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 tuning.

• During this time, traffic may drop. FRR solves this by precomputing a backup path.

• In Cisco’s world, Loop-Free Alternates (LFA) or TI-LFA (Topology Independent LFA) are used.

• If the primary next-hop fails, traffic is immediately redirected to the backup (pre-installed in forwarding plane), before IGP converges.

• Once convergence finishes, the IGP installs the new best path as usual.

Key Point: FRR is a local repair mechanism to protect traffic during the IGP convergence window.

🚦 IGP + SR-TE (Segment Routing - Traffic Engineering)

• SR-TE allows explicit path steering based on segments (SID list) rather than relying only on the IGP’s shortest path.

• While IGP still computes SPT, SR-TE policies can override that with custom paths for SLA, bandwidth, or avoidance.

• Even after IGP convergence, the SR-TE policy takes precedence for flows that match its criteria.

• If a failure occurs on an SR path, FRR can be used within SR-MPLS to protect it during convergence (via TI-LFA).

Key Point: SR-TE allows more flexible post-convergence routing choices beyond IGP's best path.

While IGP still computes SPT, SR-TE policies can override that with custom paths

✅ IGP (e.g., OSPF/IS-IS):

• Shortest Path Tree (SPT): IGP calculates the shortest path based on metrics (like cost).

• Static and Dynamic: It works well, but has limited control over how traffic flows.

🚧 Limitation in IGP:

• Cannot control:

  • Latency

  • Bandwidth requirements

  • Path avoidance (e.g., avoid congested or untrusted nodes)

  • Traffic differentiation (e.g., voice vs. bulk data)

🧭 Segment Routing - Traffic Engineering (SR-TE):

SR-TE builds on top of IGP, but introduces explicit path control via Segment Identifiers (SIDs). You can program traffic to follow a specific route even if it’s not the shortest path.

🎯 Use Cases:

🛠️ How SR-TE Overrides IGP:

You define a policy with:

• Source and destination

• Constraints (e.g., "path must avoid R2" or "minimum available bandwidth: 100 Mbps")

• A Segment List (i.e., list of router/node/adjacency SIDs to reach destination)

Then:

• The router installs this policy into the RIB and FIB

• Traffic matching that policy is forwarded according to the SID list, not the IGP’s SPT

🔄 If IGP recalculates (e.g., topology changes), the SR-TE path remains unchanged unless it becomes invalid, at which point a new SR-TE path can be computed or failover triggered.

📦 Use Case: Low-Latency Path for Voice Traffic

🌐 Network Topology:

        [R1]
        /   \
     10ms   50ms
     /         \
   [R2]——30ms——[R3]——10ms——[R4]

• You need to send voice traffic from R1 to R4.

• Voice requires <40ms latency.

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🧮 What IGP Would Do (e.g., OSPF):

• OSPF sees R1–R3–R4 (50ms + 10ms = 60ms) as the shortest path by cost/metric.

• So it installs that path in the RIB.

• Problem: That path exceeds your latency SLA.

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🛠️ What SR-TE Allows:

• You define a policy:

  • Source: R1

  • Destination: R4

  • Constraint: "Only allow paths with total latency < 40ms"

• SR-TE evaluates paths:

  • R1–R2–R3–R4 → 10 + 30 + 10 = 50ms ❌

  • R1–R2–R4 (via a new link) → 10 + 15 = 25ms ✅

• If this 25ms path exists (maybe R2–R4 has a direct link), SR-TE installs a segment list:

    R1 → R2 (Node SID) → R4 (Node SID)
  

• Voice traffic follows this explicit path, even though it’s not the IGP's shortest.

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🧠 Summary:

Sample configuration of Segment Routing with Traffic Engineering (SR-TE) using Cisco IOS-XR.

This example assumes:

• IS-IS is used as the IGP.

• Segment Routing is enabled.

• An SR-TE policy is created to prefer a custom path with specific SLAs (e.g., low latency).

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🔧 Step 1: Enable Segment Routing

bashCopyEditrouter isis CORE
 address-family ipv4 unicast
  segment-routing mpls
 !

---

🗺️ Step 2: Assign Segment IDs to Loopbacks

bashCopyEditinterface Loopback0
 ipv4 address 1.1.1.1 255.255.255.255
!
router isis CORE
 net 49.0001.0000.0000.0001.00
 address-family ipv4 unicast
  mpls traffic-eng router-id Loopback0
  segment-routing mpls
   connected-prefix-sid-map
    address 1.1.1.1/32 sid 16001

Repeat for other routers, adjusting the IP and SID.

---

📡 Step 3: Enable MPLS Traffic Engineering

bashCopyEditmpls traffic-eng
!
interface GigabitEthernet0/0/0/0
 mpls traffic-eng tunnels

---

🚧 Step 4: Create SR-TE Policy

bashCopyEditsegment-routing
 traffic-eng
  policy VOICE-POLICY
   color 123
   end-point ipv4 4.4.4.4
   candidate-paths
    preference 100
     explicit segment-list VOICE-PATH
      name VOICE-PATH
       index 10
        segment 16002
       index 20
        segment 16004

---

🧭 Step 5: Forward Traffic Using the Policy

bashCopyEditrouter static
 address-family ipv4 unicast
  4.4.4.4/32  segment-routing policy VOICE-POLICY

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🔍 Notes

• segment 16002 and 16004 are SIDs representing routers R2 and R4.

• This policy manually forces the traffic to take R1 → R2 → R4 instead of the IGP shortest path.

• If you use a PCE, you can also compute the path dynamically based on constraints like latency or bandwidth.

Cisco IOS XE example for configuring Segment Routing with SR-TE and PCE-based dynamic policies.

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🧱 Prerequisites

• IOS XE with SR-MPLS and PCE support (e.g., on ASR 1000, Catalyst 8000).

• You have a functioning IGP (like OSPF or IS-IS).

• Devices support SR-PCE (Path Computation Element).

---

1️⃣ Enable Segment Routing and MPLS TE

bashCopyEditrouter ospf 1
 router-id 1.1.1.1
 segment-routing mpls
 mpls traffic-eng
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 ip ospf 1 area 0
!
interface GigabitEthernet0/0/0
 ip ospf 1 area 0
 mpls ip

---

2️⃣ Enable SRGB and SID on Loopback

bashCopyEditsegment-routing
 mpls
  global-block 16000 23999
  connected-prefix-sid-map
   address 1.1.1.1/32 absolute-sid 16001

Repeat on each router with a unique loopback and SID.

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3️⃣ Configure PCE (on PCE node)

bashCopyEditpce
 segment-routing
  peer 2.2.2.2
   source-address 1.1.1.1
   lsr-id 1.1.1.1
   capability pce
   capability pcc

---

4️⃣ Enable PCC (on client router)

bashCopyEditpce
 segment-routing
  pcc
   peer 1.1.1.1
    source-address 2.2.2.2

---

5️⃣ Configure SR-TE Policy with Dynamic Path (computed by PCE)

bashCopyEditinterface Tunnel10
 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

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🔁 Summary

• PCE computes the best path dynamically (e.g., lowest latency).

• SR-TE tunnel forwards traffic based on this path.

• You don't need to define explicit segment lists—PCE handles it.