Segment Routing (SR) leverages the source paradigm. A node. steers a packet through an ordered list of instructions, called ‘ segment ‘.State is kept in the packet header, not on the router, with Segment Routing.
Resources such as the CPU and Memory are saved. If you have 100 Edge Routers in your network and if you enable MPLS Traffic Edge to Edge, you would have 100×99/2 = 4950 LSP states on your Midpoint LSR. This is prevalent in many MPLS TE enabled network.
If you enable Segment Routing and if you evaluate the same midpoint case (since you assign a Prefix/Node SID for every Edge router), Midpoint LSR would have 110 entries instead of 4500 entries.
As for the scalability, everything is perfect. However, there is a caveat. Segment list can easily get big if you use explicit routing for the purpose of OAM. If you do that, you may end up with 7-8 segments. In that case, it is pertinent that you check the hardware support. Cisco claims that they have performed the tests on a number of service provider networks and that their findings show that two or three segments would be enough for the most explicit path scenarios.
You can use Segment Routing to provide MPLS VPN service without using LDP for the transport label distribution. Segment Routing provides Traffic Engineering without having soft state RSVP-TE protocol on your network. Soft state protocols require a lot of processing power.
Although Segment Routing does not have permission control, you can use routers to specify, for instance, 50Mbs LSP path for traffic A and 30 Mbps for traffic B using centralized controller, a process that allows you to use traffic engineering. Segment Routing provides Fast Reroute without RSVP-TE, and you do not need to have thousands of forwarding state in the network, as it uses IP FRR technology, specifically Topology Independent LFA.
Segment Routing has many use cases. This article explains MPLS VPN, Traffic Engineering, and Fast Reroute even though Dual Plane topologies are other use cases for the operators. With Traffic Engineering, you can have ECMP capability, a task that is very difficult to achieve with MPLS Traffic Engineering. This is because you need to create two tunnels.
There are other use cases such as Egress peering engineering. Today, this can be achieved by the complex BGP policy or LISP . However, with Segment Routing, BGP Egress peer engineering is much easier. I will explain this process and other use cases in a separate article. Major vendors – including Alcatel, Ericson, and Juniper – support segment Routing.
If you have devices not supported by Segment Routing but by LDP, you can use Segment Routing to interwork the LDP enabled devices. Also, the Segment Routing Mapping Server provides interworking functionality. One of Cisco’s objectives is to allow Segment Routing to provide native IPv6 transport.Today, Segment Routing supports IPv6 more than MPLS.
Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021
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