MPLS L3VPN Inter-AS Option B aka MPLS VPN Inter-AS with ASBRs Exchanging VPN-IPv4 Addresses, is similar to the previous Option A example with the exception that the Inter-AS link runs a single VPNv4 EBGP peering instead of multiple VRF aware IGP or BGP instances.

In this procedure, the PE routers use IBGP to redistribute labeled VPN-IPv4 routes either to an Autonomous System Border Router (ASBR), or to a route reflector of which an ASBR is a client.  The ASBR then uses EBGP to redistribute those labeled VPN-IPv4 routes to an ASBR in another AS, which in turn distributes them to the PE routers in that AS, or perhaps to another ASBR which in turn distributes them, and so on.

We will use following network topology to discuss this feature.

PE-CE Configuration with ISP-1

R7 (CE Router) - Customer A Site 1

## EIGRP is the PE-CE routing protocol used for this customer ##

interface FastEthernet0/0
 ip address 192.168.47.7 255.255.255.0
!
interface FastEthernet0/1
 ip address 192.168.7.7 255.255.255.0
!
router eigrp 100
 network 192.168.0.0 0.0.255.255

R8 (CE Router) - Customer B Site 1

## OSPF is the PE-CE routing protocol used for this customer ##

interface FastEthernet0/0
 ip address 172.16.48.8 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
!
interface FastEthernet0/1
 ip address 172.16.8.8 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
!
router ospf 1
 router-id 172.16.8.8

R4 (PE Router) - ISP 1

vrf definition Cust-A
 rd 101:201
 !
 address-family ipv4
  route-target export 101:201
  route-target import 101:201
 exit-address-family
!
vrf definition Cust-B
 rd 102:202
 !        
 address-family ipv4
  route-target export 102:202
  route-target import 102:202
 exit-address-family
!
interface Loopback0
 ip address 1.1.0.4 255.255.255.255
 ip ospf 1 area 0
!
interface FastEthernet0/1
 ip address 1.1.34.4 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip
!
interface FastEthernet1/0
 vrf forwarding Cust-A
 ip address 192.168.47.4 255.255.255.0
!
interface FastEthernet1/1
 vrf forwarding Cust-B
 ip address 172.16.48.4 255.255.255.0
 ip ospf network point-to-point
 ip ospf 100 area 0
!
router eigrp 1
 !
 address-family ipv4 vrf Cust-A autonomous-system 100
  default-metric 10000 100 255 1 1500
  redistribute bgp 100
  network 192.168.47.0
 exit-address-family
!
router ospf 100 vrf Cust-B
 router-id 172.16.48.4
 redistribute bgp 100 subnets
!
router ospf 1
 router-id 1.1.0.4
!
router bgp 100
 bgp router-id 1.1.0.4
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 1.1.0.1 remote-as 100
 neighbor 1.1.0.1 update-source Loopback0
 !
 address-family ipv4
 exit-address-family
 !
 address-family vpnv4
  neighbor 1.1.0.1 activate
  neighbor 1.1.0.1 send-community extended
 exit-address-family
 !
 address-family ipv4 vrf Cust-A
  redistribute connected
  redistribute eigrp 100
 exit-address-family
 !
 address-family ipv4 vrf Cust-B
  redistribute connected
  redistribute ospf 100 match internal external 1 external 2
 exit-address-family
!    

R3 (P Router) - ISP 1

interface Loopback0
 ip address 1.1.0.3 255.255.255.255
 ip ospf 1 area 0
!
interface FastEthernet0/0
 ip address 1.1.13.3 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip
!
interface FastEthernet0/1
 ip address 1.1.34.3 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip  
!
router ospf 1
 router-id 1.1.0.3
!     

R1 (PE Router) - ISP 1 - ASBR connected to ISP 2

interface Loopback0
 ip address 1.1.0.1 255.255.255.255
 ip ospf 1 area 0
!
interface FastEthernet0/0
 ip address 1.1.13.1 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip
!
interface FastEthernet0/1
 ip address 12.12.12.1 255.255.255.0
 mpls bgp forwarding
!
router ospf 1
 router-id 1.1.0.1
!
router bgp 100
 bgp router-id 1.1.0.1
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 no bgp default route-target filter
 neighbor 1.1.0.4 remote-as 100
 neighbor 1.1.0.4 update-source Loopback0
 neighbor 12.12.12.2 remote-as 200
 !
 address-family ipv4
 exit-address-family
 !
 address-family vpnv4
  neighbor 1.1.0.4 activate
  neighbor 1.1.0.4 send-community extended
  neighbor 1.1.0.4 next-hop-self
  neighbor 12.12.12.2 activate
  neighbor 12.12.12.2 send-community extended
 exit-address-family
!

PE-CE Configuration with ISP-2

Option B’s VPNv4 EBGP peering between the providers means that the VPNv4 Route Distinguisher and Route Target fields have global significance between the MPLS Service Providers.

R1(config-if)#router bgp 100
R1(config-router)#neighbor 12.12.12.2 remote-as 200
R1(config-router)#address-family vpnv4
R1(config-router-af)#neighbor 12.12.12.2 activate
R1(config-router-af)#
*Sep  1 21:48:01.247: %BGP-5-ADJCHANGE: neighbor 12.12.12.2 Up 
*Sep  1 21:48:01.251: RT: updating connected 12.12.12.2/32 (0x0):
    via 0.0.0.0 Fa0/1  1048578

*Sep  1 21:48:01.259: RT: add 12.12.12.2/32 via 0.0.0.0, connected metric [0/0]
R1(config-router-af)#
*Sep  1 21:48:01.459: %BGP_LMM-6-AUTOGEN1: The mpls bgp forwarding command has been configured on interface: FastEthernet0/1
R1(config-router-af)#

PE Routers R4 and R6 in both ISP-1 and ISP-2 respectively are configured with same Route Distinguisher and Route Target for Customer A & Customer B as agreed between both MPLS providers.

vrf definition Cust-A
 rd 101:201
 !
 address-family ipv4
  route-target export 101:201
  route-target import 101:201
 exit-address-family
!
vrf definition Cust-B
 rd 102:202
 !        
 address-family ipv4
  route-target export 102:202
  route-target import 102:202
 exit-address-family

R2 (PE Router) - ISP 2 - ASBR connected to ISP 1

interface Loopback0
 ip address 2.2.0.2 255.255.255.255
 ip ospf 1 area 0
!
interface FastEthernet0/0
 ip address 2.2.25.2 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip
!
interface FastEthernet0/1
 ip address 12.12.12.2 255.255.255.0
 mpls bgp forwarding
!
router ospf 1
 router-id 2.2.0.2
!
router bgp 200
 bgp router-id 2.2.0.2
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 no bgp default route-target filter
 neighbor 2.2.0.6 remote-as 200
 neighbor 2.2.0.6 update-source Loopback0
 neighbor 12.12.12.1 remote-as 100
 !
 address-family ipv4
 exit-address-family
 !
 address-family vpnv4
  neighbor 2.2.0.6 activate
  neighbor 2.2.0.6 send-community extended
  neighbor 2.2.0.6 next-hop-self
  neighbor 12.12.12.1 activate
  neighbor 12.12.12.1 send-community extended
 exit-address-family
!

R5 (P Router) - ISP 2

interface Loopback0
 ip address 2.2.0.5 255.255.255.255
 ip ospf 1 area 0
!
interface FastEthernet0/0
 ip address 2.2.25.5 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip
!
interface FastEthernet0/1
 ip address 2.2.56.5 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip  
!
router ospf 1
 router-id 2.2.0.5

R6 (PE Router) - ISP 2

vrf definition Cust-A
 rd 101:201
 !
 address-family ipv4
  route-target export 101:201
  route-target import 101:201
 exit-address-family
!
vrf definition Cust-B
 rd 102:202
 !        
 address-family ipv4
  route-target export 102:202
  route-target import 102:202
 exit-address-family
!
interface Loopback0
 ip address 2.2.0.6 255.255.255.255
 ip ospf 1 area 0
!
interface FastEthernet0/1
 ip address 2.2.56.6 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip
!
interface FastEthernet1/0
 vrf forwarding Cust-A
 ip address 192.168.69.6 255.255.255.0
!
interface FastEthernet1/1
 vrf forwarding Cust-B
 ip address 172.16.106.6 255.255.255.0
 ip ospf network point-to-point
 ip ospf 200 area 0
!
!
router eigrp 1
 !
 address-family ipv4 vrf Cust-A autonomous-system 100
  default-metric 10000 100 255 1 1500
  redistribute bgp 200
  network 192.168.69.0
 exit-address-family
!
router ospf 200 vrf Cust-B
 router-id 172.16.106.6
 redistribute bgp 200 subnets
!
router ospf 1
!
router bgp 200
 bgp router-id 2.2.0.6
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 2.2.0.2 remote-as 200
 neighbor 2.2.0.2 update-source Loopback0
 !
 address-family ipv4
 exit-address-family
 !
 address-family vpnv4
  neighbor 2.2.0.2 activate
  neighbor 2.2.0.2 send-community extended
 exit-address-family
 !
 address-family ipv4 vrf Cust-A
  redistribute connected
  redistribute eigrp 100
 exit-address-family
 !
 address-family ipv4 vrf Cust-B
  redistribute connected
  redistribute ospf 200 match internal external 1 external 2
 exit-address-family
!

R9 (CE Router) - Customer A Site 2

## EIGRP is the PE-CE routing protocol used for this customer ##

interface FastEthernet0/0
 ip address 192.168.69.9 255.255.255.0
!
interface FastEthernet0/1
 ip address 192.168.9.9 255.255.255.0
!
router eigrp 100
 network 192.168.0.0 0.0.255.255

R10 (CE Router) - Customer B Site 2

## OSPF is the PE-CE routing protocol used for this customer ##

interface FastEthernet0/0
 ip address 172.16.106.10 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
!
interface FastEthernet0/1
 ip address 172.16.10.10 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0
!
router ospf 1
 router-id 172.16.10.10

Verification & Testing

R7#sh ip route | beg Gateway
Gateway of last resort is not set

      192.168.7.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.7.0/24 is directly connected, FastEthernet0/1
L        192.168.7.7/32 is directly connected, FastEthernet0/1
D     192.168.9.0/24 [90/33280] via 192.168.47.4, 00:46:10, FastEthernet0/0
      192.168.47.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.47.0/24 is directly connected, FastEthernet0/0
L        192.168.47.7/32 is directly connected, FastEthernet0/0
D     192.168.69.0/24 [90/30720] via 192.168.47.4, 00:46:10, FastEthernet0/0

Ping Customer-A Site 2 network

R7#ping 192.168.9.9 source 192.168.7.7
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.9.9, timeout is 2 seconds:
Packet sent with a source address of 192.168.7.7 
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 232/240/248 ms

Traceroute Customer-A Site 2 network

R7#traceroute 192.168.9.9 source 192.168.7.7
Type escape sequence to abort.
Tracing the route to 192.168.9.9
VRF info: (vrf in name/id, vrf out name/id)
  1 192.168.47.4 28 msec 60 msec 56 msec
  2 1.1.34.3 [MPLS: Labels 302/109 Exp 0] 220 msec 208 msec 260 msec
  3 1.1.13.1 [MPLS: Label 109 Exp 0] 232 msec 240 msec 208 msec
  4 12.12.12.2 [MPLS: Label 206 Exp 0] 200 msec 256 msec 200 msec
  5 2.2.25.5 [MPLS: Labels 502/604 Exp 0] 196 msec 260 msec 244 msec
  6 192.168.69.6 [MPLS: Label 604 Exp 0] 208 msec 184 msec 228 msec
  7 192.168.69.9 256 msec *  204 msec

We can see from the above output there are three different LSP (Label Switch Path) are being used to provide end-to-end reachability.

LSP-1: From R4 to R1 using VPN Label 109

R4#sh bgp vpnv4 unicast vrf Cust-A 192.168.9.0
BGP routing table entry for 101:201:192.168.9.0/24, version 10
Paths: (1 available, best #1, table Cust-A)
  Not advertised to any peer
  Refresh Epoch 1
  200
    1.1.0.1 (metric 3) from 1.1.0.1 (1.1.0.1)
      Origin incomplete, metric 0, localpref 100, valid, internal, best
      Extended Community: RT:101:201 0x8800:32768:0 0x8801:100:5120 
        0x8802:65281:25600 0x8803:65281:1500 0x8806:0:3232253193
      mpls labels in/out nolabel/109
      rx pathid: 0, tx pathid: 0x0

LSP-2: From R1 to R2 using VPN Label 206

R1#sh bgp vpnv4 unicast all 192.168.9.0
BGP routing table entry for 101:201:192.168.9.0/24, version 6
Paths: (1 available, best #1, no table)
  Advertised to update-groups:
     2         
  Refresh Epoch 1
  200
    12.12.12.2 from 12.12.12.2 (2.2.0.2)
      Origin incomplete, localpref 100, valid, external, best
      Extended Community: RT:101:201 0x8800:32768:0 0x8801:100:5120 
        0x8802:65281:25600 0x8803:65281:1500 0x8806:0:3232253193
      mpls labels in/out 109/206
      rx pathid: 0, tx pathid: 0x0


LSP-3: From R2 to R6 using VPN label 604

R2#sh bgp vpnv4 unicast all 192.168.9.0
BGP routing table entry for 101:201:192.168.9.0/24, version 3
Paths: (1 available, best #1, no table)
  Advertised to update-groups:
     1         
  Refresh Epoch 1
  Local
    2.2.0.6 (metric 3) from 2.2.0.6 (2.2.0.6)
      Origin incomplete, metric 30720, localpref 100, valid, internal, best
      Extended Community: RT:101:201 Cost:pre-bestpath:128:30720 
        0x8800:32768:0 0x8801:100:5120 0x8802:65281:25600 0x8803:65281:1500 
        0x8806:0:3232253193
      mpls labels in/out 206/604
      rx pathid: 0, tx pathid: 0x0

Finally unlabelled traffic from ISP 2 PE Router (R6) to CE Router (R9)

R6#sh bgp vpnv4 unicast vrf Cust-A 192.168.9.0
BGP routing table entry for 101:201:192.168.9.0/24, version 2
Paths: (1 available, best #1, table Cust-A)
  Advertised to update-groups:
     1         
  Refresh Epoch 1
  Local
    192.168.69.9 from 0.0.0.0 (2.2.0.6)
      Origin incomplete, metric 30720, localpref 100, weight 32768, valid, sourced, best
      Extended Community: RT:101:201 Cost:pre-bestpath:128:30720 
        0x8800:32768:0 0x8801:100:5120 0x8802:65281:25600 0x8803:65281:1500 
        0x8806:0:3232253193
      mpls labels in/out 604/nolabel
      rx pathid: 0, tx pathid: 0x0

Similarly, end-to-end reachability can be verified for Customer B sites.

R8#sh ip route | beg Gateway
Gateway of last resort is not set

      172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
C        172.16.8.0/24 is directly connected, FastEthernet0/1
L        172.16.8.8/32 is directly connected, FastEthernet0/1
O E2     172.16.10.0/24 [110/1] via 172.16.48.4, 01:29:16, FastEthernet0/0
C        172.16.48.0/24 is directly connected, FastEthernet0/0
L        172.16.48.8/32 is directly connected, FastEthernet0/0
O E2     172.16.106.0/24 [110/1] via 172.16.48.4, 01:29:16, FastEthernet0/0

R8#ping 172.16.10.10 source 172.16.8.8
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.10.10, timeout is 2 seconds:
Packet sent with a source address of 172.16.8.8 
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 184/227/260 ms

R8#traceroute 172.16.10.10 source 172.16.8.8
Type escape sequence to abort.
Tracing the route to 172.16.10.10
VRF info: (vrf in name/id, vrf out name/id)
  1 172.16.48.4 40 msec 40 msec 36 msec
  2 1.1.34.3 [MPLS: Labels 302/111 Exp 0] 204 msec 252 msec 252 msec
  3 1.1.13.1 [MPLS: Label 111 Exp 0] 240 msec 220 msec 220 msec
  4 12.12.12.2 [MPLS: Label 211 Exp 0] 248 msec 200 msec 232 msec
  5 2.2.25.5 [MPLS: Labels 502/606 Exp 0] 208 msec 256 msec 236 msec
  6 172.16.106.6 [MPLS: Label 606 Exp 0] 184 msec 232 msec 188 msec
  7 172.16.106.10 256 msec *  240 msec

R4#sh bgp vpnv4 unicast vrf Cust-B 172.16.10.0
BGP routing table entry for 102:202:172.16.10.0/24, version 12
Paths: (1 available, best #1, table Cust-B)
  Not advertised to any peer
  Refresh Epoch 1
  200
    1.1.0.1 (metric 3) from 1.1.0.1 (1.1.0.1)
      Origin incomplete, metric 0, localpref 100, valid, internal, best
      Extended Community: RT:102:202 OSPF DOMAIN ID:0x0005:0x000000C80200 
        OSPF RT:0.0.0.0:2:0 OSPF ROUTER ID:172.16.106.6:0
      mpls labels in/out nolabel/111
      rx pathid: 0, tx pathid: 0x0

R1#sh bgp vpnv4 unicast all 172.16.10.0       
BGP routing table entry for 102:202:172.16.10.0/24, version 8
Paths: (1 available, best #1, no table)
  Advertised to update-groups:
     2         
  Refresh Epoch 1
  200
    12.12.12.2 from 12.12.12.2 (2.2.0.2)
      Origin incomplete, localpref 100, valid, external, best
      Extended Community: RT:102:202 OSPF DOMAIN ID:0x0005:0x000000C80200 
        OSPF RT:0.0.0.0:2:0 OSPF ROUTER ID:172.16.106.6:0
      mpls labels in/out 111/211
      rx pathid: 0, tx pathid: 0x0

R2#sh bgp vpnv4 unicast all 172.16.10.0
BGP routing table entry for 102:202:172.16.10.0/24, version 7
Paths: (1 available, best #1, no table)
  Advertised to update-groups:
     1         
  Refresh Epoch 1
  Local
    2.2.0.6 (metric 3) from 2.2.0.6 (2.2.0.6)
      Origin incomplete, metric 2, localpref 100, valid, internal, best
      Extended Community: RT:102:202 OSPF DOMAIN ID:0x0005:0x000000C80200 
        OSPF RT:0.0.0.0:2:0 OSPF ROUTER ID:172.16.106.6:0
      mpls labels in/out 211/606
      rx pathid: 0, tx pathid: 0x0

R6#sh bgp vpnv4 unicast vrf Cust-B 172.16.10.0
BGP routing table entry for 102:202:172.16.10.0/24, version 4
Paths: (1 available, best #1, table Cust-B)
  Advertised to update-groups:
     1         
  Refresh Epoch 1
  Local
    172.16.106.10 from 0.0.0.0 (2.2.0.6)
      Origin incomplete, metric 2, localpref 100, weight 32768, valid, sourced, best
      Extended Community: RT:102:202 OSPF DOMAIN ID:0x0005:0x000000C80200 
        OSPF RT:0.0.0.0:2:0 OSPF ROUTER ID:172.16.106.6:0
      mpls labels in/out 606/nolabel
      rx pathid: 0, tx pathid: 0x0