---
title: "Multiprotocol support and MPLS"
subject: "Internet and Intranet"
chapter: 2
key: "semester_8/internet_and_intranet:multiprotocol-support-mpls"
source: https://www.bctengineer.com/q/semester_8/internet_and_intranet/multiprotocol-support-mpls
site: BCT Engineering
---

# Multiprotocol support and MPLS

Internet and Intranet · Chapter 2: Internet Protocol Overview

- Asked in: 2071 Bhadra, 2+8 marks
- Video: [What is MPLS and How Does it Work? | CBT Nuggets](https://www.youtube.com/watch?v=E5Ud1m9h0yc) — CBT Nuggets
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/internet_and_intranet/multiprotocol-support-mpls
- Whole chapter: https://www.bctengineer.com/md/semester_8/internet_and_intranet/chapter-2

## Question

What do you mean by multiprotocol support? Explain how MPLS works?

## Short answer

Multiprotocol support means carrying multiple network-layer protocols such as IPv4, IPv6, IPX, or AppleTalk over the same links. MPLS forwards packets using short fixed labels inserted between layer 2 and IP headers; edge LERs push/pop labels, core LSRs swap labels along an LSP, with labels distributed by LDP or RSVP-TE.

## Answer

#### Multiprotocol support

**Multiprotocol support** means that a network device or technology can carry and handle traffic of more than one network-layer protocol at the same time, such as IPv4, IPv6, IPX or AppleTalk, over the same links. A multiprotocol router keeps a separate routing table for each protocol; a dual-stack router runs IPv4 and IPv6 together. In MPLS, multiprotocol means the technology works independently of both the network-layer protocol above it (IP, IPv6, Ethernet frames, ATM cells) and the link-layer technology below it.

#### How MPLS works

**Multiprotocol Label Switching (MPLS)** forwards packets using short fixed-length **labels** instead of looking up the IP destination address at every router. It is called a "layer 2.5" technology because the label is inserted between the layer 2 header and the IP header.

**MPLS header (32 bits):** Label (20 bits), Traffic Class (3 bits, QoS), S bit (1 bit, bottom of label stack), TTL (8 bits).

**Terms**
- **LER (Label Edge Router):** at the edge of the MPLS domain; adds (push) or removes (pop) labels.
- **LSR (Label Switch Router):** core router that forwards by label (swap).
- **FEC (Forwarding Equivalence Class):** group of packets forwarded the same way (e.g. same destination prefix).
- **LSP (Label Switched Path):** the one-way path through the network.
- **LDP / RSVP-TE:** protocols to distribute labels. The routers build a label forwarding table (LFIB).

**Operation**
1. IGP (OSPF/IS-IS) builds routes and LDP assigns labels for each prefix, building the LSP.
2. **Ingress LER** classifies the incoming IP packet into a FEC and **pushes** a label.
3. Each **LSR** looks at only the label, finds the outgoing label and interface in its table, **swaps** the label and forwards.
4. **Egress LER** **pops** the label and forwards the packet by a normal IP lookup. (With **penultimate hop popping**, the last LSR before the egress removes the label instead, so the egress LER does only the IP lookup.)

```text
 IP pkt        LSP
  -->[LER1]-->[LSR2]-->[LSR3]-->[LER4]--> IP
     push 17   swap     swap     pop
              17->22   22->31
```

**Advantages:** fast forwarding with a simple lookup, traffic engineering (explicit paths), VPNs (layer 2 and layer 3), QoS and fast reroute. It is widely used in ISP and enterprise WAN backbones.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does multiprotocol support mean for MPLS?
   A. It works only with IPv4 over Ethernet
   B. It works independently of both the network-layer protocol above and the link-layer technology below
   C. It converts all packets to ATM
   D. It runs several routing protocols on one table
   Answer: B. MPLS can carry IP, IPv6, Ethernet frames or ATM cells over various link technologies. (Key point: Multiprotocol meaning)

2. What does each core LSR do when it receives a labelled packet?
   A. Looks up the IP destination, then pushes a new label
   B. Looks only at the label, swaps it with the outgoing label and forwards
   C. Pops the label and routes by IP
   D. Classifies the packet into a FEC
   Answer: B. LSRs use the label forwarding table (LFIB) to swap labels; push is done by the ingress LER and pop at the end. (Key point: MPLS label operations)

3. Where is the label removed in penultimate hop popping, and what does the egress LER then do?
   A. Removed by the ingress LER; egress swaps labels
   B. Removed by the last-but-one router; egress LER forwards using normal IP lookup
   C. Removed by the egress LER; penultimate router pushes a label
   D. Never removed
   Answer: B. The penultimate router pops the label so the egress LER only needs a normal IP lookup. (Key point: Penultimate hop popping)

4. How large is the label field in the 32-bit MPLS header?
   A. 8 bits
   B. 20 bits
   C. 3 bits
   D. 32 bits
   Answer: B. The header has Label (20 bits), Traffic Class (3), S bit (1) and TTL (8). (Key point: MPLS header)

## Related in other subjects

- [Advantages of closed loop with mathematical support](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Acl-advantages-math) — Control Systems, chapter 1
- [Selecting support type for underground opening](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Asupport-type-underground) — Engineering Geology II, chapter 6