MPLS LDP and SR MPLS to provide VPN using Inter AS option C (IOS-XR)

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Updated:August 5, 2021
Document ID:217289

Bias-Free Language

The documentation set for this product strives to use bias-free language. For the purposes of this documentation set, bias-free is defined as language that does not imply discrimination based on age, disability, gender, racial identity, ethnic identity, sexual orientation, socioeconomic status, and intersectionality. Exceptions may be present in the documentation due to language that is hardcoded in the user interfaces of the product software, language used based on RFP documentation, or language that is used by a referenced third-party product. Learn more about how Cisco is using Inclusive Language.

    Introduction

    This document describes how a partial SR deployment can be used to provide SR benefits to LDP-based traffic including a possible application of SR in the context of inter-domain MPLS use-cases.

    Prerequisite

    Requirements

    Cisco recommends that you have knowledge of these topics:

    • Segment Routing
    • MPLS
    • Inter-AS
    • LDP

    Components Used

    The information in this document is based on these software and hardware versions:

    • Supporting Segment Routing
    • NCS5500, ASR9000

    The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

    Background Information

    This document outlines the mechanisms through which SR interworks with LDP in cases where a mix of SR-capable and non-SR-capable routers co-exist within the same network and more precisely in the same routing domain.

    A Multicast Control Plane Client (MCC), operating at a node, must ensure that the incoming label it installs in the MPLS data plane of Node has been uniquely allocated and Segment Routing makes use of the Segment Routing Global Block (SRGB) for the label allocation. The use of the SRGB allows SR to co-exist with any other MCC.

    Tip: This information helps you with the Inter-AS solution for MPLS SR and MPLS LDP using Inter-AS Option C.

    Problem

    SR deployment in a heterogeneous environment with SR MPLS Control plane interoperate with MPLS LDP Control Plane using Inter-AS option C defined in RFC 4364.

    This document describes a method by which Service provider Network using MPLS LDP and SR MPLS to provide Virtual Private Network using Inter-AS option C.

    Solution 

    A brief overview of Inter-AS Option C:

    Inter-AS Option C is the third option for interconnecting multi-AS backbones covered in RFC 4364. It’s the most scalable option of the three so far and it has its own applicability scenarios that we must be aware of to apply this design properly.

    Option C is a good candidate because it is scalable. ASBR’s don’t carry any VPN routes and they just take care of the distribution of labeled IPv4 routes of the PE’s within their own AS.

    To improve scalability, one MP-EBGP VPNv4 session transports all VPN routes (external routes) between PEs or RR. In the case of using RR to exchange the external routes, the next-hop of the VPNv4 routes must be preserved.

    The ASBR uses EBGP to exchange the internal PE routing information between AS (internal routes). These internal routes correspond to the BGP next-hops of the external routes advertised through the multi-hop MP-EBGP session between PEs or RRs. The internal routes advertised by the ASBRs can be used to establish the MP-EBGP sessions between PEs and allows for LSP setup from the ingress to the egress PE.

    Option C is a very good solution from the scalability point of view and is the way to go for the same SP multi-AS networks.

    A brief overview of Segment Routing:

    Segment Routing (SR) leverages the source routing and tunneling paradigms. A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header. A segment can represent any instruction, topological or service-based. SR allows enforcing a flow through any topological path and service chain while maintaining a per-flow state only at the ingress node of the SR domain. The Segment Routing architecture can be directly applied to the MPLS data plane with a little change on the forwarding plane. It requires minor extensions to the existing link-state routing protocols. Segment Routing can also be applied to IPv6 with a new type of routing extension header.

    A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack.  Segment-routing MPLS data-plane operations are push, swap, and pop as per the traditional MPLS forwarding. Following segment types are defines in Segment Routing:

    • Prefix Segment
    • Adjacency Segment
    • BGP Peering Segment
    • BGP Prefix Segment

    SR Global Block

    The Segment Routing Global Block (SRGB) is the range of label values preserved for segment routing in the LSD. The SRGB label values are assigned as prefix segment identifiers (SIDs) to SR-enabled nodes and have global significance throughout the domain.

    • SR label range can NOT start below 16,000. The default SR global block is : 16,000 - 24,000.
    • SRGB configuration is NOT address-family specific because the “SR-Capabilities Sub-TLV” of router capability TLV defined in is not address-family specific.
    • If  CLI results in enlarging or moving the default SRGB, then it is OK to require a reload but only if there are clients who have labels in the new range.
    • Before configuring SRGB, the administrator needs to make sure that a portion of the label base that is being configured for Segment-Routing is free and is not being used by any other MPLS LSD clients.

    SR Mapping Server

    Mapping Server centrally assigns prefix-SIDs for some or all of the known prefixes. A router must be able to act as a mapping server, mapping client, or both.

    • A mapping server is a control plane mechanism and its position is comparable to a BGP Route-reflector
    • Allows the user to configure non-overlapping SID mapping entries to specify the prefix-SIDs for some or all prefixes.
    • ISIS advertises the local SID-mapping policy in 'SID/Label Binding TLV'
    • The mapping server must be resilient, redundancy should be provided

    Main functions of Mapping Server include:

    • Advertise Prefix-to-SID mappings in IGP on behalf of other non-SR-capable nodes prefix-to-sid mappings are configured on the Mapping Server
    • Enable SR-capable nodes to interwork with (non-SR-capable) LDP nodes, a Mapping Server is required for SR/LDP interworking

    Mapping Client

    • Receives and parses remotely received SID/Label Binding TLV to create remote SID-mapping entries. Using the remotely learned and locally configured mapping entries, construct the non-overlapping consistent active mapping policy.
    • IGP instance uses the active mapping policy to (re)calculate the prefix-SIDs of some or all prefixes.

    This section helps you to understand & configuring L3 Virtual Private Network (VPN) service between Provider network with SR Capable Network peering/Connecting with Non-SR capable Network. In this section, you learn how to Configure Option C Defined in RFC “4364” and Use Cases.

    Topology – Inter-AS Option C (with Route-Reflectors)

    Brief Overview of the Topology Diagram

    Towards the top of the topology diagram, we have an SR-enabled network comprising of Route-reflectors, Provider Edge routers, and Customer Edge routers.

    The customer edge routers CE1 and CE2 have VRF A and VRF B respectively. This belongs to AS 65002.

    Towards the bottom of the topology diagram, we have an LDP-enabled network comprising of Route-reflectors, Provider Edge routers, and customer edge routers.

    The customer edge routers CE3 and CE4 have VRF A and VRF B respectively. This belongs to AS65001.

    VRF A and VRF B on either end of the SR and LDP enabled networks to need to communicate with each other.

    IP Addressing Schema

    Hostname IP Address
    RR-01 10.0.0.1
    RR-02 10.0.0.2
    PE1 10.0.0.3
    PE2 10.0.0.4
    CE1 10.0.0.5
    CE2 10.0.0.6
    RR-03 10.0.1.1
    RR-04 10.0.1.2
    PE5 10.0.1.3
    PE6 10.0.1.4
    CE3 10.0.1.5
    CE4 10.0.1.6

    Configurations

    The configurations of the devices are described.

     RR-1

     segment-routing mpls

     !       

     mapping-server

      !

      prefix-sid-map

       address-family ipv4

        10.0.0.1/32 index 200 range 10

       exit-address-family

      !

     !




    interface Loopback0

     description Loopback0

     ip address 10.0.0.1 255.255.255.255

     ip router isis 65002

    !




    !

    router isis 65002

     net xx.xxxx.xxxx.xxxx.xx

     metric-style wide

     segment-routing mpls

     segment-routing prefix-sid-map advertise-local

     !

    router bgp 65002

     bgp router-id 10.0.0.1

     neighbor 10.0.0.3 remote-as 65002

     neighbor 10.0.0.3 description rr client

     neighbor 10.0.0.3 update-source Loopback0

     neighbor 10.0.0.4 remote-as 65002

     neighbor 10.0.0.4 description rr client

     neighbor 10.0.0.4 update-source Loopback0

     neighbor 10.0.0.2 remote-as 65002

     neighbor 10.0.0.2 description iBGP peer

     neighbor 10.0.0.2 update-source Loopback0

     neighbor 10.0.1.1 remote-as 65001

     neighbor 10.0.1.1 ebgp-multihop 255

     neighbor 10.0.1.1 update-source Loopback0

     neighbor 10.0.1.2 remote-as 65001

     neighbor 10.0.1.2 ebgp-multihop 255

     neighbor 10.0.1.2 update-source Loopback0

     !       

    address-family ipv4

      neighbor 10.0.0.3 activate

      neighbor 10.0.0.3 route-reflector-client

      neighbor 10.0.0.4 activate

      neighbor 10.0.0.4 route-reflector-client

      neighbor 10.0.0.2 activate

      neighbor 10.0.1.1 activate

      neighbor 10.0.1.2 activate

     exit-address-family

     !

     address-family vpnv4

      neighbor 10.0.0.3 activate

      neighbor 10.0.0.3 send-community extended

      neighbor 10.0.0.3 route-reflector-client

      neighbor 10.0.0.4 activate

      neighbor 10.0.0.4 send-community extended

      neighbor 10.0.0.4 route-reflector-client

      neighbor 10.0.0.2 activate

      neighbor 10.0.0.2 send-community extended

      neighbor 10.0.1.1 activate

      neighbor 10.0.1.1 send-community both

      neighbor 10.0.1.1 next-hop-unchanged

      neighbor 10.0.1.2 activate

      neighbor 10.0.1.2 send-community both

      neighbor 10.0.1.2 next-hop-unchanged

    exit-address-family

     !

    RR-2

    segment-routing mpls

     !       

     mapping-server

      !

      prefix-sid-map

       address-family ipv4

        10.0.0.2/32 index 200 range 10

       exit-address-family

      !

     !




    interface Loopback0

     description Loopback0

     ip address 10.0.0.2 255.255.255.255

     ip router isis 65002

    !




    !

    router isis 65002

     net xx.xxxx.xxxx.xxxx.xx

     metric-style wide

     segment-routing mpls

     segment-routing prefix-sid-map advertise-local

     !

    router bgp 65002

     bgp router-id 10.0.0.2

     neighbor 10.0.0.3 remote-as 65002

     neighbor 10.0.0.3 description rr client

     neighbor 10.0.0.3 update-source Loopback0

     neighbor 10.0.0.4 remote-as 65002

     neighbor 10.0.0.4 description rr client

     neighbor 10.0.0.4 update-source Loopback0

     neighbor 10.0.0.1 remote-as 65002

     neighbor 10.0.0.1 description iBGP peer

     neighbor 10.0.0.1 update-source Loopback0

     neighbor 10.0.1.1 remote-as 65001

     neighbor 10.0.1.1 ebgp-multihop 255

     neighbor 10.0.1.1 update-source Loopback0

     neighbor 10.0.1.2 remote-as 65001

     neighbor 10.0.1.2 ebgp-multihop 255

     neighbor 10.0.1.2 update-source Loopback0

     !       

    address-family ipv4

      neighbor 10.0.0.3 activate

      neighbor 10.0.0.3 route-reflector-client

      neighbor 10.0.0.4 activate

      neighbor 10.0.0.4 route-reflector-client

      neighbor 10.0.0.1 activate

      neighbor 10.0.1.1 activate

      neighbor 10.0.1.2 activate

     exit-address-family

     !

     address-family vpnv4

      neighbor 10.0.0.3 activate

      neighbor 10.0.0.3 send-community extended

      neighbor 10.0.0.3 route-reflector-client

      neighbor 10.0.0.4 activate

      neighbor 10.0.0.4 send-community extended

      neighbor 10.0.0.4 route-reflector-client

      neighbor 10.0.0.1 activate

      neighbor 10.0.0.1 send-community extended

      neighbor 10.0.1.1 activate

      neighbor 10.0.1.1 send-community both

      neighbor 10.0.1.1 next-hop-unchanged

      neighbor 10.0.1.2 activate

      neighbor 10.0.1.2 send-community both

      neighbor 10.0.1.2 next-hop-unchanged

    exit-address-family

     !

    PE-1



    interface Loopback0

     description Loopback0

     ip address 10.0.0.3 255.255.255.255

     ip router isis 65002

    !





    vrf A                               # Define VRF A

     address-family ipv4 unicast

      import route-target

       65000:1

      !

      export route-target

       65000:1

      !

     !




    vrf B                               # Define VRF B

     address-family ipv4 unicast

      import route-target

       65000:2

      !

      export route-target

       65000:2

      !

     !




    router isis 65002                   # ISIS Level 2

     is-type level-2-only

     net xx.xxxx.xxxx.xxxx.xx

     address-family ipv4 unicast

      metric-style wide

      advertise link attributes

      mpls traffic-eng level-2-only

      mpls traffic-eng router-id Loopback0

      router-id Loopback0

      segment-routing mpls sr-prefer

     !




    router bgp 65002                    # BGP

     bgp router-id 10.0.0.3

     neighbor-group RR

      remote-as 65002

      update-source Loopback0

      address-family vpnv4 unicast

      !

     !

    neighbor 10.0.0.1

      use neighbor-group RR

     !

     neighbor 10.0.0.2

      use neighbor-group RR




    vrf A

      rd 65000:1

      address-family ipv4 unicast

       redistribute connected

       allocate-label all

      !

      neighbor 10.0.0.5           # IP address of CE1

       remote-as 61001

       ebgp-multihop 255

       update-source Loopback100

       address-family ipv4 unicast

       !

    vrf B

      rd 65000:2

      address-family ipv4 unicast

       redistribute connected

       allocate-label all

      !

      neighbor 10.0.0.6           # IP address of CE2

       remote-as 61001

       ebgp-multihop 255

       update-source Loopback101

       address-family ipv4 unicast

       !

    interface GigabitEthernet1         # Link to CE-01

    vrf A

     ipv4 address x.x.x.x 255.255.255.0

    !

    interface GigabitEthernet2         # Link to CE-02

     vrf B

     ipv4 address x.x.x.x 255.255.255.0

    !

    segment-routing

    global-block 16000 23999

    !

    RR-3

    interface Loopback0

     description Loopback0

     ip address 10.0.1.1 255.255.255.255

     ip router isis 65001

    !




    !

    router isis 65001

     net xx.xxxx.xxxx.xxxx.xx

     metric-style wide

     segment-routing mpls

     segment-routing prefix-sid-map advertise-local

     !




    router bgp 65001

     bgp router-id 10.0.1.1

     neighbor 10.0.1.3 remote-as 65001

     neighbor 10.0.1.3 description rr client

     neighbor 10.0.1.3 update-source Loopback0

     neighbor 10.0.1.4 remote-as 65001

     neighbor 10.0.1.4 description rr client

     neighbor 10.0.1.4 update-source Loopback0

     neighbor 10.0.1.2 remote-as 65001

     neighbor 10.0.1.2 description iBGP peer

     neighbor 10.0.1.2 update-source Loopback0

     neighbor 10.0.0.1 remote-as 65002

     neighbor 10.0.0.1 ebgp-multihop 255

     neighbor 10.0.0.1 update-source Loopback0

     neighbor 10.0.0.2 remote-as 65002

     neighbor 10.0.0.2 ebgp-multihop 255

     neighbor 10.0.0.2 update-source Loopback0

     !       

    address-family ipv4

      neighbor 10.0.1.3 activate

      neighbor 10.0.1.3 route-reflector-client

      neighbor 10.0.1.4 activate

      neighbor 10.0.1.4 route-reflector-client

      neighbor 10.0.1.2 activate

      neighbor 10.0.0.1 activate

      neighbor 10.0.0.2 activate

     exit-address-family

     !

     address-family vpnv4

      neighbor 10.0.1.3 activate

      neighbor 10.0.1.3 send-community extended

      neighbor 10.0.1.3 route-reflector-client

      neighbor 10.0.1.4 activate

      neighbor 10.0.1.4 send-community extended

      neighbor 10.0.1.4 route-reflector-client

      neighbor 10.0.1.2 activate

      neighbor 10.0.1.2 send-community extended

      neighbor 10.0.0.1 activate

      neighbor 10.0.0.1 send-community both

      neighbor 10.0.0.1 next-hop-unchanged

      neighbor 10.0.0.2 activate

      neighbor 10.0.0.2 send-community both

      neighbor 10.0.0.2 next-hop-unchanged

    exit-address-family

     !




    RR-4

    interface Loopback0

     description Loopback0

     ip address 10.0.1.2 255.255.255.255

     ip router isis 65001

    !




    !

    router isis 65001

     net xx.xxxx.xxxx.xxxx.xx

     metric-style wide

     segment-routing mpls

     segment-routing prefix-sid-map advertise-local

     !




    router bgp 65001

     bgp router-id 10.0.1.2

     neighbor 10.0.1.3 remote-as 65001

     neighbor 10.0.1.3 description rr client

     neighbor 10.0.1.3 update-source Loopback0

     neighbor 10.0.1.4 remote-as 65001

     neighbor 10.0.1.4 description rr client

     neighbor 10.0.1.4 update-source Loopback0

     neighbor 10.0.1.1 remote-as 65001

     neighbor 10.0.1.1 description iBGP peer

     neighbor 10.0.1.1 update-source Loopback0

     neighbor 10.0.0.1 remote-as 65002

     neighbor 10.0.0.1 ebgp-multihop 255

     neighbor 10.0.0.1 update-source Loopback0

     neighbor 10.0.0.2 remote-as 65002

     neighbor 10.0.0.2 ebgp-multihop 255

     neighbor 10.0.0.2 update-source Loopback0

     !       

    address-family ipv4

      neighbor 10.0.1.3 activate

      neighbor 10.0.1.3 route-reflector-client

      neighbor 10.0.1.4 activate

      neighbor 10.0.1.4 route-reflector-client

      neighbor 10.0.1.1 activate

      neighbor 10.0.0.1 activate

      neighbor 10.0.0.2 activate

     exit-address-family

     !

     address-family vpnv4

      neighbor 10.0.1.3 activate

      neighbor 10.0.1.3 send-community extended

      neighbor 10.0.1.3 route-reflector-client

      neighbor 10.0.1.4 activate

      neighbor 10.0.1.4 send-community extended

      neighbor 10.0.1.4 route-reflector-client

      neighbor 10.0.1.1 activate

      neighbor 10.0.1.1 send-community extended

      neighbor 10.0.0.1 activate

      neighbor 10.0.0.1 send-community both

      neighbor 10.0.0.1 next-hop-unchanged

      neighbor 10.0.0.2 activate

      neighbor 10.0.0.2 send-community both

      neighbor 10.0.0.2 next-hop-unchanged

    exit-address-family

     !


    PE-6   



    interface Loopback0

     description Loopback0

     ip address 10.0.1.3 255.255.255.255

     ip router isis 65001

    !



    vrf A                               # Define VRF A

     address-family ipv4 unicast

      import route-target

       65000:1

      !

      export route-target

       65000:1

      !




    vrf B                               # Define VRF B

     address-family ipv4 unicast

      import route-target

       65000:2

      !

      export route-target

       65000:2

      !




    router isis 65001                  

     is-type level-2-only

     net xx.xxxx.xxxx.xxxx.xx

     address-family ipv4 unicast

      metric-style wide

      advertise link attributes

      mpls traffic-eng level-2-only

      mpls traffic-eng router-id Loopback0

      router-id Loopback0

      segment-routing mpls sr-prefer

     !




    router bgp 65001                   

     bgp router-id 10.0.1.3

     neighbor-group RR

      remote-as 65002

      update-source Loopback0

      address-family vpnv4 unicast

      !

     !

    neighbor 10.0.1.1

      use neighbor-group RR

     !

     neighbor 10.0.1.2

      use neighbor-group RR




    vrf A

      rd 65000:1

      address-family ipv4 unicast

       redistribute connected

       allocate-label all

      !

      neighbor 10.0.1.5           # IP address of CE3

       remote-as 61001

       ebgp-multihop 255

       update-source Loopback100

       address-family ipv4 unicast

       !

    vrf B

      rd 65000:2

      address-family ipv4 unicast

       redistribute connected

       allocate-label all

      !

      neighbor 10.0.1.6           # IP address of CE4

       remote-as 61001

       ebgp-multihop 255

       update-source Loopback101

       address-family ipv4 unicast

       !

    interface GigabitEthernet1         # Link to CE3

    vrf A

     ipv4 address x.x.x.x 255.255.255.0

    !

    interface GigabitEthernet2         # Link to CE4

     vrf B

     ipv4 address x.x.x.x 255.255.255.0

    !

    Related Information

    Revision History

    Revision Publish Date Comments
    1.0
    05-Aug-2021
    Initial Release
    TAC Authored

    Contributed by Cisco Engineers

    • Akshay Mahajan
      Cisco Professional Services
    • Gopal Prasad Singh
      Cisco Professional Services

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