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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.
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In this blog, I'll show how to use Ansible to automate interface configuration across multiple Cisco IOS-XE routers using a loop. This is especially useful for bulk interface provisioning and ensuring consistency across your edge routers. Objective C...
How IGP convergence ties into Fast Reroute (FRR) and Segment Routing - Traffic Engineering (SR-TE) 🔁 IGP + Fast Reroute (FRR) IGP convergence usually takes tens to hundreds of milliseconds—even up to a few seconds depending on the topology and tuni...
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Azure Load Balancer vs. Application Gateway
A Deep Dive into Secure Cloud Connectivity
[ 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
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.
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
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
show pce peer
show pce lsp
show segment-routing traffic-eng policy
show mpls forwarding-table
[R1 - PCC]
|
(IGP + SR)
|
[R2 - PCE]
/ \
/ \
[R3]----(IGP)----[R4]
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.
R1 uses IGP (e.g., OSPF or IS-IS) with Segment Routing enabled to advertise and receive labels (SIDs).
R1, as PCC, sends a path computation request (PCEP) to R2.
R2, as PCE, computes a TE path using available topology and policy constraints.
R1 receives the path and installs it as an SR-TE policy.
Traffic follows the SR policy instead of IGP’s default shortest path.
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.
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.
segment-routing
traffic-eng
affinity-map green 0x2
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
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
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.
! ! 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 ```