The highlighted text 10.20.30.40:1 represents the Route Distinguisher (RD). In BGP/MPLS IP VPNs, the RD is an 8-byte value prepended to a customer's non-unique IPv4 prefix (e.g., 192.168.1.0/24) to create a globally unique 12-byte address known as a VPN-IPv4 address. This process allows service providers to support multiple customers who may use overlapping private IP address spaces. The format shown, IP-address:number, is a standard representation for a Type 1 RD. The show router bgp routes vpn-ipv4 command on the Nokia 7750 SR displays these unique VPN-IPv4 prefixes, which are composed of the RD and the customer's original IPv4 prefix.

A. The LDP identifier is used to establish and maintain Label Distribution Protocol (LDP) sessions between routers and is not part of a BGP route prefix.

B. The Service identifier is used in the context of service transport tunnels (Service Distribution Paths - SDPs), not for distinguishing Layer 3 VPN routes.

C. The Route Target (RT) is a BGP extended community attribute that governs the import and export of routes into VRFs, but it is not prepended to the prefix itself.

E. The tunnel label is the MPLS label used for transport across the provider core. The BGP update carries a VPN label, but neither is represented by the RD.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VPRN Services": In the section "VPN-IPv4 and VPN-IPv6 Address Formats", the guide states, "A VPN-IPv4 address is a 12-byte quantity, beginning with an 8-byte Route Distinguisher (RD) and ending with a 4-byte IPv4 address... The purpose of the RD is to allow the service provider to create distinct routes to the same IPv4 address prefix." This document confirms the structure and purpose of the RD as displayed in the command output.

2. IETF RFC 4364, "BGP/MPLS IP Virtual Private Networks (VPNs)": Section 4.2, "The VPN-IPv4 Address", defines a VPN-IPv4 address as a 12-octet quantity composed of an 8-octet Route Distinguisher (RD) and a 4-octet IPv4 address. It specifies that the RD's only purpose is to make IPv4 prefixes from different VPNs unique. The format IP-address:number corresponds to the Type 1 RD defined in this RFC.

3. Nokia 7750 SR OS Routing Protocols Guide, Release 21.10.R1, "BGP": The section on "Displaying BGP Routing Information" describes the output of commands like show router bgp routes. The format shown in the question, RD:prefix, is consistent with the command output examples for VPN-IPv4 routes provided in this guide.