The command maximum-paths eibgp 6 is used within the BGP routing process on Cisco IOS XE devices to enable equal-cost multipathing (ECMP) for up to six parallel paths. This specific command variant allows the router to install multiple routes for the same destination prefix into the Routing Information Base (RIB), including a mix of paths learned from both internal BGP (iBGP) and external BGP (eBGP) peers. It is the single command that directly satisfies the requirement to load-balance across both iBGP and eBGP routes simultaneously.

A. The syntax is incorrect; the command is maximum-paths, and bgp is not part of the command itself. The closest valid command, maximum-paths 6, only enables multipathing for eBGP routes by default.

B. The command keyword multipath is incorrect for configuring BGP ECMP in this context on Cisco IOS devices; the correct keyword is maximum-paths.

C. The syntax is incorrect. The keyword routers is not a valid argument for the maximum-paths command in the BGP configuration context.

1. Cisco IOS XE Fuji 16.9.x BGP Command Reference, "BGP Commands: M to N":

Section: maximum-paths eibgp

Content: "To control the maximum number of parallel Interior Border Gateway Protocol (iBGP) and external BGP (eBGP) routes that can be installed in a routing table, use the maximum-paths eibgp command in router configuration mode." This directly confirms that option D is the correct single command for both iBGP and eBGP multipathing.

2. Cisco IOS IP Routing: BGP Configuration Guide, "BGP Multipath":

Section: "BGP Multipath Load Sharing for Both eBGP and iBGP in an L3VPN"

Content: This guide explains that without the eibgp keyword, separate commands are needed: maximum-paths for eBGP and maximum-paths ibgp for iBGP. This demonstrates that option A, even if syntactically corrected to maximum-paths 6, would be incomplete as it only addresses eBGP paths.