FRAME MODE MPLS
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Protocol Header
Conceptual Components
8
16
24
32
Label
TC S
TTL
L2
IP
Label stack
Label (20 bits) · Unique label value
Control Plane
Facilitates label exchange between neighboring
LSRs using LDP or TDP (includes the LIB)
Forwarding/Data Plane
Forwards packets based on label or destination
IP address (includes the FIB and LFIB)
Label Protocols
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LDP
TDP
Traffic Class (3 bits) · CoS-mapped QoS marking
Hello Address
224.0.0.2
255.255.255.255
Bottom of Stack (1 bit) · Indicates label is last in the stack
Hello Port
UDP/646
UDP/711
Time To Live (8 bits) · Hop counter mapped from IP TTL
Adjacency Port
TCP/646
TCP/711
Label Switched Path
Provider Network
PE
P
PE
P
P
LSP
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Customer Network
CE
C
C
CE
Customer (C) · IP-only routers internal to customer network
Customer Edge (CE) · C routers which face PE routers
Provider Edge (PE) · LSRs on the MPLS-IP boundary
Provider (P) · MPLS-only LSRs in provider network
MPLS Configuration
! Enable CEF
ip cef
! Select label protocol
mpls label protocol ldp
! Enable MPLS on IP interfaces
interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.252
mpls ip
! Raise MPLS MTU to accommodate multilabel stack
mpls mtu 1512
Proprietary
No
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Cisco
Terminology
Label Distribution Protocol (LDP)
Standards-based label distribution protocol
defined in RFC 3036
Tag Distribution Protocol (TDP)
Cisco's proprietary predecessor to LDP
Label Switching Router (LSR)
Any router performing label switching (MPLS)
Label-Switched Path (LSP)
The unidirectional path through one or more
LSRs taken by a label-switched packet
belonging to an FEC
Forwarding Equivalence Class (FEC)
A group of packets which are forwarded in an
identical manner, typically by destination prefix
and/or traffic class
Label Information Base (LIB)
Contains all labels learned by an LSR via a label
distribution protocol
Forwarding Information Base (FIB)
Routing database for unlabeled (IP) packets
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Label FIB (LFIB)
Routing database for labeled (MPLS) packets
Interim Packet Propagation
An LSR temporarily falls back to IP routing
while waiting to learn the necessary MPLS
label(s)
Penultimate Hop Popping (PHP)
The second-to-last LSR in an LSP removes the
MPLS label so the last LSR only has to perform
an IP lookup
show mpls interfaces
show mpls ldp bindings [detail] (LIB)
show ip cef [detail] (FIB)
show mpls ldp neighbors
show mpls forwarding-table [detail] (LFIB)
debug mpls […]
Troubleshooting
by Jeremy Stretch
v2.0