📘CCNP Enterprise – ENARSI (300-410)
1. What Is MPLS? (Quick Revision)
MPLS (Multiprotocol Label Switching) is a forwarding technology used by service providers and large enterprises to move traffic efficiently across a network.
Instead of routing packets based on IP addresses at every hop, MPLS uses short labels that are attached to packets. These labels tell routers exactly where to send traffic next, making forwarding faster and more predictable.
2. Key MPLS Components (Must Know for Exam)
2.1 Label Switch Router (LSR)
An LSR is any router inside an MPLS network that:
- Receives packets with MPLS labels
- Uses the label to forward traffic
- Does not look at the IP address during forwarding
Types of LSRs:
- Ingress LSR – First MPLS router, adds a label
- Transit LSR – Middle router, swaps labels
- Egress LSR – Last router, removes the label
2.2 Label Distribution Protocol (LDP)
LDP is the protocol used by MPLS routers to:
- Exchange label information
- Build label forwarding tables
- Create label paths automatically
Key points:
- Runs over TCP (port 646)
- Uses UDP for discovery
- Works with the IGP (OSPF / IS-IS)
📌 Exam Tip:
LDP does not decide paths. The IGP decides the path; LDP only assigns labels.
2.3 Label Switching
Label switching replaces IP lookup with label lookup.
Steps:
- Ingress router adds a label
- Transit routers swap labels
- Egress router removes the label
This process is:
- Faster than IP routing
- Predictable
- Scalable
2.4 Label Switched Path (LSP)
An LSP is the path that labeled packets follow through the MPLS network.
Important points:
- One-way path
- Built using IGP + LDP
- Similar to a virtual circuit
📌 Exam Tip:
LSPs are unidirectional, even though traffic flows both ways.
3. Why Policies Are Needed in MPLS
Policies allow network engineers to:
- Control which routes enter or leave the MPLS network
- Decide which path traffic should take
- Improve security and traffic control
- Enforce business and traffic engineering rules
In MPLS, policies are mainly applied at:
- Ingress LSR
- Egress LSR
- PE routers (Provider Edge)
4. Inbound and Outbound Filtering in MPLS
Filtering controls which routes are accepted or advertised.
4.1 Inbound Filtering
Inbound filtering means:
- Filtering routes as they are received
- Applied on incoming routing updates
Where it is used:
- On PE routers
- On LDP neighbors
- On BGP neighbors (MPLS VPN)
What it can do:
- Block unwanted routes
- Prevent routing table overload
- Improve security
📌 Important Exam Point:
Inbound filtering does not change the routes already sent by the neighbor. It only affects what this router accepts.
4.2 Outbound Filtering
Outbound filtering means:
- Filtering routes before sending them
- Applied on outgoing updates
Where it is used:
- When advertising routes to other MPLS routers
- When controlling customer route advertisement
What it can do:
- Hide internal routes
- Control route visibility
- Prevent route leakage
📌 Exam Tip:
Outbound filtering affects what neighbors see, not what the local router knows.
5. Path Manipulation in MPLS
Path manipulation means forcing traffic to use a specific path instead of the default IGP path.
This is very important in MPLS networks.
5.1 Why Path Manipulation Is Needed
Default IGP paths may:
- Cause congestion
- Waste bandwidth
- Not meet traffic requirements
Path manipulation allows:
- Traffic engineering
- Load balancing
- Controlled traffic flow
5.2 MPLS Traffic Engineering (High-Level)
While deep TE is not required here, you must understand:
- MPLS can steer traffic using labels
- Paths can be influenced without changing IP routing
- MPLS TE uses additional information like bandwidth and priority
📌 Exam Focus:
Understand the concept, not the configuration.
5.3 Policy-Based Path Control
Common methods:
- Label preference
- Route filtering
- Route selection control
- Prefix-based decisions
Policies are usually applied at:
- Ingress routers
- PE routers
6. How Policies Interact with MPLS Forwarding
- Routing protocol selects the path
- LDP assigns labels for that path
- Policies filter or modify route availability
- MPLS forwards traffic using labels
- Traffic follows the desired LSP
📌 Key Concept:
Policies influence routing decisions, which indirectly influence LSP creation.
7. Common Exam Traps and Clarifications
| Topic | Important Clarification |
|---|---|
| MPLS vs IP Routing | MPLS forwards based on labels, not IP |
| LDP | Does not select paths |
| LSP | One-way only |
| Inbound filtering | Affects received routes |
| Outbound filtering | Affects advertised routes |
| Path manipulation | Happens at routing/policy level |
8. Exam Summary (Very Important)
To pass this section, remember:
- MPLS uses labels instead of IP lookup
- LSRs forward traffic using label switching
- LDP distributes labels, IGP decides paths
- LSPs are one-directional paths
- Policies control:
- Which routes are accepted
- Which routes are advertised
- Which path traffic follows
- Inbound filtering affects received routes
- Outbound filtering affects sent routes
- Path manipulation helps control traffic flow in MPLS
9. Final One-Line Exam Takeaway
MPLS policies allow engineers to control route visibility and traffic paths by filtering routing updates and influencing how labeled paths are built and used across the MPLS network.
