There are three flavors of Inter-AS BGP MPLS VPN implementation. Options A, B, and C. Option C is the most preferred solution for large ISP networks because of its scalability and ease of implementation.

Check out the implementation of option A and B below links:

Inter-AS BGP MPLS VPN Option A implementation on Huawei routers

Inter-AS BGP MPLS VPN Option B implementation on Huawei routers

Both Option A and Option B can meet inter-AS VPN communication requirements. Their main drawback is that ASBRs need to maintain and distribute VPN-IPv4 routes which may hinder network extension when each AS needs to exchange a large number of VPN routes. There are two implementations of Inter-AS BGP Option C:

• The PE devices directly establish a multi-hop EBGP connection to exchange VPN-IPv4 routes. This is not scalable when you have many PEs and have to establish multi-hop EBGP peering between them.
• Inter-Provider Backbones Option C with an RR. To improve network scalability, you can specify an RR in each AS. The RR stores all VPN-IPv4 routes and exchanges VPN-IPv4 routes with the PE devices in the local AS. The RRs in two ASs establish an MP-EBGP connection to advertise VPN-IPv4 routes.

In this article, we will focus on the first implementation where RRs are not deployed. Below is our topology diagram.

Lab objective: Establish multi-hop EBGP peering between PE1 and PE2, ensure they can exchange customer routes, and test communication between the two customer sites.

1. Configure IP Address, Enable MPLS and LDP on interfaces in the two ASes 100, and 200. Configure IGP and enable on the interfaces in the backbone network.

***************************PE01
sys
sysname PE1
#
isis 20
 is-level level-2
 cost-style wide
 network-entity 49.0000.0020.0200.2002.00
 is-name PE01
#
mpls lsr-id 2.2.2.2
mpls
mpls ldp
#
interface LoopBack0
 ip address 2.2.2.2 255.255.255.255
 isis enable 20
#
interface GigabitEthernet0/0/3
 ip address 10.251.250.10 255.255.255.254
 isis enable 20
 isis circuit-type p2p
 isis cost 100
 mpls
 mpls ldp
#
commit

*******************************P1
sys
sysname P1
#
mpls lsr-id 3.3.3.3
mpls
#
mpls ldp
#
isis 20
 is-level level-2
 cost-style wide
 network-entity 49.0000.0030.0300.3003.00
 is-name P01
#
interface LoopBack0
 ip address 3.3.3.3 255.255.255.255
 isis enable 20
#
interface GigabitEthernet0/0/0
 ip address 10.251.250.11 255.255.255.254
 isis enable 20
 isis circuit-type p2p
 isis cost 100
 mpls
 mpls ldp
#
interface GigabitEthernet0/0/1
 ip address 10.251.250.8 255.255.255.254
 isis enable 20
 isis circuit-type p2p
 isis cost 100
 mpls
 mpls ldp
#
commit

****************************ASBR01
sys
sysname ASBR01
#
mpls lsr-id 4.4.4.4
mpls
#
mpls ldp
#
isis 20
 is-level level-2
 cost-style wide
 network-entity 49.0000.0040.0400.4004.00
 is-name ASBR01
#
interface LoopBack0
 ip address 4.4.4.4 255.255.255.255
 isis enable 20
#
interface GigabitEthernet0/0/2
 ip address 10.251.250.9 255.255.255.254
 isis enable 20
 isis circuit-type p2p
 isis cost 100
 mpls
 mpls ldp
#
interface GigabitEthernet0/0/1
 ip address 172.16.16.3 255.255.255.254
 mpls
#
commit

*******************************ASBR02
sysname ASBR02
#
mpls lsr-id 5.5.5.5
mpls
#
mpls ldp
#
isis 20
 is-level level-2
 cost-style wide
 network-entity 49.0000.0050.0500.5005.00
 is-name ASBR02
#
interface LoopBack0
 ip address 5.5.5.5 255.255.255.255
 isis enable 20
#
interface GigabitEthernet0/0/0
 description TO_ASBR_02
 ip address 172.16.16.2 255.255.255.254
 mpls
 mpls ldp
#
interface GigabitEthernet0/0/2
 ip address 10.251.250.12 255.255.255.254
 isis enable 20
 isis circuit-type p2p
 isis cost 100
 mpls
 mpls ldp
#
commit

*************************************P2
sysname P2
#
mpls lsr-id 6.6.6.6
mpls
#
mpls ldp
#
isis 20
 is-level level-2
 cost-style wide
 network-entity 49.0000.0060.0600.6006.00
 is-name P02
#
interface LoopBack0
 ip address 6.6.6.6 255.255.255.255
 isis enable 20
#
interface GigabitEthernet0/0/0
 ip address 10.251.250.13 255.255.255.254
 isis enable 20
 isis circuit-type p2p
 isis cost 100
 mpls
 mpls ldp
#
interface GigabitEthernet0/0/1
 ip address 10.251.250.14 255.255.255.254
 isis enable 20
 isis circuit-type p2p
 isis cost 100
 mpls
 mpls ldp
#

**********************************PE2
mpls lsr-id 7.7.7.7
mpls
#
mpls ldp
#
isis 20
 is-level level-2
 cost-style wide
 network-entity 49.0000.0070.0700.7007.00
 is-name PE02
#
interface LoopBack0
 ip address 7.7.7.7 255.255.255.255
 isis enable 20
#
interface GigabitEthernet0/0/0
 ip address 10.251.250.8 255.255.255.254
 isis enable 20
 isis circuit-type p2p
 isis cost 100
 mpls
 mpls ldp
#
interface GigabitEthernet0/0/2
 ip address 10.251.250.13 255.255.255.254
 isis enable 20
 isis circuit-type p2p
 isis cost 100
 mpls
 mpls ldp
#
commit

Verify IGP status in AS 100 and AS 200. Ensure PEs can ping ASBRs.

Configure iBGP between ASBRs and PE routers. Configure EBGP between ASBRs

****************************PE1
bgp 200
 router-id 2.2.2.2
 peer 4.4.4.4 as-number 200
 peer 4.4.4.4 connect-interface LoopBack0
 #
 ipv4-family unicast
  undo synchronization

  peer 4.4.4.4 enable
  peer 4.4.4.4 label-route-capability
 #

*****************************ASBR01
bgp 200
 router-id 4.4.4.4
 peer 2.2.2.2 as-number 200
 peer 2.2.2.2 connect-interface LoopBack0
 peer 172.16.16.2 as-number 100
 #
 ipv4-family unicast
  undo synchronization
  network 2.2.2.2 255.255.255.255

  peer 2.2.2.2 enable
  peer 2.2.2.2 route-policy policy2 export
  peer 2.2.2.2 label-route-capability
  peer 172.16.16.2 enable
  peer 172.16.16.2 route-policy policy1 export
  peer 172.16.16.2 label-route-capability
#

************************PE2
bgp 100
 router-id 7.7.7.7

 peer 5.5.5.5 as-number 100
 peer 5.5.5.5 connect-interface LoopBack0
 #
 ipv4-family unicast
  undo synchronization

  peer 5.5.5.5 enable
  peer 5.5.5.5 label-route-capability
 #

***************************ASBR02
bgp 100
 router-id 5.5.5.5
 peer 7.7.7.7 as-number 100
 peer 7.7.7.7 connect-interface LoopBack0
 peer 172.16.16.3 as-number 200
 #
 ipv4-family unicast
  undo synchronization
  network 7.7.7.7 255.255.255.255

  peer 7.7.7.7 enable
  peer 7.7.7.7 route-policy policy2 export
  peer 7.7.7.7 label-route-capability
  peer 172.16.16.3 enable
  peer 172.16.16.3 route-policy policy1 export
  peer 172.16.16.3 label-route-capability
#

Verify BGP status on PEs and ASBRs. Configure reachability between PE routers

Configure EBGP between PE routers

****************************PE1
bgp 200

 peer 7.7.7.7 as-number 100
 peer 7.7.7.7 ebgp-max-hop 10
 peer 7.7.7.7 connect-interface LoopBack0
 #
 ipv4-family unicast

  undo peer 7.7.7.7 enable
 #
 ipv4-family vpnv4
  undo policy vpn-target
  peer 7.7.7.7 enable
 #

****************************PE2
bgp 100
 peer 2.2.2.2 as-number 200
 peer 2.2.2.2 ebgp-max-hop 10
 peer 2.2.2.2 connect-interface LoopBack0

 #
 ipv4-family unicast

  undo peer 2.2.2.2 enable

 #
 ipv4-family vpnv4
  undo policy vpn-target
  peer 2.2.2.2 enable
 #

Configure VPN Instances on PE routers and bind the VPN to the interfaces facing the customers.

**************************PE1
ip vpn-instance VRF1
 ipv4-family
  route-distinguisher 200:1
  vpn-target 200:1 export-extcommunity
  vpn-target 200:1 import-extcommunity
#
interface GigabitEthernet0/0/0
 ip binding vpn-instance VRF1
 ip address 10.10.10.2 255.255.255.254
#

**************************PE2
ip vpn-instance VRF1
 ipv4-family
  route-distinguisher 65000:1
  vpn-target 200:1 export-extcommunity
  vpn-target 200:1 import-extcommunity
#
interface GigabitEthernet0/0/1
 ip binding vpn-instance VRF1
 ip address 20.20.20.2 255.255.255.254
#

Configure EBGP between PEs and CEs.

*************************PE1
bgp 200
 ipv4-family vpn-instance VRF1

  peer 10.10.10.3 as-number 65500
 #

*************************CE01
bgp 65500
 router-id 1.1.1.1
 peer 10.10.10.2 as-number 200
 #
 ipv4-family unicast
  undo synchronization
  network 1.1.1.1 255.255.255.255

  peer 10.10.10.2 enable
#

*************************PE2
bgp 100
 ipv4-family vpn-instance VRF1
  peer 20.20.20.3 as-number 65000
#

************************CE02
bgp 65000
 peer 20.20.20.2 as-number 100
 #
 ipv4-family unicast
  undo synchronization
  network 8.8.8.8 255.255.255.255

  peer 20.20.20.2 enable
#

Verify the eBGP status between PEs and CEs

Testing communication between customer sites.