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