Inter-AS Option A

Scenario
This comparison shows an MPLS Inter-AS Option A setup as follows:

This setup shows both the ASBR1s configuration with the following conditions:

• An iBGP VPNv4 multi-hop session to local the route-reflectors.
• A point-to-point OSPF session between the ASBRs inside the customer VRF
• The customer VRF and dot1q sub-interface
• VRF-Lite configure to avoid OSPF down-bit from preventing ASBRs learning routes from one another (this only need to be configured on one of the ASBRs – regardless of IOS/XR setup).
• Only host routes have labels assigned for the IGP and a label range has been configured.

For more information on how Inter-AS options work, including Option A with BGP, see this document.

IOS Config

hostname ASBR1
!
ip vrf CUST_1
 description CUST_1_VRF
 rd 500:1
 vpn id 500:1
 route-target export 500:1
 route-target import 500:1
!
!
interface Loopback0
 ip address 2.2.2.2 255.255.255.255
!
interface GigabitEthernet1
 description Inter-AS Link to ISP1
 no ip address
 negotiation auto
!
interface GigabitEthernet1.100
 description Sub-interface for CUST_1
 encapsulation dot1Q 100
 ip vrf forwarding CUST_1
 ip address 192.168.117.2 255.255.255.252
 ip ospf network point-to-point
 ip ospf 1 area 0
!
interface GigabitEthernet2
 description link to P1
 ip address 10.10.24.2 255.255.255.0
 ip router isis LAB
 mpls ip
 isis circuit-type level-2-only
!
router ospf 1 vrf CUST_1
 router-id 2.2.2.2
 capability vrf-lite
 redistribute bgp 500 subnets
!
router isis LAB
 net 49.0500.2222.2222.2222.00
 is-type level-2-only
 metric-style wide
 passive-interface Loopback0
!
router bgp 500
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 3.3.3.3 remote-as 500
 neighbor 3.3.3.3 update-source Loopback0
 !
 address-family ipv4
 exit-address-family
 !
 address-family vpnv4
  neighbor 3.3.3.3 activate
  neighbor 3.3.3.3 send-community extended
  neighbor 3.3.3.3 next-hop-self
 exit-address-family
 !
 address-family ipv4 vrf CUST_1
  redistribute ospf 1
 exit-address-family
!

XR Config

hostname ASBR1
!
vrf CUST_1
 address-family ipv4 unicast
  import route-target
   500:1
  !
  export route-target
   500:1
  !
 !
!
interface Loopback0
 ipv4 address 2.2.2.2 255.255.255.255
!
interface GigabitEthernet0/0/0/1
 description link to ASBR1
!
interface GigabitEthernet0/0/0/1.100
 vrf CUST_1
 ipv4 address 192.168.117.1 255.255.255.252
 encapsulation dot1q 100
!
interface GigabitEthernet0/0/0/2
 description link to P1
 ipv4 address 10.10.24.2 255.255.255.0
!
!
router isis CORE
 is-type level-2-only
 net 49.0500.2222.2222.2222.00
 address-family ipv4 unicast
 !
 interface Loopback0
  passive
  address-family ipv4 unicast
  !
 !
 interface GigabitEthernet0/0/0/2
  address-family ipv4 unicast
  !
 !
!
router ospf INTER-AS
 vrf CUST_1
  router-id 2.2.2.2
  capability vrf-lite
  redistribute bgp 500
  area 0
   interface GigabitEthernet0/0/0/1.100
    network point-to-point
   !
  !
 !
!
router bgp 500
 bgp router-id 2.2.2.2
 bgp log neighbor changes detail
 address-family ipv4 unicast
 !
 address-family vpnv4 unicast
 !
 neighbor 3.3.3.3
  remote-as 500
  update-source Loopback0
  address-family vpnv4 unicast
   next-hop-self
  !
 !
 vrf CUST_1
  rd 500:1
  address-family ipv4 unicast
   redistribute ospf INTER-AS
  !
 !
!
mpls ldp
 router-id 2.2.2.2
 address-family ipv4
  label
   local
    allocate for host-routes
   !
  !
 !
 interface GigabitEthernet0/0/0/2
 !
!

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