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Here the connection between SwitchB and SwitchA is EBGP, and the connection
between SwitchC and SwitchD is IBGP. The BGP connection can be processed between
SwitchB and SwitchD without physical link, but the premise is a route which reaches from
one switch to the other switch. The route can be obtained by static routing or IGP.
14.9.4 MBGP4+ Troubleshooting Help
It is the same as corresponding section of BGP