The following sections describe how to configure the Inter-AS Option AB feature on an ASBR for either an MPLS VPN or an MPLS VPN that supports CSC:

Note

If Inter-AS Option AB is already deployed in your network and you want to do Option B style peering for some prefixes (that is, implement Inter-AS Option AB+), configure the inter-as-hybrid global command as described in the “Configuring the Routing Policy for VPNs that Need Inter-AS Connections” section.

Use the following steps to configure the VRFs on the ASBR interface for each VPN customer so that these VPNs have connectivity over the MPLS VPN--Inter-AS Option AB network.

Note	The mpls bgp forwarding command is used only on the ASBR interface for VRFs that support CSC.

Use all of the steps in the following procedure to configure additional VRFs that need to be configured on the ASBR interface and the VRFs that need to be configured on the peer ASBR interface.

1. Enable privileged EXEC mode. Enter your password if prompted.

   enable
   Example:
   Router> enable

2. Enter global configuration mode.

   configure terminal
   Example:
   Router# configure terminal

3. Specify the interface to configure and enter the interface configuration mode.

   • The type argument specifies the type of interface to be configured.

   • The number argument specifies the port, connector, or interface card number.

     interface type number
     Example:
      Router(config)# interface Ethernet 5/0

4. Associate a VRF with the specified interface or subinterface.

   • The vrf-name argument is the name assigned to a VRF.

     ip vrf forwarding vrf-name
     Example:
      Router(config-if)# ip vrf forwarding vpn1

5. (Optional) Configures BGP to enable MPLS forwarding on connecting interfaces for VRFs that must support MPLS traffic.

   • This step applies to a CSC network only.

   mpls bgp forwarding
   Example:
    Router(config-if)# mpls bgp forwarding

6. (Optional) Exits to privileged EXEC mode.

   end
   Example:
    Router(config-if)# end

BGP propagates reachability information for VPN-IPv4 prefixes among PE routers by means of the BGP multiprotocol extensions (see RFC 2283, Multiprotocol Extensions for BGP-4 ), which define support for address families other than IPv4. Using the extensions ensures that the routes for a given VPN are learned only by other members of that VPN, enabling members of the VPN to communicate with each other.

Follow the steps in this section to configure the MP-BGP session on the ASBR.

Use all of the steps in the following procedure to configure the MP BGP session on the peer ASBR.

1. Enable privileged EXEC mode. Enter your password if prompted.

   enable
   Example:
   Router> enable

2. Enter global configuration mode.

   configure terminal
   Example:
   Router# configure terminal

3. Configures a BGP routing process and places the router in router configuration mode.

   • The as-number argument indicates the number of an autonomous system that identifies the router to other BGP routers and tags the routing information passed along. Valid numbers are from 0 to 65535. Private autonomous system numbers that can be used in internal networks range from 64512 to 65535.

   router bgp as-number
   Example:
    Router(config)# router bgp 100

4. Adds an entry to the BGP or multiprotocol BGP neighbor table.

   • The ip-address argument specifies the IP address of the neighbor.

   • The peer-group-name argument specifies the name of a BGP peer group.

   • The as-number argument specifies the autonomous system to which the neighbor belongs.

   neighbor {ip-address | peer-group-name} remote-as as-number
   Example:
    Router(config-router)# neighbor 192.168.0.1 
   remote-as 200

5. Enters address family configuration mode for configuring routing sessions, such as BGP, that use standard VPNv4 address prefixes.

   • The unicast keyword specifies IPv4 unicast address prefixes.

   address-family vpnv4 [unicast]
   Example:
    Router(config-router)# address-family vpnv4

6. Enables the exchange of information with a neighboring router.

   • The ip-address argument specifies the IP address of the neighbor.

   • The peer-group-name argument specifies the name of a BGP peer group.

   neighbor {ip-address | peer-group-name} activate
   Example:
    Router(config-router-af)# neighbor 192.168.0.1 
   activate

7. Configures eBGP peer router (ASBR) as an Inter-AS Option AB peer.

   • The ip-address argument specifies the IP address of the neighbor.

   • The peer-group-name argument specifies the name of a BGP peer group.

   • If any prefixes are imported into Option AB VRFs, then the imported paths are advertised to this peer.

   • If any prefixes are received from this peer and are imported into Option AB VRFs, then the imported paths are advertised to iBGP peers.

   Note	Advertised routes have RTs that are configured on the VRF. Advertised routes do not have their original RTs.

   neighbor {ip-address | peer-group-name} inter-as-hybrid
   Example:
    Router(config-router-af)# neighbor 192.168.0.1 
   inter-as-hybrid

8. Exits from address family configuration mode.

   exit-address-family
   Example:
    Router(config-router-af)# exit-address-family

9. (Optional) Exits to privileged EXEC mode.

   end
   Example:
    Router(config-af)# end

Use the steps in this section to configure VRFs for the VPNs that need Inter-AS connections between ASBR peers, by configuring the appropriate routing policy and Option AB configuration.

Use all of the steps in the following procedure to configure additional VPNs that need Inter-AS Option AB connectivity on this ASBR and the peer ASBR.

1. Enable privileged EXEC mode. Enter your password if prompted.

   enable
   Example:
   Router> enable

2. Enter global configuration mode.

   configure terminal
   Example:
   Router# configure terminal

3. Defines the VPN routing instance by assigning a VRF name and enters VRF configuration mode.

   • The vrf-name argument is the name assigned to a VRF.

   vrf definition vrf-name
   Example:
    Router(config)# vrf definition vpn1 

4. Creates routing and forwarding tables.

   • The route-distinguisher argument adds an 8-byte value to an IPv4 prefix to create a VPN IPv4 prefix. You can enter an RD in either of these formats:

     • 16-bit autonomous system number: your 32-bit number, for example, 101:3
     • 32-bit IP address: your 16-bit number, for example, 192.168.122.15:1

   rd route-distinguisher
   Example:
    Router(config-vrf)# rd 100:1

5. Enters VRF address family configuration mode to specify an address family for a VRF.

   • The ipv4 keyword specifies an IPv4 address family for a VRF.

   address-family ipv4
   Example:
   Router(config-vrf)# address-family ipv4

6. Creates a route-target extended community for a VRF.

   • The import keyword imports routing information from the target VPN extended community.

   • The export keyword exports routing information to the target VPN extended community.

   • The both keyword imports routing information from and exports routing information to the target VPN extended community.

   • The route-target-ext-community argument adds the route-target extended community attributes to the VRF list of import, export, or both (import and export) route-target extended communities.

   route-target {import | export | both} 
   route-target-ext-community
   Example:
    Router(config-vrf-af)# route-target import 
   100:1

7. For Inter-AS Option AB+, go to Step 10; otherwise, go to Step 8.

8. Specifies the VRF as an Option AB VRF, which has the following effects:

   • Routes imported to this VRF can be advertised to Option AB peers and VPNv4 iBGP peers.

   • When routes received from Option AB peers and are imported into the VRF, the next hop table ID of the route is set to the table ID of the VRF.

   • If the csc keyword is not used, a per-VRF label is allocated for imported routes.

   • When routes are received from Option AB peers and are imported next into the VRF, the learned out label can be installed only in forwarding when the csc keyword is used.

   The csc keyword implies the following:

   • A per-prefix label is allocated for imported routes.

   • For routes received from Option AB peers that are imported into the VRF, the learned out label is installed in forwarding.

   inter-as-hybrid [csc]
   Example:
    Router(config-vrf-af)# inter-as-hybrid

9. (Optional) Specifies the next hop IP address to be set on paths that are imported into the VRF and that are received from an Option AB peer.

   • The next hop context is also set to the VRF, which imports these paths.

   • The csc keyword implies the following:

     • A per-prefix label is allocated for imported routes.
     • For routes received from Option AB peers that are imported into the VRF, the learned out label is installed in forwarding.

   inter-as-hybrid next-hop global
   Example:
    Router(config-vrf-af)# inter-as-hybrid next-hop 
   global

10. (For Option AB+) Enables Inter-AS Option AB+.

    • Specifies that the next-hop address for BGP updates to be set on paths that are imported to the VRF and that are received from an Option AB+ peer are placed in the global routing table.

    • The address used is the address of the interface that is at the remote end of the external BGP (eBGP) global shared link. The next-hop context is retained as global and not modified to that of the importing VRF.

    inter-as-hybrid next-hop global
    Example:
     Router(config-vrf-af)# inter-as-hybrid next-hop 
    global

11. (Optional) Exits to privileged EXEC mode.

    end
    Example:
     Router(config-vrf-af)# end