Information About InterAS
An autonomous system (AS) is a single network or group of networks that is controlled by a common system administration group and using a single, clearly defined protocol. In many cases, virtual private networks (VPNs) extend to different ASes in different geographical areas. Some VPNs must extend across multiple service providers; these VPNs are called overlapping VPNs. The connection between ASes must be seamless to the customer, regardless of the complexity or location of the VPNs.
InterAS and ASBRs
Separate ASes from different service providers can communicate by exchanging information in the form of VPN IP addresses. The ASBRs use eBGP to exchange that information. The iBGP distributes the network layer information for IP prefixes throughout each VPN and each AS. The following protocols are used for sharing routing information:
Within an AS, routing information is shared using iBGP.
Between ASes, routing information is shared using eBGP. eBGP allows service providers to set up an interdomain routing system that guarantees loop-free exchange of routing information between separate ASes.
The primary function of eBGP is to exchange network reachability information between ASes, including information about the list of AS routes. The ASes use eBGP border edge routers to distribute the routes, which includes label-switching information. Each border edge router rewrites the next-hop and MPLS labels.
InterAS configuration supported in this MPLS VPN can include an interprovider VPN, which is MPLS VPNs that include two or more ASes, connected by separate border edge routers. The ASes exchange routes use eBGP, and no iBGP or routing information is exchanged between the ASes.
InterAS Options use iBGP and eBGP peering to allow VPNs in different AS to communicate with each other. Cisco Catalyst 9400 Series Switches support the interAS option B.
Overview of InterAS Option B
Two methods are supported to distribute the next hop for VPNv4 routes between ASBRs. There is no requirement for LDP or any IGP to be enabled on the link connecting the two ASBRs. The MP-eBGP session between directly connected interfaces on the ASBRs enables the interfaces to forward labeled packets. To ensure this MPLS forwarding for directly connected BGP peers, you must configure mpls bgp forwarding command on the interface connecting to ASBR. This command is implemented in the IOS for directly connected interfaces. Upto 200 BGP neighbors can be configured.
Next-hop-self Method: Changing next-hop to that of the local ASBR for all VPNv4 routes learnt from the other ASBR.
Redistribute Connected Subnets Method: Redistributing the next hop address of the remote ASBR into the local IGP using redistribute connected subnets command , i.e., the next hop is not changed when the VPNv4 routes are redistributed into the local AS.
The label switch path forwarding sections described below has AS200 configured with the Next-hop-self method and the AS300 is configured with Redistribute-subnet method.
Next-Hop Self Method
The following figure shows the label forwarding path for next-hop-self method. The labels get pushed, swapped and popped on the stack as packet makes its way from PE-200 in AS 200 to PE-300 in AS 300. In step 5, ASBR-A300 receives labeled frame, replaces label 164 with label 161 pushes IGP label 162 onto the label stack.
Redistribute Connected Subnet Method
The following figure shows the label forwarding path for Redistribute connected subnets method. The labels get pushed, swapped and popped on the stack as packet travels from PE- 300 in AS 300 to PE-200 in AS 200. In step 5, ASBR-A200 receives frame with BGP label 20, swaps it with label 29 and pushes label 17.