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Updated:August 5, 2021
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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.
Cisco recommends that you have knowledge of these topics:
The information in this document is based on these software and hardware versions:
Supporting Segment Routing
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.
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.
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.
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:
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
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.