Inter-AS Option AB Guide to Connect MPLS L3VPN Networks

« 2023 Sept 29 »

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What is MPLS inter-AS option AB?

Inter-AS option AB provides a mechanism to interconnect two Service Provider networks and allow VPN services such as MPLS L3VPN to function across autonomous system boundaries. Inter-AS option AB combines features from option A and B. The interconnection between the SP networks is not label switched and each customer receives one or more dedicated VRF-aware subinterfaces. Meanwhile, there is a single interface (not VRF-aware) dedicated to control plane BGP VPNv4 traffic.

Inter-AS option AB uses a single interface for BGP VPNv4 control plane and uses VRF-aware subinterfaces for customer traffic on the data plane

Inter-AS option AB is also referred to as inter-AS option D, and it is described in the informational Internet Draft titled "Inter-AS Option D for BGP/MPLS IP VPN". The following packet capture is taken between the ASBR PE routers, and shows that a customer-dedicated VLAN is assigned between VRF-aware subinterfaces, but traffic is not label switched.

Inter-AS option AB interconnection between ASBR PE routers is not label switched

Inter-AS option AB advantages

The following three benefits apply to inter-AS option AB when compared to inter-AS options A and B separately.

  1. Granular QoS control can be configured on the dedicated customer VRF-subinterfaces over the interconnection between the SP networks. Because the inter-AS link is not label switched, each customer's VRF-aware subinterface can use a service policy to apply QoS with the DSCP field in the IP header.
  2. With inter-AS option AB a single BGP VPNv4 neighborship is established which carries control plane traffic (VPN route updates) for all customer prefixes. This means, only one BGP VPNv4 neighborship is needed on the inter-AS link between the ASBR PE routers.
  3. Additionally, the Route Target (RT) values are assigned by the ASBR PE routers with import/export rules for each customer VRF. This means, the RT value is not inherited from the neighboring ASBR PE and is not passed along between SP MPLS domains. Instead, a route is imported and exported on each ASBR PE based on RT value configurations.

How to configure MPLS inter-AS option AB?

In the following example topology Inter-AS option AB is configured between the ASBR PE R5 and PE R6. With this method, the two sites that belong to the same customer can use VRF Gold to communicate with each other through different Service Provider MPLS networks.

Inter-AS option AB uses an unlabbeled VRF-aware subinterface for the customer traffic, this is essentially the data plane. Each customer needs a subinterface on the inter-AS link, and the interconnection is not label switched. Additionally, there is a single control plane eBGP neighborship configured between the inter-AS PE R5 and PE R6 using another subinterface. The subinterface used for control plane traffic is not VRF-aware.

Inter-AS option AB also called inter-AS option D configuration with MPLS L3VPN network and separate interconnection interface for BGP VPNv4 control plane and VRF subinterface for data plane customer traffic

Configuration:

R5 (ASBR PE) 
    R5#show run all | sec ^mpls ip__
mpls ip

R5#show run | sec ^mpls
mpls label range 500 599
mpls ldp router-id Loopback10 force

R5#show run int Lo10 | sec int
interface Loopback10
 ip address 5.5.5.5 255.255.255.255

R5#show run | sec ^vrf     
vrf definition Gold
 rd 65010:2
 !
 address-family ipv4
  route-target export 65010:2
  route-target import 65010:1
  route-target import 65020:2
  inter-as-hybrid next-hop 172.16.2.2
 exit-address-family

R5#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to R4 **
 ip address 10.3.0.2 255.255.255.252
 ip router isis 
 duplex auto
 speed auto
 media-type rj45
 mpls ip
 isis circuit-type level-2-only
 isis network point-to-point 

R5#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
 description ** to ASBR PE R6 **
 no ip address
 duplex auto
 speed auto
 
R5#show run int Gi0/1.10 | sec int
interface GigabitEthernet0/1.10
 description ** control plane BGP VPNv4 **
 encapsulation dot1Q 10
 ip address 172.16.1.1 255.255.255.252
 mpls bgp forwarding

R5#show run int Gi0/1.20 | sec int
interface GigabitEthernet0/1.20
 description ** data plane customer VRF Gold **
 encapsulation dot1Q 20
 vrf forwarding Gold
 ip address 172.16.2.1 255.255.255.252

R5#show run | sec ^router
router isis
 net 49.0010.0000.0000.0005.00
 is-type level-2-only
 advertise passive-only
 metric-style wide
 log-adjacency-changes
 passive-interface Loopback10
router bgp 65010
 bgp router-id 5.5.5.5
 bgp log-neighbor-changes
 neighbor 1.1.1.1 remote-as 65010
 neighbor 1.1.1.1 update-source Loopback10
 neighbor 172.16.1.2 remote-as 65020
 !
 address-family vpnv4
  neighbor 1.1.1.1 activate
  neighbor 1.1.1.1 send-community extended
  neighbor 1.1.1.1 next-hop-self
  neighbor 172.16.1.2 activate
  neighbor 172.16.1.2 send-community extended
  neighbor 172.16.1.2 inter-as-hybrid
 exit-address-family
R6 (ASBR PE) 
    R6#show run all | sec ^mpls ip__
mpls ip

R6#show run | sec ^mpls         
mpls label range 600 699
mpls ldp router-id Loopback10 force

R6#show run int Lo10 | sec int
interface Loopback10
 ip address 6.6.6.6 255.255.255.255

R6#show run | sec ^vrf
vrf definition Gold
 rd 65020:2
 !
 address-family ipv4
  route-target export 65020:2
  route-target import 65020:1
  route-target import 65010:2
  inter-as-hybrid next-hop 172.16.2.1
 exit-address-family

R6#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to R7 **
 ip address 10.4.0.2 255.255.255.252
 ip ospf network point-to-point
 duplex auto
 speed auto
 media-type rj45
 mpls ip
 
R6#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
 description ** to ASBR PE R5 **
 no ip address
 duplex auto
 speed auto

R6#show run int Gi0/1.10 | sec int
interface GigabitEthernet0/1.10
 description ** control plane BGP VPNv4 **
 encapsulation dot1Q 10
 ip address 172.16.1.2 255.255.255.252
 mpls bgp forwarding

R6#show run int Gi0/1.20 | sec int
interface GigabitEthernet0/1.20
 description ** data plane customer VRF Gold **
 encapsulation dot1Q 20
 vrf forwarding Gold
 ip address 172.16.2.2 255.255.255.252

R6#show run | sec ^router
router ospf 10
 router-id 6.6.6.6
 network 6.6.6.6 0.0.0.0 area 0
 network 10.4.0.0 0.0.0.3 area 0
router bgp 65020
 bgp router-id 6.6.6.6
 bgp log-neighbor-changes
 neighbor 9.9.9.9 remote-as 65020
 neighbor 9.9.9.9 update-source Loopback10
 neighbor 172.16.1.1 remote-as 65010
 !
 address-family vpnv4
  neighbor 9.9.9.9 activate
  neighbor 9.9.9.9 send-community extended
  neighbor 9.9.9.9 next-hop-self
  neighbor 172.16.1.1 activate
  neighbor 172.16.1.1 send-community extended
  neighbor 172.16.1.1 inter-as-hybrid
 exit-address-family
R1 (RR) 
    R1#show run int Lo10 | sec int
interface Loopback10
 ip address 1.1.1.1 255.255.255.255

R1#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to PE R2 **
 ip address 10.1.0.1 255.255.255.252
 ip router isis 
 duplex auto
 speed auto
 media-type rj45
 isis circuit-type level-2-only
 isis network point-to-point

R1#show run | sec ^router
router isis
 net 49.0010.0000.0000.0001.00
 is-type level-2-only
 advertise passive-only
 metric-style wide
 log-adjacency-changes
 passive-interface Loopback10
router bgp 65010
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 neighbor 2.2.2.2 remote-as 65010
 neighbor 2.2.2.2 update-source Loopback10
 neighbor 5.5.5.5 remote-as 65010
 neighbor 5.5.5.5 update-source Loopback10
 !
 address-family vpnv4
  neighbor 2.2.2.2 activate
  neighbor 2.2.2.2 send-community extended
  neighbor 2.2.2.2 route-reflector-client
  neighbor 5.5.5.5 activate
  neighbor 5.5.5.5 send-community extended
  neighbor 5.5.5.5 route-reflector-client
 exit-address-family
R9 (RR) 
    R9#show run int Lo10 | sec int
interface Loopback10
 ip address 9.9.9.9 255.255.255.255

R9#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to PE R8 **
 ip address 10.6.0.1 255.255.255.252
 ip ospf network point-to-point
 duplex auto
 speed auto
 media-type rj45

R9#show run | sec ^router
router ospf 10
 router-id 9.9.9.9
 network 9.9.9.9 0.0.0.0 area 0
 network 10.6.0.0 0.0.0.3 area 0
router bgp 65020
 bgp router-id 9.9.9.9
 bgp log-neighbor-changes
 neighbor 6.6.6.6 remote-as 65020
 neighbor 6.6.6.6 update-source Loopback10
 neighbor 8.8.8.8 remote-as 65020
 neighbor 8.8.8.8 update-source Loopback10
 !
 address-family vpnv4
  neighbor 6.6.6.6 activate
  neighbor 6.6.6.6 send-community extended
  neighbor 6.6.6.6 route-reflector-client
  neighbor 8.8.8.8 activate
  neighbor 8.8.8.8 send-community extended
  neighbor 8.8.8.8 route-reflector-client
 exit-address-family
R2 (PE) 
    R2#show run all | sec ^mpls ip__
mpls ip

R2#show run | sec ^mpls
mpls label range 200 299
mpls ldp router-id Loopback10 force

R2#show run | sec ^vrf
vrf definition Gold
 rd 65010:1
 !
 address-family ipv4
  route-target export 65010:1
  route-target import 65010:2
 exit-address-family

R2#show run int Lo10 | sec int
interface Loopback10
 ip address 2.2.2.2 255.255.255.255

R2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to CE R3 **
 no ip address
 duplex auto
 speed auto
 media-type rj45

R2#show run int Gi0/0.10 | sec int
interface GigabitEthernet0/0.10
 description ** to CE R3 in VRF Gold **
 encapsulation dot1Q 10
 vrf forwarding Gold
 ip address 10.0.1.1 255.255.255.252

R2#show run int Gi0/1 | sec int   
interface GigabitEthernet0/1
 description ** to RR R1 **
 ip address 10.1.0.2 255.255.255.252
 ip router isis 
 duplex auto
 speed auto
 media-type rj45
 isis circuit-type level-2-only
 isis network point-to-point

R2#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
 description ** to R4 **
 ip address 10.2.0.1 255.255.255.252
 ip router isis 
 duplex auto
 speed auto
 media-type rj45
 mpls ip
 isis circuit-type level-2-only
 isis network point-to-point

R2#show run | sec ^router
router isis
 net 49.0010.0000.0000.0002.00
 is-type level-2-only
 advertise passive-only
 metric-style wide
 log-adjacency-changes
 passive-interface Loopback10
router bgp 65010
 bgp router-id 2.2.2.2
 bgp log-neighbor-changes
 neighbor 1.1.1.1 remote-as 65010
 neighbor 1.1.1.1 update-source Loopback10
 !
 address-family vpnv4
  neighbor 1.1.1.1 activate
  neighbor 1.1.1.1 send-community extended
 exit-address-family
 !
 address-family ipv4 vrf Gold
  network 10.0.1.0 mask 255.255.255.252
  neighbor 10.0.1.2 remote-as 65001
  neighbor 10.0.1.2 activate
 exit-address-family
R8 (PE) 
    R8#show run all | sec ^mpls ip__
mpls ip

R8#show run | sec ^mpls
mpls label range 800 899
mpls ldp router-id Loopback10 force

R8#show run | sec ^vrf
vrf definition Gold
 rd 65020:1
 !
 address-family ipv4
  route-target export 65020:1
  route-target import 65020:2
 exit-address-family

R8#show run int Lo10 | sec int
interface Loopback10
 ip address 8.8.8.8 255.255.255.255

R8#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to CE R10 **
 no ip address
 duplex auto
 speed auto
 media-type rj45

R8#show run int Gi0/0.10 | sec int
interface GigabitEthernet0/0.10
 description ** to CE R10 in VRF Gold **
 encapsulation dot1Q 10
 vrf forwarding Gold
 ip address 10.0.2.1 255.255.255.252

R8#show run int Gi0/1 | sec int   
interface GigabitEthernet0/1
 description ** to RR R9 **
 ip address 10.6.0.2 255.255.255.252
 ip ospf network point-to-point
 duplex auto
 speed auto
 media-type rj45

R8#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
 description ** to R7 **
 ip address 10.5.0.1 255.255.255.252
 ip ospf network point-to-point
 duplex auto
 speed auto
 media-type rj45
 mpls ip

R8#show run | sec ^router
router ospf 10
 router-id 8.8.8.8
 network 8.8.8.8 0.0.0.0 area 0
 network 10.5.0.0 0.0.0.3 area 0
 network 10.6.0.0 0.0.0.3 area 0
router bgp 65020
 bgp router-id 8.8.8.8
 bgp log-neighbor-changes
 neighbor 9.9.9.9 remote-as 65020
 neighbor 9.9.9.9 update-source Loopback10
 !
 address-family vpnv4
  neighbor 9.9.9.9 activate
  neighbor 9.9.9.9 send-community extended
 exit-address-family
 !
 address-family ipv4 vrf Gold
  network 10.0.2.0 mask 255.255.255.252
  neighbor 10.0.2.2 remote-as 65002
  neighbor 10.0.2.2 activate
 exit-address-family
R4 
    R4#show run all | sec ^mpls ip__
mpls ip

R4#show run | sec ^mpls
mpls label range 400 499
mpls ldp router-id Loopback10 force

R4#show run int Lo10 | sec int
interface Loopback10
 ip address 4.4.4.4 255.255.255.255

R4#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to PE R2 **
 ip address 10.2.0.2 255.255.255.252
 ip router isis 
 duplex auto
 speed auto
 media-type rj45
 mpls ip
 isis circuit-type level-2-only
 isis network point-to-point 

R4#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
 description ** to ASBR PE R5 **
 ip address 10.3.0.1 255.255.255.252
 ip router isis 
 duplex auto
 speed auto
 media-type rj45
 mpls ip
 isis circuit-type level-2-only
 isis network point-to-point 

R4#show run | sec ^router
router isis
 net 49.0010.0000.0000.0004.00
 is-type level-2-only
 advertise passive-only
 metric-style wide
 log-adjacency-changes
 passive-interface Loopback10
R7 
    R7#show run all | sec ^mpls ip__
mpls ip

R7#show run | sec ^mpls
mpls label range 700 799
mpls ldp router-id Loopback10 force

R7#show run int Lo10 | sec int
interface Loopback10
 ip address 7.7.7.7 255.255.255.255

R7#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to PE R8 **
 ip address 10.5.0.2 255.255.255.252
 ip ospf network point-to-point
 duplex auto
 speed auto
 media-type rj45
 mpls ip

R7#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
 description ** to ASBR PE R6 **
 ip address 10.4.0.1 255.255.255.252
 ip ospf network point-to-point
 duplex auto
 speed auto
 media-type rj45
 mpls ip

R7#show run | sec ^router
router ospf 10
 router-id 7.7.7.7
 network 7.7.7.7 0.0.0.0 area 0
 network 10.4.0.0 0.0.0.3 area 0
 network 10.5.0.0 0.0.0.3 area 0
R3 (CE) 
    R3#show run | sec ^vrf
vrf definition Gold
 rd 1:1
 !
 address-family ipv4
 exit-address-family

R3#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to Host1 **
 no ip address
 duplex auto
 speed auto
 media-type rj45

R3#show run int Gi0/0.1 | sec int
interface GigabitEthernet0/0.1
 description ** to Host1 in VRF Gold (VLAN 1) **
 encapsulation dot1Q 1 native
 vrf forwarding Gold
 ip address 192.168.1.2 255.255.255.0

R3#show run int Gi0/1 | sec int   
interface GigabitEthernet0/1
 description ** to PE R2 **
 no ip address
 duplex auto
 speed auto
 media-type rj45

R3#show run int Gi0/1.10 | sec int
interface GigabitEthernet0/1.10
 description ** to PE R2 in VRF Gold **
 encapsulation dot1Q 10
 vrf forwarding Gold
 ip address 10.0.1.2 255.255.255.252

R3#show run | sec ^router
router bgp 65001
 bgp router-id 3.3.3.3
 bgp log-neighbor-changes
 !
 address-family ipv4 vrf Gold
  network 192.168.1.0
  neighbor 10.0.1.1 remote-as 65010
  neighbor 10.0.1.1 activate
 exit-address-family
R10 (CE) 
    R10#show run | sec ^vrf
vrf definition Gold
 rd 1:1
 !
 address-family ipv4
 exit-address-family

R10#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to Host2 **
 no ip address
 duplex auto
 speed auto
 media-type rj45

R10#show run int Gi0/0.1 | sec int
interface GigabitEthernet0/0.1
 description ** to Host2 in VRF Gold (VLAN 1) **
 encapsulation dot1Q 1 native
 vrf forwarding Gold
 ip address 192.168.2.2 255.255.255.0

R10#show run int Gi0/1 | sec int   
interface GigabitEthernet0/1
 description ** to PE R8 **
 no ip address
 duplex auto
 speed auto
 media-type rj45

R10#show run int Gi0/1.10 | sec int
interface GigabitEthernet0/1.10
 description ** to PE R8 in VRF Gold **
 encapsulation dot1Q 10
 vrf forwarding Gold
 ip address 10.0.2.2 255.255.255.252

R10#show run | sec ^router
router bgp 65002
 bgp router-id 10.10.10.10
 bgp log-neighbor-changes
 !
 address-family ipv4 vrf Gold
  network 192.168.2.0
  neighbor 10.0.2.1 remote-as 65020
  neighbor 10.0.2.1 activate
 exit-address-family
Host1 
    Host1#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to CE R3 **
 ip address 192.168.1.1 255.255.255.0
 duplex auto
 speed auto
 media-type rj45

Host1#show run | sec ^ip route
ip route 0.0.0.0 0.0.0.0 192.168.1.2
Host2 
    Host2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
 description ** to CE R10 **
 ip address 192.168.2.1 255.255.255.0
 duplex auto
 speed auto
 media-type rj45

Host2#show run | sec ^ip route
ip route 0.0.0.0 0.0.0.0 192.168.2.2

Host1#trace 192.168.2.1 probe 1
Type escape sequence to abort.
Tracing the route to 192.168.2.1
VRF info: (vrf in name/id, vrf out name/id)
  1 192.168.1.2 2 msec
  2 10.0.1.1 23 msec
  3 10.2.0.2 [MPLS: Labels 402/508 Exp 0] 52 msec
  4 172.16.2.1 [MPLS: Label 508 Exp 0] 26 msec
  5 172.16.2.2 46 msec                   « Inter-AS link is not label switched
  6 10.4.0.1 [MPLS: Labels 703/807 Exp 0] 69 msec
  7 10.0.2.1 [MPLS: Label 807 Exp 0] 49 msec
  8 10.0.2.2 70 msec
  9 192.168.2.1 71 msec
 



Host1#ping 192.168.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 69/70/72 ms





Host2#trace 192.168.1.1 probe 1
Type escape sequence to abort.
Tracing the route to 192.168.1.1
VRF info: (vrf in name/id, vrf out name/id)
  1 192.168.2.2 2 msec
  2 10.0.2.1 24 msec
  3 10.5.0.2 [MPLS: Labels 702/609 Exp 0] 46 msec
  4 172.16.2.2 [MPLS: Label 609 Exp 0] 24 msec
  5 172.16.2.1 46 msec
  6 10.3.0.1 [MPLS: Labels 403/206 Exp 0] 69 msec
  7 10.0.1.1 [MPLS: Label 206 Exp 0] 48 msec
  8 10.0.1.2 70 msec
  9 192.168.1.1 70 msec



Host2#ping 192.168.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 71/72/74 ms

As visible in the above outputs, the inter-AS interconnection is not label switched. With inter-AS option AB routing information is exchanged on a single and separate subinterface using eBGP. Due to the VRF configurations appplied on the ASBR PE routers the Route Target (RT) values are assigned by each ASBR PE router. As a result, they need not be changed (rewritten) when advertising inter-AS routes into the local MPLS network. This is shown in the following outputs.

R5#show ip bgp vpnv4 all sum | beg Ne
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
1.1.1.1         4        65010      19      18        9    0    0 00:10:44        2
172.16.1.2      4        65020      18      18        9    0    0 00:12:24        2       « Two prefixes received from ASBR PE R6




R5#show ip bgp vpnv4 all | beg Ne    
     Network          Next Hop            Metric LocPrf Weight Path
Route Distinguisher: 65010:1
 *>i  10.0.1.0/30      2.2.2.2                  0    100      0 i
 *>i  192.168.1.0      2.2.2.2                  0    100      0 65001 i
Route Distinguisher: 65010:2 (default for vrf Gold)
 *>i  10.0.1.0/30      2.2.2.2                  0    100      0 i
 *>   10.0.2.0/30      172.16.1.2                             0 65020 i
 *>i  192.168.1.0      2.2.2.2                  0    100      0 65001 i
 *>   192.168.2.0      172.16.1.2                             0 65020 65002 i
Route Distinguisher: 65020:2
 *>   10.0.2.0/30      172.16.1.2                             0 65020 i
 *>   192.168.2.0      172.16.1.2                             0 65020 65002 i             « Site 2 prefix received from ASBR PE R6






R5#show ip bgp vpnv4 rd 65010:2 192.168.2.0 
BGP routing table entry for 65010:2:192.168.2.0/24, version 9
Paths: (1 available, best #1, table Gold)
  Advertised to update-groups:
     2         
  Refresh Epoch 1
  65020 65002, imported path from 65020:2:192.168.2.0/24 (global)        « Site 2 prefix with RT 65020:2 is imported into BGP table on ASBR PE R5
    172.16.1.2 (via default) from 172.16.1.2 (6.6.6.6)
      Origin IGP, localpref 100, valid, external, best
      Extended Community: RT:65020:2                                     « RT value 65020:2 is assigned by ASBR PE R6 during route export
      mpls labels in/out 508/610
      rx pathid: 0, tx pathid: 0x0
	  






R1#show ip bgp vpnv4 all 192.168.2.0
BGP routing table entry for 65010:2:192.168.2.0/24, version 5            « ASBR PE R5 exports prefixes with RD/RT 65010:2
Paths: (1 available, best #1, no table)
Flag: 0x100
  Advertised to update-groups:
     1         
  Refresh Epoch 1
  65020 65002, (Received from a RR-client)
    5.5.5.5 (metric 30) (via default) from 5.5.5.5 (5.5.5.5)
      Origin IGP, metric 0, localpref 100, valid, internal, best
      Extended Community: RT:65010:2                                    « Thus, the RR R1 receives Site 2 prefix with RT 65010:2 from ASBR PE R5
      mpls labels in/out nolabel/508
      rx pathid: 0, tx pathid: 0x0

As visible above, the RD/RT value is assigned by the ASBR PE routers, and it is not inherited from the remote Service Provider, in other words it is not passed along within the local MPLS networks. This creates a more predictable control plane environmnet when compared to inter-AS option B, because there is no need to rewrite RT values.