Multi-Layer Routing

---

• Overview
• How It Works
• Building a Router Hierarchy
• Root Routers
• Intermediate Routers
• Leaf Routers
• Configuration Example

---

Hierarchical Router Relationships for Advanced Routing Scenarios.

Overview

Multi-layer routing enables you to create hierarchical relationships between routers, where parent routers can process requests through middleware before child routers make final routing decisions.

This feature allows middleware at the parent level to modify requests (adding headers, performing authentication, etc.) that influence how child routers evaluate their rules and route traffic to services.

Multi-layer routing is particularly useful for progressive request enrichment, where each layer adds context to the request, enabling increasingly specific routing decisions:

• Authentication-Based Routing: Parent router authenticates requests and adds user context (roles, permissions) as headers, child routers route based on these headers
• Staged Middleware Application: Apply common middleware (rate limiting, CORS) at parent level (for a given domain/path), but specific middleware at child level

!!! info "Provider Support"

Multi-layer routing is supported by the following providers:

- **File provider** (YAML, TOML, JSON)
- **KV stores** (Consul, etcd, Redis, ZooKeeper)
- **Kubernetes CRD** (IngressRoute)

Multi-layer routing is not available for other providers (Docker, Kubernetes Ingress, Gateway API, etc.).

How It Works

Request → EntryPoint → Parent Router → Middleware → Child Router A → Service A
                                          ↓       → Child Router B → Service B
                                     Modify Request
                                  (e.g., add headers)

1. Request arrives at an entrypoint
2. Parent router matches based on its rule (e.g., Host(`example.com`))
3. Parent middleware executes, potentially modifying the request
4. One child router matches based on its rule (which may use modified request attributes)
5. Request is forwarded to the matching child router's service

Building a Router Hierarchy

Root Routers

• Have no parentRefs (top of the hierarchy)
• Can have tls, observability, and entryPoints configuration
• Can be either parent routers (with children) or standalone routers (with service)
• Can have models applied (non-root routers cannot have models)

Intermediate Routers

• Reference their parent router(s) via parentRefs
• Have one or more child routers
• Must not have a service defined
• Must not have entryPoints, tls, or observability configuration

Leaf Routers

• Reference their parent router(s) via parentRefs
• Must have a service defined
• Must not have entryPoints, tls, or observability configuration

Configuration Example

??? example "Authentication-Based Routing"

```yaml tab="File (YAML)"
## Dynamic configuration
http:
  routers:
    # Parent router with authentication
    api-parent:
      rule: "PathPrefix(`/api`)"
      middlewares:
        - auth-middleware
      entryPoints:
        - websecure
      tls: {}
      # Note: No service defined - this is a parent router

    # Child router for admin users
    api-admin:
      rule: "HeaderRegexp(`X-User-Role`, `admin`)"
      service: admin-service
      parentRefs:
        - api-parent

    # Child router for regular users
    api-user:
      rule: "HeaderRegexp(`X-User-Role`, `user`)"
      service: user-service
      parentRefs:
        - api-parent

  middlewares:
    auth-middleware:
      forwardAuth:
        address: "http://auth-service:8080/auth"
        authResponseHeaders:
          - X-User-Role
          - X-User-Name

  services:
    admin-service:
      loadBalancer:
        servers:
          - url: "http://admin-backend:8080"

    user-service:
      loadBalancer:
        servers:
          - url: "http://user-backend:8080"
```

```toml tab="File (TOML)"
## Dynamic configuration
[http.routers]
  # Parent router with authentication
  [http.routers.api-parent]
    rule = "PathPrefix(`/api`)"
    middlewares = ["auth-middleware"]
    entryPoints = ["websecure"]
    [http.routers.api-parent.tls]
    # Note: No service defined - this is a parent router

  # Child router for admin users
  [http.routers.api-admin]
    rule = "HeaderRegexp(`X-User-Role`, `admin`)"
    service = "admin-service"
    parentRefs = ["api-parent"]

  # Child router for regular users
  [http.routers.api-user]
    rule = "HeaderRegexp(`X-User-Role`, `user`)"
    service = "user-service"
    parentRefs = ["api-parent"]

[http.middlewares]
  [http.middlewares.auth-middleware.forwardAuth]
    address = "http://auth-service:8080/auth"
    authResponseHeaders = ["X-User-Role", "X-User-Name"]

[http.services]
  [http.services.admin-service.loadBalancer]
    [[http.services.admin-service.loadBalancer.servers]]
      url = "http://admin-backend:8080"

  [http.services.user-service.loadBalancer]
    [[http.services.user-service.loadBalancer.servers]]
      url = "http://user-backend:8080"
```

```txt tab="KV (Consul/etcd/Redis/ZK)"
| Key                                                                    | Value                           |
|------------------------------------------------------------------------|---------------------------------|
| `traefik/http/routers/api-parent/rule`                                 | `PathPrefix(\`/api\`)`          |
| `traefik/http/routers/api-parent/middlewares/0`                        | `auth-middleware`               |
| `traefik/http/routers/api-parent/entrypoints/0`                        | `websecure`                     |
| `traefik/http/routers/api-parent/tls`                                  | `true`                          |
| `traefik/http/routers/api-admin/rule`                                  | `HeaderRegexp(\`X-User-Role\`, \`admin\`)` |
| `traefik/http/routers/api-admin/service`                               | `admin-service`                 |
| `traefik/http/routers/api-admin/parentrefs/0`                          | `api-parent`                    |
| `traefik/http/routers/api-user/rule`                                   | `HeaderRegexp(\`X-User-Role\`, \`user\`)` |
| `traefik/http/routers/api-user/service`                                | `user-service`                  |
| `traefik/http/routers/api-user/parentrefs/0`                           | `api-parent`                    |
| `traefik/http/middlewares/auth-middleware/forwardauth/address`         | `http://auth-service:8080/auth` |
| `traefik/http/middlewares/auth-middleware/forwardauth/authresponseheaders/0` | `X-User-Role`         |
| `traefik/http/middlewares/auth-middleware/forwardauth/authresponseheaders/1` | `X-User-Name`         |
| `traefik/http/services/admin-service/loadbalancer/servers/0/url`       | `http://admin-backend:8080`     |
| `traefik/http/services/user-service/loadbalancer/servers/0/url`        | `http://user-backend:8080`      |
```

**How it works:**

1. Request to `/api/endpoint` matches `api-parent` router
2. `auth-middleware` (ForwardAuth) validates the request and adds `X-User-Role` header
3. Modified request is evaluated by child routers
4. If `X-User-Role: admin`, `api-admin` router matches and forwards to `admin-service`
5. If `X-User-Role: user`, `api-user` router matches and forwards to `user-service`

{% include-markdown "includes/traefik-for-business-applications.md" %}