inclusion or exclusion of the system IP address.
LSR load balancing is configured using the config>system>lsr-load-balancing or
config>router>if>lsr-load-balancing command. Configuration at the router
interface level overrides the system-level configuration for the specified interface.
If an ELI is found in the label stack, the entropy label is used as the hash result.
Hashing continues based on the configuration of label-only (lbl-only), label-IP
(lbl-ip), or label-IP with Layer 4 header and TEID (lbl-ip-l4-teid) options.
3.2.14.3.1 LSR Label-only Hashing
ECMP operation consists of an initial hash based on the system IP address, then on
the global port number if the use-ingress-port option is enabled.
Each label in the stack is then hashed separately with the result of the previous hash,
up to a maximum of 10 labels. The net result is used to select which LDP FEC next
hop to send the packet to using a threshold hashing operation of the net result with
the number of next hops. Threshold hashing is described in RFC 2992, Analysis of
an Equal-Cost Multi-Path Algorithm.
If an ELI is found in the label stack, the entropy label replaces the MPLS label stack
hashing result and hashing continues.
Note: The global IF index is no longer a hash input for LSR ECMP load balancing. It has
been replaced with the use-ingress-port configurable option in the lsr-load-balancing
command. As well, the default treatment of the MPLS label stack has changed to focus on
the bottom-of-stack label (VC label). In previous releases, all labels had equal influence.
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