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[SR-MPLS] LAB Segment Routing with SR-TE and PCE

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•5 min read
[SR-MPLS] LAB Segment Routing with SR-TE and PCE
N
Nam Nguyen

Visit to see more: https://linktr.ee/nddnam

I am an enthusiastic Network Engineer with 7+ years of experience working on MPLS L3VPN Network projects, Cisco SDWAN Deployment, and Enterprise Networks.

I love to automate every daily task and think Dev-Ops as always. Thus, I am entering the DevNet world.

Lab topology and sample configs for testing Segment Routing with SR-TE and PCE on Cisco IOS XE routers.

🖥️ Topology

[ R1 ]---[ R2 ]---[ R3 ]---[ R4 ]
   |                         |
   |-------------------------|

  • R1: PCC (client), Source

  • R4: Destination

  • R2 & R3: Transit

  • Loopbacks:

    • R1 = 1.1.1.1/32, SID 16001

    • R2 = 2.2.2.2/32, SID 16002

    • R3 = 3.3.3.3/32, SID 16003

    • R4 = 4.4.4.4/32, SID 16004

🔧 Common OSPF & SR Config (on all routers)

hostname R1
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface Gig0/0
 ip address 10.0.12.1 255.255.255.0
 mpls ip
 ip ospf 1 area 0
!
router ospf 1
 router-id 1.1.1.1
 segment-routing mpls
!
mpls traffic-eng
!
segment-routing
 mpls
  global-block 16000 23999
  connected-prefix-sid-map
   address 1.1.1.1/32 absolute-sid 16001

Change IPs and SIDs accordingly for R2–R4.

🧠 On PCE Node (e.g., R2)

pce
 segment-routing
  peer 1.1.1.1
   source-address 2.2.2.2
   lsr-id 2.2.2.2
   capability pcc
   capability pce
!
pcc-address-family ipv4
 stateful-client
!
pce-address-family ipv4
 source-address 2.2.2.2

🧭 On PCC Node (R1)

pce
 segment-routing
  pcc
   peer 2.2.2.2
    source-address 1.1.1.1

🚀 Tunnel (SR-TE Policy on R1 to R4)

interface Tunnel100
 ip unnumbered Loopback0
 tunnel mode mpls traffic-eng
 tunnel destination 4.4.4.4
 tunnel mpls traffic-eng path-option 10 dynamic
 tunnel mpls traffic-eng autoroute announce
 segment-routing mpls

📋 Verification

  • show pce peer

  • show pce lsp

  • show segment-routing traffic-eng policy

  • show mpls forwarding-table

More details

     [R1 - PCC]
        |
     (IGP + SR)
        |
     [R2 - PCE]
      /      \
     /        \
[R3]----(IGP)----[R4]

Roles:

  • R1 (PCC): Requests a path from the PCE.

  • R2 (PCE): Computes the best SR-TE path based on constraints (e.g., low latency, bandwidth).

  • R3 & R4: Intermediate and destination routers.

Traffic Flow:

  1. R1 uses IGP (e.g., OSPF or IS-IS) with Segment Routing enabled to advertise and receive labels (SIDs).

  2. R1, as PCC, sends a path computation request (PCEP) to R2.

  3. R2, as PCE, computes a TE path using available topology and policy constraints.

  4. R1 receives the path and installs it as an SR-TE policy.

  5. Traffic follows the SR policy instead of IGP’s default shortest path.

Benefits of SR-TE with PCE:

  • Centralized path computation (PCE).

  • Dynamic re-optimization when network state changes.

  • SLA-aware routing (e.g., avoid congested links or meet latency targets).

  • Supports automation via NETCONF/YANG or CLI.

🧱 Example Network Setup

R1 -- R2 -- R3 -- R4

R1 = PCC (headend)
R2 = PCE (central controller)
R4 = destination
Links: SR-MPLS enabled, OSPF/ISIS as IGP

Suppose you want traffic to use only "green" links.

interface GigabitEthernet0/0/0/1
 description Link to R3 (green)
 segment-routing
  forwarding
  affinity 0x2

Here:

  • affinity 0x2 marks the link as "green".

  • Affinity is a 32-bit bitmask — you define meanings separately.

2️⃣ Define Affinity Mapping (on PCE or router where policy is configured)

segment-routing
 traffic-eng
  affinity-map green 0x2

3️⃣ Define SR-TE Policy with Constraints (on R1 — PCC)

segment-routing
 traffic-eng
  policy SRTE-R1-R4
   color 100
   end-point ipv4 10.0.0.4
   candidate-paths
    preference 100
     constraints
      affinity include green
      bandwidth 1000000  ! in Kbps (1 Gbps)
     explicit segment-list R1-R2-R3-R4

4️⃣ Define Explicit Segment List (Optional for hard path pinning)

segment-routing
 traffic-eng
  segment-list R1-R2-R3-R4
   index 10
    mpls label 16002  ! R2 SID
   index 20
    mpls label 16003  ! R3 SID
   index 30
    mpls label 16004  ! R4 SID

✅ What This Does:

  • Bandwidth: Allocates bandwidth-aware paths via the PCE.

  • Affinity: Ensures only “green” tagged links are used.

  • Explicit path: You can pin the SR path manually using segment SIDs.

Here is the full configuration for a Cisco SR-TE setup using a PCE-PCC model across four routers (R1 to R4). R1 is the PCC, R2 is the PCE, and R3/R4 are transit/destination nodes.

  • ```plaintext ! ! R1 Configuration (PCC) ! hostname R1 ! interface Loopback0 ip address 1.1.1.1 255.255.255.255 ! interface GigabitEthernet0/0 ip address 10.0.12.1 255.255.255.0 mpls ip ip ospf 1 area 0 ! interface GigabitEthernet0/1 ip address 10.0.14.1 255.255.255.0 mpls ip ip ospf 1 area 0 ! router ospf 1 router-id 1.1.1.1 network 10.0.12.0 0.0.0.255 area 0 network 10.0.14.0 0.0.0.255 area 0 network 1.1.1.1 0.0.0.0 area 0 segment-routing mpls ! mpls traffic-eng ! segment-routing mpls global-block 16000 23999 connected-prefix-sid-map address 1.1.1.1/32 absolute-sid 16001 ! pce segment-routing pcc peer 2.2.2.2 source-address 1.1.1.1 ! interface Tunnel100 ip unnumbered Loopback0 tunnel mode mpls traffic-eng tunnel destination 4.4.4.4 tunnel mpls traffic-eng path-option 10 dynamic tunnel mpls traffic-eng autoroute announce segment-routing mpls

! ! R2 Configuration (PCE) ! hostname R2 ! interface Loopback0 ip address 2.2.2.2 255.255.255.255 ! interface GigabitEthernet0/0 ip address 10.0.12.2 255.255.255.0 mpls ip ip ospf 1 area 0 ! interface GigabitEthernet0/1 ip address 10.0.23.1 255.255.255.0 mpls ip ip ospf 1 area 0 ! router ospf 1 router-id 2.2.2.2 network 10.0.12.0 0.0.0.255 area 0 network 10.0.23.0 0.0.0.255 area 0 network 2.2.2.2 0.0.0.0 area 0 segment-routing mpls ! mpls traffic-eng ! segment-routing mpls global-block 16000 23999 connected-prefix-sid-map address 2.2.2.2/32 absolute-sid 16002 ! pce segment-routing peer 1.1.1.1 source-address 2.2.2.2 lsr-id 2.2.2.2 capability pcc capability pce ! pcc-address-family ipv4 stateful-client ! pce-address-family ipv4 source-address 2.2.2.2

! ! R3 Configuration (Transit) ! hostname R3 ! interface Loopback0 ip address 3.3.3.3 255.255.255.255 ! interface GigabitEthernet0/0 ip address 10.0.23.2 255.255.255.0 mpls ip ip ospf 1 area 0 ! interface GigabitEthernet0/1 ip address 10.0.34.1 255.255.255.0 mpls ip ip ospf 1 area 0 ! router ospf 1 router-id 3.3.3.3 network 10.0.23.0 0.0.0.255 area 0 network 10.0.34.0 0.0.0.255 area 0 network 3.3.3.3 0.0.0.0 area 0 segment-routing mpls ! mpls traffic-eng ! segment-routing mpls global-block 16000 23999 connected-prefix-sid-map address 3.3.3.3/32 absolute-sid 16003

! ! R4 Configuration (Destination) ! hostname R4 ! interface Loopback0 ip address 4.4.4.4 255.255.255.255 ! interface GigabitEthernet0/0 ip address 10.0.34.2 255.255.255.0 mpls ip ip ospf 1 area 0 ! interface GigabitEthernet0/1 ip address 10.0.14.2 255.255.255.0 mpls ip ip ospf 1 area 0 ! router ospf 1 router-id 4.4.4.4 network 10.0.34.0 0.0.0.255 area 0 network 10.0.14.0 0.0.0.255 area 0 network 4.4.4.4 0.0.0.0 area 0 segment-routing mpls ! mpls traffic-eng ! segment-routing mpls global-block 16000 23999 connected-prefix-sid-map address 4.4.4.4/32 absolute-sid 16004 ```

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