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Hi,

when we do as-path prepending on SW2 as per SG, we face the following problem: SW2 now selects AS_54 routes via R6 and next-hop value as 54.1.1.254. The network 54.1.1.0/24 is known on SW2 via SW1 via EIGRP 100. SW1 know AS_100 networks via SW2 via EBGP - we have an ordinate loop.

My workaround was:

Rack1SW2(config-if)#do sh run | b bgp
router bgp 100
 neighbor 129.1.78.7 remote-as 200
 neighbor 129.1.78.7 ebgp-multihop 255
 neighbor 129.1.78.7 next-hop-unchanged

This way SW1 will learn AS_100 networks with next-hop 54.1.1.254, and network 54.1.1.0/24 is known to it via EIGRP from R1->R4->R6.

This is a good question actually :) You correctly pointed at the routing loop here. However, from a verification script standpoint, it might be enough to have just valid best-path in BGP table. If there is no requirement to obtain connectivity to the mentioned network, you may not need to bother with that. Thus, in the real exam, you may save some precious time by asking if only the BGP table contents matters (most likely they dont ask for full reachability to BGP backbone networks).