Chapter 5. Migrating to Segment Routing
Network operators face the ongoing challenge of staying competitive, which often requires proactively adopting new technologies. This brings us to the concept of migration, which involves shifting to newer technological landscapes. But what does migration really entail? It is a concept that is deceptively simple to state yet complex to execute.
At its core, migration involves moving from an existing network configuration to an enhanced, desired state. This journey unfolds a tapestry of complexity as it demands adaptations in hardware, software, configurations, IT systems, personnel, and business processes to accommodate the new technology. For network operators, especially those managing large-scale or diverse networks, the transformation is multifaceted and challenging. It can range from deploying entirely new infrastructures and overhauling networks to replacing outdated equipment, upgrading existing hardware, implementing new software solutions, and embracing novel protocols.
With the introduction of two segment routing (SR) technologies and their advantages in the previous chapters, this chapter provides a practical roadmap for implementing SR, whether you’re starting afresh with a greenfield approach, or using a brownfield method, which involves integration with existing systems. This chapter presents simple steps to mitigate the risks associated with transitioning services from MPLS to SR.
This chapter covers the following topics:
Deployment models and strategies for achieving connectivity between MPLS and SR networks during the migration phase, which is a critical period during which a network incrementally adopts SR
Specifics of migrating from an LDP network to SR-MPLS and details of both deployment models and MPLS and SR interconnectivity options
Three distinct strategies for migrating from MPLS to SRv6
A roadmap for migration to an SRv6 network for four different MPLS networks
The migration process typically concerns the transition of customer edge (CE) or provider edge (PE) devices from one network environment to another. Although this chapter uses IPv4 to illustrate service migrations, the concepts are equally applicable to IPv6 services.
Deployment Models
Greenfield network deployment refers to the installation of a network where previously there was none. This term is derived from the construction industry, where new development on previously undeveloped land is termed a greenfield development. An important advantage of greenfield deployment is the opportunity to implement cutting-edge technology solutions from the ground up, free from the constraints or dependencies of existing infrastructure, software, biases, or business processes. In the context of SR, a greenfield deployment refers to building a separate SR network and then migrating the services from the MPLS network to the SR network. During the migration phase, services may connect with both the SR and MPLS networks, and so interworking is essential to maintain seamless service connectivity across the networks. Implementing interworking might necessitate additional hardware and software, which can be phased out after the completion of the service migration.
In contrast to a greenfield deployment, a brownfield deployment involves an upgrade or expansion of an existing network. This type of deployment involves installation and configuration of new hardware or network technology that is designed to coexist with the legacy network. One benefit of brownfield development is the ability to enhance existing technology solutions within established business processes; in addition, an organization can avoid extra capital expenditure on new infrastructure by undertaking a brownfield development. However, brownfield projects also come with their own challenges, including the need for a comprehensive and accurate understanding of the existing network's limitations and issues with legacy infrastructure that can potentially slow the development process and inflate overall costs.
A brownfield deployment involves activating SR in the MPLS network. This process can be conducted in phases during several maintenance windows. Throughout these periods, SR and MPLS coexist within the network, which introduces increased complexity in terms of features and configurations. However, it also reduces the effort and mitigates risks typically associated with maintenance windows.
Migration Strategies
Transitioning the services to SR-MPLS or SRv6 in a network can be achieved through a multitude of viable methodologies, and this section illustrates several strategies. When embarking on a greenfield deployment and establishing a new SR core alongside metro and access layers, there are two principal strategies that stand out for migrating services from MPLS to SR network. The first strategy relies on the ability of the PE routers to simultaneously support MPLS and SR during the migration phase, and the second strategy depends on interworking.
Figure 5-1 illustrates the first strategy, which leverages an interworking gateway to sequentially transition the MPLS PE devices to the SR network in multiple maintenance windows. This strategy assumes that all the MPLS PE devices support SR.
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Figure 5.1 Interworking Greenfield Strategy: Migrating PE Devices to a New SR Network
In cases where the MPLS PE devices do not support SR, the CE devices can be migrated from the MPLS PE devices to the SR PE devices, as shown in Figure 5-2.
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Figure 5.2 Interworking Greenfield Strategy: Migrating CE Devices to the New SR Network
Later sections of this chapter covering the interworking strategy involve the migration of PE devices or CE devices:
The section “Building a New SR-MPLS Network” covers migration from LDP to SR-MPLS, where a border node is the gateway between the LDP and SR-MPLS networks
The section “Building a New SRv6 Network Using an SRv6 IWG” covers migration from MPLS to SRv6 where an SRv6 interworking gateway (IWG) acts as the IWG between the MPLS and SRv6 networks, and the section “Building a New SRv6 Network Using Inter-AS Option A" presents a solution using ASBRs as IWGs between the two networks. The information on the migration of CE devices is also valid for the migration of PE devices.
Figure 5-3 shows the second strategy, where PE devices can be concurrently connected to both the MPLS and SR networks, thus removing the necessity for an interworking gateway between the two networks.
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Figure 5.3 Dual-Homed Greenfield Strategy : PE Devices Connected to the MPLS and SR Networks
The PE devices must have sufficient port capacity, CPU, and memory to connect to the MPLS and SR networks simultaneously. The dual-homed strategy for migration from MPLS to SR is described in the section "SRv6 Network Using Dual-Connected PE Devices." The MPLS network can be decommissioned once all the MPLS PE devices have been migrated to the SR network.
A brownfield deployment uses a coexistence strategy, incorporating SR into the existing network as illustrated in Figure 5-4. This strategy entails activating SR in specific parts of the MPLS network and progressively expanding its scope in multiple migration windows.
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Figure 5.4 Coexistence Brownfield Strategy: Enabling SR in an MPLS Network
For this approach to work, the existing network must be able to support SR. Throughout the transition phase, PE and P devices are configured to support MPLS and SR concurrently. When all the devices in the network are migrated to SR, the MPLS-related configuration can be removed from the network.
The coexistence strategy is discussed in the following sections:
LDP to SR-MPLS migration is presented in the section "Enabling SR-MPLS in an Existing Network (Coexistence)."
MPLS to SRv6 migration is explained in the section "SRv6 Network Using Dual-Connected PE Devices." Although this section shows the migration strategy for a greenfield deployment, it is also valid for a brownfield deployment.
Figure 5-5 shows another SRv6-specific brownfield strategy, which involves expanding the coverage of SRv6-based network services between PE devices connected to an existing IPv6 network.
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Figure 5.5 IPv6 Backhaul Brownfield Strategy: SRv6-Based Services over an IPv6 Network
Since the data packets from an SRv6 PE device are IPv6 packets with optional extension headers, they can be transported across any IPv6 network to facilitate SRv6-based service connectivity. However, this strategy comes with certain limitations, like the lack of TI-LFA and SRv6 traffic engineering in the native IPv6 network.