Inter-AS Option C

The third method of configuring inter-AS services and inter-AS VPNs is known as inter-AS option C or 2547bis option C. This method is described in RFC 4364—BGP/MPLS IP Virtual Private Networks (VPNs) (February 2006).

Inter-AS option C, similarly to the carrier-of-carriers configuration, requires a label-switched path from a packet's ingress PE router to its egress PE router. Option C introduces multihop EBGP redistribution of labeled VPN-IPv4 routes between source and destination autonomous systems. Labeled IPv4 routes are redistributed by EBGP between neighboring autonomous systems. Inter-AS option C uses BGP as the label distribution protocol.

In an inter-AS option C network, ASBRs do not maintain or distribute VPN-IPv4 routes. Each ASBR maintains labeled IPv4 /32 routes to the PE routers within its AS. The ASBR distributes these routes to other autonomous systems with EBGP. If transit autonomous systems are included in the topology, their ASBRs must also distribute the labeled /32 routes with EBGP. This configuration creates a label-switched path from the ingress PE router to the egress PE router. This configuration enables the PE routers in different autonomous systems to establish multihop EBGP connections to each other, and to exchange VPN-IPv4 routes over those connections.

Two different configuration scenarios are possible with option C, one employing a two-label stack and the other a three-label stack.

Figure 79 illustrates the three-label stack scenario. PHP is not used in this example.

Figure 79: Topology for Three-label Stack Configuration for Inter-AS Option C

In this topology, you can use either LDP or RSVP-TE to establish an LSP between each ASBR router and the PE router in an autonomous system. A labeled MP-IBGP session exists between the ASBR and the PE router in each autonomous system. A labeled MP-EBGP session exists between the two ASBR routers. The ASBR routers advertise the loopback IP addresses of their PE routers and associates the prefixes with labels.

When PE 1 learns the PE 2 loopback address and PE 2 learns the PE 1 loopback address, these PE routers can establish a multihop MP-EBGP session in order to exchange VPN-IPv4 routes. Because VPN-IPv4 routes are only exchanged between end PE routers, no other router on the path from PE 1 to PE 2 needs to keep or install VPN routes in its RIB or FIB.

1. P 2 learns label L2 for the route to the loopback address on PE 2 by means of LDP or RSVP-TE from PE 2.
2. ASBR 2 learns label L3 for the route to the loopback address on PE 2 by means of LDP or RSVP-TE from P 2.

   Each ASBR builds its own MPLS forwarding table with the received and advertised routes and labels. ASBR 2 uses its own IP address as the next hop.

3. ASBR 2 uses an MP-EBGP labeled unicast session to advertise label L4 for the route to the loopback address on PE 2 to neighboring ASBR 1.
4. ASBR 1 receives this route and the associated label L4.
5. ASBR 1 assigns label L6 to the route to the loopback address on PE 2 and changes the next-hop address to its own address.
6. ASBR 1 then uses an MP-IBGP session to advertise that address to PE 1. PE 1 therefore has an update with the label information and a next hop to ASBR 1.
7. P 1 learns label L7 for the route to the loopback address on ASBR 1 by means of LDP or RSVP-TE from ASBR 1.
8. PE 1 learns label L5 for the route to the loopback address on ASBR 1 by means of LDP or RSVP-TE from P 1.
9. PE 1 learns label L1 for the VPN-IPv4 route from the multihop EBGP session with PE 2.

Because the routes to the PE routers are unknown to all P routers other than the ASBRs, the ingress PE must push a three-label stack on packets received from the VPN end users. This is illustrated in Figure 79 as follows:

1. The first (innermost or bottom) label, L1, is assigned by the egress PE router, PE 2. This label is obtained from the multihop MP-EBGP session. It corresponds to the packet's destination address in a particular VRF at the remote PE router.
2. The middle label, L6, is assigned by ASBR 1. This label is obtained from the MP-IBGP labeled unicast session from the ASBR. It corresponds to the /32 route to the egress PE router, PE 2.
3. The top (outermost) label, L5, is assigned by the ingress PE router’s IGP next hop. P 1. This label is obtained from an LDP or RSVP-TE session with the next hop. It corresponds to the /32 route to ASBR 1.

While the packet travels across the VPN from ingress router PE 1, labels are swapped as follows:

1. P 1 swaps outermost label L5 for L7 to get to its next hop, ASBR 1.
2. ASBR 1 pops outermost label L7 and swaps the middle label L6 for L4 to get to ASBR 2.
3. ASBR 2 swaps outer label L4 for L3 to get to its next hop, P 2.
4. P 2 swaps outer label L3 for L2 to get to its next hop, PE 2.
5. PE 2 pops outer label L2 and inner label L1 and then processes the IP data packet.

In contrast to the three-label stack scenario described previously, in a two-label stack scenario, BGP labeled unicast is not used inside the autonomous system. Instead, only LDP is used as the label distribution protocol. A PE router in one AS has a direct LSP to a PE in another AS, achieved by using LDP labels within the AS and BGP labels across the AS boundary.

For a two-label stack scenario to work, you must issue the mpls ldp redistribute bgp command on the ASBRs. This command enables the BGP prefixes to be advertised by LDP inside the autonomous systems. For more information on this command, see Configuring MPLS.

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