Penultimate hop popping (PHP) is an LSR label stack processing optimization feature. When enabled, the LSR can pop (or discard) the remaining label stack and forward the packet to the last router along the LSP as a normal Ethernet packet.
By popping the label stack one hop prior to the LSP egress router, the egress router is spared having to do two lookups. After the label stack has been popped by the penultimate hop LSR, the LSP egress router must only perform an address lookup to forward the packet to the destination.
PHP label advertisements using implicit NULL labels can be optionally enabled. Support for receiving implicit NULL label advertisements by neighbor LSRs is always enabled. For example, if an LSR advertises implicit NULL labels for IP prefixes, the neighbor LSRs must support PHP.
Note
PHP should not be enabled on egress LER when there is a Summit X670-G2 or X770 in a network acting as a PHP LSR.
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