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The purpose of the RT attribute is to identify the destination VPN on the egress PE. RT stands for route target, which is a BGP extended community attribute that is used in MPLS VPNs. RT is attached to VPN routes by the ingress PE router and is used to control the import and export of routes between different VPNs. The egress PE router uses the RT value to determine which VPN routes belong to which VPN customers and installs them in the appropriate VRF table56.

References: IP Routing: BGP Configuration Guide - BGP-RT and VPN … - Cisco, : The Ruby Text element - MDN Web Docs

Penultimate hop popping (PHP) is a function performed by certain routers in an MPLS enabled network. It refers to the process whereby the outermost label of an MPLS tagged packet is removed by a label switch router (LSR) before the packet is passed to an adjacent label edge router (LER). The purpose of PHP is to reduce the load on the LER, as it saves one cycle of label lookup. The LER can forward the packet based on its IP header or another label in the stack, without having to process the outer label. PHPfunctionality is achieved by the LER advertising a label with a value of 3 to its neighbors. This label is defined as "implicit-null" and informs the neighboring LSRs to perform PHP12.

References: Penultimate hop popping - Wikipedia, MPLS Penultimate hop popping - NetworkLessons.com

The router function that advertises external routes in OSPF is ASBR, which stands for Autonomous System Boundary Router. An ASBR is a router that connects an OSPF domain to another routing domain, such as another OSPF domain or a different routing protocol domain. An ASBR can redistribute routes from other routing domains into OSPF as external routes, which are carried in type 5 or type 7 LSAs. External routes have two types: E1 and E2. E1 routes include the cost from the ASBR to the destination, while E2 routes only include the cost advertised by the ASBR34.

References: OSPF External Routes ⋆ IpCisco, OSPF External Route selection algorithm - Cisco Community