digraph G {
    node [ shape=box ]

    upstream [ label="Upstream Router" ];
    upstreamswitch [ label="Upstream Switch" ];
    router1 [ label="Router1" ];
    router2 [ label="Router2" ];
    clients [ label="Clients" ];

    upstreamswitch->{router1,router2} [ label="Receive packets from 'outside'" ]

    {router1,router2}->upstream [ label="BGP peering" ]
    clients->{router1,router2} [ label="Connect to either router via 'virtual' IP address" ]

    router1->router2 [ label="Exchange of session database" ]
    router2->router1 [ label="Exchange of session database" ]
}