Изначальная конфигурация BGP:

samovarov@imhtst1# show protocols bgp   
traceoptions {
    file bgp.log size 1m files 5;
}
group internal-peers {
    type external;
    description Internal-peers;
    local-address 10.2.2.253;
    export imhtst1-to-ALL;
    local-as 65532;
    neighbor 10.2.2.3 {
        description PMH;
        family inet {
            unicast;
        }
        peer-as 65101;
    }
    neighbor 10.252.1.2 {
        description jun100;
        local-address 10.252.1.1;
        family inet {
            unicast;
        }
        peer-as 65533;
    }
}

Здесь мы выполняем экспорт обоим соседям согласно правилам:

samovarov@imhtst1# show policy-options policy-statement imhtst1-to-ALL 
term net-obn-routes {
    from {
        prefix-list-filter net-imhtst1-prfx orlonger;
    }
    then accept;
}
term else {
    then reject;
}

[edit]
samovarov@imhtst1# show policy-options prefix-list net-imhtst1-prfx       
10.253.10.0/24;

Cо стороны jun100_obn36a маршрут до внутренней сети imhtst1 выглядит следующим образом:

samovarov@jun100_obn36a> show route 10.253.10.0 

inet.0: 30 destinations, 32 routes (30 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.253.10.0/24     *[BGP/170] 00:02:38, localpref 100
                      AS path: 65532 I
                    > to 10.252.1.1 via st0.1
                    [BGP/170] 00:02:38, localpref 100, from 10.2.2.3
                      AS path: 65101 65532 I
                    > to 10.2.2.253 via st0.0

Т.е. более предпочтителен маршрут через 10.252.1.1 via st0.1, поскольку здесь короче AS path.

Использование as-path-prepend

Повлияем на данный маршрут, используя as-path-prepend.

Создадим новую policy-statement imhtst1-to-ALL2:

samovarov@imhtst1# show policy-options policy-statement imhtst1-to-ALL2 
term net-obn-routes {
    from {
        prefix-list-filter net-imhtst1-prfx orlonger;
    }
    then {
        as-path-prepend "65532 65532 65532 65532 65532";
        accept;
    }
}
term else {
    then reject;
}

Здесь мы добавили сразу 5 штук номеров local-as 65532 для imhtst1.
Далее добавляем экспорт к настройке neighbor 10.252.1.2:

samovarov@imhtst1# show protocols bgp                                                                   
traceoptions {
    file bgp.log size 1m files 5;
}
group internal-peers {
    type external;
    description Internal-peers;
    local-address 10.2.2.253;
    export imhtst1-to-ALL;
    local-as 65532;
    neighbor 10.2.2.3 {
        description PMH;
        family inet {
            unicast;
        }
        peer-as 65101;
    }
    neighbor 10.252.1.2 {
        description jun100;
        local-address 10.252.1.1;
        family inet {
            unicast;
        }
        export imhtst1-to-ALL2;
        peer-as 65533;
    }
}

С такой настройкой на neighbor 10.2.2.3 будет влиять export imhtst1-to-ALL.
Но на neighbor 10.252.1.2 повлияет наша новая export imhtst1-to-ALL2.

Тогда со стороны jun100_obn36a маршрут до внутренней сети imhtst1 поменяется:

samovarov@jun100_obn36a> show route 10.253.10.0    

inet.0: 30 destinations, 32 routes (30 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.253.10.0/24     *[BGP/170] 00:18:43, localpref 100, from 10.2.2.3
                      AS path: 65101 65532 I
                    > to 10.2.2.253 via st0.0
                    [BGP/170] 00:02:57, localpref 100
                      AS path: 65532 65532 65532 65532 65532 65532 I
                    > to 10.252.1.1 via st0.1

Для симметричной маршрутизации необходимо сделать аналогичную конструкцию и на jun100_obn36a

samovarov@jun100_obn36a# show protocols bgp 
traceoptions {
    file bgp.log size 1m files 5;
}
group internal-peers {
    type external;
    description Internal-peers;
    local-address 10.2.2.254;
    export OBN-to-ALL;
    local-as 65533;
    neighbor 10.2.2.3 {
        description PMH;
        family inet {
            unicast;
        }
        peer-as 65101;
    }
    neighbor 10.252.1.1 {
        description imhtst1;
        local-address 10.252.1.2;
        family inet {
            unicast;
        }
        export OBN-to-ALL2;
        peer-as 65532;
    }
    inactive: neighbor 10.252.100.1 {
        description imhtst1;
        local-address 10.252.100.2;
        family inet {
            unicast;
        }
        peer-as 65532;
    }
}

samovarov@jun100_obn36a# show policy-options policy-statement OBN-to-ALL 
term net-obn-routes {
    from {
        prefix-list-filter customers orlonger;
    }
    then accept;
}
term others {
    then reject;
}

[edit]
samovarov@jun100_obn36a# show policy-options policy-statement OBN-to-ALL2   
term net-obn-routes {
    from {
        prefix-list-filter customers orlonger;
    }
    then {
        as-path-prepend "65533 65533 65533 65533 65533";
        accept;
    }
}
term others {
    then reject;
}

samovarov@jun100_obn36a# show policy-options prefix-list customers           
192.168.254.0/24;

Тогда со стороны imhtst1 маршрут до внутренней сети jun100_obn36a также поменяется:

samovarov@imhtst1> show route 192.168.254.0    

inet.0: 30 destinations, 32 routes (29 active, 0 holddown, 1 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.254.0/24   *[BGP/170] 21:46:32, localpref 100, from 10.2.2.3
                      AS path: 65101 65533 I, validation-state: unverified
                    > to 10.2.2.254 via st0.0
                    [BGP/170] 00:03:55, localpref 100
                      AS path: 65533 65533 65533 65533 65533 65533 I, validation-state: unverified
                    > to 10.252.1.2 via st0.1

Источник:
https://www.juniper.net/documentation/en_US/junos/topics/example/routing...