IOS XR · SR-TE

IOS XR: SR-TE policy with BGP color steering

The core runs segment routing, and the top path through P1 has TE metric 100 per link. PE2 advertises two customer LANs over iBGP. Mark LAN 20 with the BGP color 10. On PE1, build SR-TE policy GREEN for color 10 toward PE2: a dynamic candidate on the TE metric, then an explicit one. Traffic to LAN 20 rides the policy; LAN 10 has no color and keeps the default path, the IGP shortest path through P1.

IOS XR track · IOS XR · 5 objectives

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Objectives

The simulator ticks each one as soon as the network meets it.

On PE2 create extcommunity-set opaque COLOR10 (value 10) and route-policy COLOR10 that sets it, and attach it to network 192.168.20.0/24.
On PE1 create policy GREEN, color 10, end-point 5.5.5.5, candidate preference 100 dynamic with metric type te. It comes up through P2 and P3 (show segment-routing traffic-eng policy).
PC1 → PC3 (LAN 20, color 10) rides policy GREEN.
PC1 → PC2 (LAN 10, no color) takes the default path through P1.
Add candidate preference 200: explicit segment list VIA-P1 (16002, 16005). It wins, so GREEN now runs through P1.

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Before you start

A BGP route is steered into the policy whose color equals the route’s color and whose end-point equals its BGP next hop. Every other route uses the default path. Among valid candidate paths, the highest preference is active. A dynamic path is computed on the TE metric here, so it avoids the expensive top links.

Concept

Steering traffic into a policy: color and autoroute

Routes are steered into an SR-TE policy in two ways. With automated steering, a BGP route whose color extended community equals the policy’s color, and whose next hop equals the policy’s end-point, uses the policy. With autoroute, every IGP route through the end-point uses it, color or not. Everything else takes the default path: the SR (or LDP) shortest path.

Concept

SR-TE policies and candidate paths

An SR-TE policy is identified by a color and an end-point. It carries a list of candidate paths with preferences. A dynamic candidate is computed on the head end over the TE database. An explicit candidate is a segment list of prefix-SIDs and adjacency-SIDs. The valid candidate with the highest preference is active.