Architecture · Topic 1 of 6
Service Provider Core Architectures
Compare Metro Ethernet, MPLS, unified MPLS, segment routing, SR-TE and SRv6 core designs, and see how labels and segments steer provider traffic.
- 5 min
- An MPLS label is 20 bits in a 32 bit shim header, with a 3 bit TC, a 1 bit S and an 8 bit TTL.
- A PE adds and removes labels, and a P router mainly swaps them.
- Unified MPLS joins several IGP domains with BGP labelled unicast.
What it is
A service provider core is the shared transport network that carries many customers' traffic between points of presence.
The core must be fast, resilient and able to keep customers apart. Providers meet these needs with a small set of designs. Each design answers the same question in a different way: how does a packet find its path, and how are customers kept separate?
The designs you must be able to describe are Metro Ethernet, MPLS, unified MPLS, segment routing (SR), SR traffic engineering (SR-TE) and SRv6.
How it works
Each design moves traffic through the core with a different forwarding label or identifier.
Metro Ethernet
Metro Ethernet carries customer traffic over an Ethernet based provider network. The services are defined by the MEF, and they include E-Line (point to point), E-LAN (multipoint) and E-Tree (rooted multipoint).
The provider usually stacks a second VLAN tag on customer frames, so customer VLAN numbers stay private. This is IEEE 802.1ad, also called Q-in-Q. It is covered in more detail in the Services chapter.
MPLS
MPLS forwards packets by a short label instead of looking up the IP destination at every hop. A label is part of a 32 bit shim header placed between the Layer 2 and Layer 3 headers.
| Field | Size | Purpose |
|---|---|---|
| Label | 20 bits | The value used for forwarding |
| TC | 3 bits | Traffic class, used for QoS (formerly EXP) |
| S | 1 bit | Marks the bottom of the label stack |
| TTL | 8 bits | Loop prevention |
Routers have roles. A CE is the customer edge, a PE is the provider edge that adds and removes labels, and a P router is a core router that mainly swaps labels. The label stack lets MPLS carry a transport label and a service label at the same time.
RP/0/RP0/CPU0:P1# show mpls forwarding
RP/0/RP0/CPU0:P1# show mpls ldp neighborThe first command is the label forwarding table on IOS XR. The second shows the LDP sessions that distribute labels.
Unified MPLS
Unified MPLS, also called seamless MPLS, extends one end to end LSP across several IGP domains or areas. Each access, aggregation and core domain keeps its own small IGP, so the network scales. BGP labelled unicast (BGP-LU) carries loopback addresses and their labels between the domains.
The result is that a PE in one domain can reach a PE in another domain over a single label switched path. No IGP has to carry every loopback in the whole network.
Segment routing
Segment routing (SR) is source routing. The ingress router puts a list of segments on the packet, and each segment is an instruction such as "go to this node" or "use this link".
In SR-MPLS the segments are MPLS labels, and the IGP (IS-IS or OSPF) or BGP advertises them. Because the IGP distributes the labels, LDP and RSVP-TE are not needed to build the core.
SR-TE adds a policy. A policy is an ordered list of segments that forces traffic along a chosen path, such as one that avoids a congested link. The path is held at the ingress router, so the core keeps no per path state.
SRv6
SRv6 uses the IPv6 data plane instead of MPLS labels. A segment identifier (SID) is a 128 bit IPv6 address, and the path is carried in a Segment Routing Header (SRH).
SRv6 can express the transport path and the service in one address. A compressed form, micro-segment (uSID), shortens the list of SIDs so that more fit in a packet.
Comparing the designs
| Design | Data plane | Path signalling | Notes |
|---|---|---|---|
| Metro Ethernet | Ethernet, VLAN tags | Spanning tree or Ethernet rings | Layer 2 services |
| MPLS | Labels | LDP or RSVP-TE | Mature and widely deployed |
| Unified MPLS | Labels | IGP plus BGP LU | Scales across domains |
| SR-MPLS | Labels | IGP or BGP segments | No LDP or RSVP-TE |
| SRv6 | IPv6 with SRH | IGP SRv6 locators | No MPLS needed |
What to remember for the exam
Seven facts cover most questions on this topic.
- An MPLS label is 20 bits in a 32 bit shim header, with a 3 bit TC, a 1 bit S and an 8 bit TTL.
- A PE adds and removes labels, and a P router mainly swaps them.
- Unified MPLS joins several IGP domains with BGP labelled unicast.
- SR is source routing: the ingress puts a segment list on the packet.
- SR-MPLS needs no LDP and no RSVP-TE, because the IGP or BGP advertises the labels.
- SR-TE steers traffic by a policy that holds a list of segments.
- SRv6 uses an IPv6 address as the SID and the Segment Routing Header to carry the list.
When a question says one LSP must cross several IGP areas without redistributing loopbacks, think unified MPLS and BGP-LU.
Common mistakes
These four mistakes appear often.
Confusing the label and the TC field. The label forwards the packet, and the TC field is for QoS.
Thinking SR needs LDP. SR distributes labels through the IGP, so LDP is not required for SR-MPLS.
Placing path state in every core router for SR-TE. The policy lives on the ingress router, and the core follows the segments.
Treating SRv6 as MPLS with IPv6 transport. SRv6 has no MPLS labels. The SID is an IPv6 address.
IOS XR and IOS XE both support these technologies, but the configuration trees differ. Check the platform before choosing a command.
Check yourself
Work through these three questions.
Which router role mainly swaps labels in the core?
A P router mainly swaps labels, while a PE adds and removes them.
What carries loopback labels between IGP domains in unified MPLS?
BGP labelled unicast carries the loopbacks with their labels, so one LSP spans the domains.
What is an SRv6 segment identifier?
It is a 128 bit IPv6 address, and the Segment Routing Header carries the list of them.
Topic 1 of 6 in Architecture
Test yourself on Architecture
The full Cisco CCNP 350-501 SPCOR question bank has 738 questions, with timed practice exams and an explanation for every answer.
This study guide is independent and is not affiliated with, endorsed by, or sponsored by Cisco. Vendor names, certification names and exam codes are trademarks of their respective owners and are used for identification only.