WAN & SD-WAN

Connecting sites across distance — from dedicated circuits and MPLS to transport-agnostic SD-WAN overlays.

// MPLS label-switched path

→

PE
push

17 →

P
swap

22 →

PE
pop (PHP)

→

The ingress PE pushes a label; P routers swap labels (fast switching, no IP lookup); the penultimate hop pops it.

// MPLS shim header (32 bits)

Label

20 bit

EXP / TC

3 bit

1 bit

TTL

8 bit

Sits between the L2 and L3 headers ("frame mode"). Label distributed by LDP (TCP/UDP 646).

// label operations

Push	Ingress PE adds a label (imposition)
Swap	P router replaces the label
Pop	Remove the label (disposition)
PHP	Penultimate hop pops — egress gets a clean packet

// terminology

LSR	Label Switch Router — swaps labels in the core (P)
LER / PE	Edge router — pushes / pops labels
FEC	Forwarding Equivalence Class — packets treated alike
LSP	Label-Switched Path — the end-to-end label route
LIB / LFIB	Label databases — control / forwarding
LDP	Label Distribution Protocol — TCP/UDP 646

Leased line	Dedicated T1/E1, fixed bandwidth
MPLS	Label-switched, L3 VPN, QoS-friendly
Metro Ethernet	Carrier Ethernet across a metro
DSL / Cable	Broadband over phone / coax
Cellular 4G/5G	Wireless WAN & failover
Satellite	Remote sites, high latency

// encapsulation

PPP	Auth (PAP/CHAP), multilink
HDLC	Cisco default on serial
Ethernet	Most modern WAN handoffs

// SD-WAN

Overlay across any transport (MPLS, broadband, LTE)
Centralized controller & policy
Application-aware path selection
Cheaper and more resilient than pure MPLS

ppp (pap/chap + multilink) · cisco ios

username R2 password S3cret
!
interface Serial0/0
 encapsulation ppp
 ppp authentication chap
 ip address negotiated
 compress stac
 ppp multilink group 1

frame-mode mpls · cisco ios

ip cef
mpls label protocol ldp
!
interface Gi0/0
 ip address 10.0.0.1 255.255.255.252
 mpls ip
 mpls mtu 1512

verify

show ppp multilink
show mpls interfaces
show mpls ldp neighbors
show mpls forwarding-table