mpls10s03-label assignment and distribution
TRANSCRIPT
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
1/92
2001, Cisco Systems, Inc.
Module 3Label Assignment
and Distribution
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
2/92
MPLS v1.03-2 2001, Cisco Systems, Inc.
Objectives
Upon completion of this chapter, you will beable to perform the following tasks:
Describe the need for LDP/TDP in MPLSnetwork.
Describe the LDP/TDP neighbor discovery andsession establishment procedures.
Explain the needs for different LDP/TDP labeldistribution modes.
Explain the difference between independentand ordered control.
Describe various LDP/TDP retention modes(conservative and liberal).
Explain the functions and benefits of
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
3/92
LDP Role in Unicast
IP Routing
2001, Cisco Systems, Inc. MPLS v1.03-3
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
4/92
MPLS v1.03-4 2001, Cisco Systems, Inc.
Objectives
Upon completion of this section,you will be able to perform thefollowing tasks:
Explain the need for the LabelDistribution Protocol (LDP) in unicastIP routing MPLS application.
Describe the LDPs interaction withother Label Switch Router (LSR)components.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
5/92
MPLS v1.03-5 2001, Cisco Systems, Inc.
MPLS Unicast IP Routing
MPLS introduces a new field that is used forforwarding decisions.
Although labels are locally significant, theyhave to be advertised to directly reachable
peers.
One option would be to include thisparameter in existing IP routing protocols.
The other option is to create a newprotocol to exchange labels.
The second option has been used becausethere are too many existing IP routing
protocols that would have to be modified tocarry labels.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
6/92
MPLS v1.03-6 2001, Cisco Systems, Inc.
MPLS Unicast IP RoutingArchitecture
LSR
Control Plane
Data Plane
Routing Protocol
LDP
Label Forwarding Table
IP Routing Table
Exchange of
routing information
Exchange of
labels
Incoming
labeled packets
Outgoing
labeled packets
IP Forwarding Table
Incoming
IP packets
Outgoing
IP packets
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
7/92MPLS v1.03-7 2001, Cisco Systems, Inc.
MPLS Unicast IP Routing:Example
LSR
Control Plane
Data Plane
OSPF:
RT:
LIB:
FIB:
LFIB:
OSPF: 10.0.0.0/810.0.0.0/8 1.2.3.4
10.0.0.0/8 1.2.3.4
10.0.0.0/8 1.2.3.4
L=5 10.1.1.1
10.1.1.1 10.1.1.1
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
8/92MPLS v1.03-8 2001, Cisco Systems, Inc.
MPLS Unicast IP Routing:Example
LSR
Control Plane
Data Plane
OSPF:
RT:
LIB:
FIB:
LFIB:
OSPF: 10.0.0.0/810.0.0.0/8 1.2.3.4
10.0.0.0/8 1.2.3.4
10.0.0.0/8 1.2.3.410.1.1.1
LDP: 10.0.0.0/8, L=3
L=5 10.1.1.1
10.0.0.0/8 Next-hop L=3, Local L=5LDP: 10.0.0.0/8, L=5
L=3 10.1.1.1
L=3 10.1.1.1L=5 L=3
, L=3
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
9/92MPLS v1.03-9 2001, Cisco Systems, Inc.
Summary
After completing this section, youshould be able to perform the
following tasks:Explain the need for the LDP in unicastIP routing MPLS application.
Describe the LDPs interaction withother LSR components.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
10/92MPLS v1.03-10 2001, Cisco Systems, Inc.
Review Questions
Why is the LDP/TDP needed?
What is the forwarding equivalence
class in MPLS unicast IP forwarding?Where is this forwarding equivalenceclass taken from?
What is the output of the LDP/TDP?
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
11/92
Typical LabelDistribution in Packet-
mode MPLS
2001, Cisco Systems, Inc. MPLS v1.03-11
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
12/92MPLS v1.03-12 2001, Cisco Systems, Inc.
Objectives
Upon completion of this section,you will be able to perform thefollowing tasks:
Describe label allocation in packet-mode MPLS environments.
Describe TDP/LDP label distribution in
packet-mode MPLS environments.Explain how the MPLS data structuresare built based on label allocation anddistribution.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
13/92MPLS v1.03-13 2001, Cisco Systems, Inc.
Label Allocation in a Packet-Mode MPLS Environment
Label allocation and distribution in apacket-mode MPLS environment followsthese steps:
IP routing protocols build the IP routingtable.
Each LSR assigns a label to every
destination in the IP routing tableindependently.
LSRs announce their assigned labels to all
other LSRs.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
14/92MPLS v1.03-14 2001, Cisco Systems, Inc.
Building the IP Routing Table
IP routing protocols are used to build IProuting tables on all LSRs.
FIBs are built based on IP routing tables withno labeling information.
Network Next-hop
X B
Routing table of A
Network Next-hop
X C
Routing table of B
Network Next-hop
X D
Routing table of C
Network Next-hop
X C
Routing table of ENetwork Next hop Label
X B
FIB on A
A B C D
E
Network X
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
15/92MPLS v1.03-15 2001, Cisco Systems, Inc.
Allocating Labels
Every LSR allocates a label for everydestination in the IP routing table.
Labels have local significance. Label allocations are asynchronous.
Network Next-hop
X C
Routing table of B
A B C D
E
Network X
Router B assigns label 25 to
destination X.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
16/92MPLS v1.03-16 2001, Cisco Systems, Inc.
Network Next-hop
X C
Routing table of B
A B C D
E
Network X
Router B assigns label 25 to
destination X.
LIB and LFIB Setup
LIB and LFIB structures have to be initialized onthe LSR allocating the label.
Network LSR label
X local 25
LIB on B Local label is stored in LIB.
Label Action Next hop
25 pop C
LFIB on B Outgoing action is pop, as B
has received no label for X
from C.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
17/92MPLS v1.03-17 2001, Cisco Systems, Inc.
A B C D
E
Network X
Label Distribution
The allocated label is advertised to all neighborLSRs, regardless of whether the neighbors areupstream or downstream LSRs for the
destination.
Network LSR label
X local 25
LIB on B
X = 25X = 25
X=25
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
18/92MPLS v1.03-18 2001, Cisco Systems, Inc.
Receiving LabelAdvertisement
Every LSR stores the received label in its LIB. Edge LSRs that receive the label from their
next-hop also store the label information in the
FIB.
X = 25X = 25
Network LSR label
X B 25
LIB on A
Network LSR label
X B 25
LIB on C
Network LSR label
X B 25
LIB on E
Network Next hop Label
X B 25
FIB on A
A B C D
E
X=25
Network X
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
19/92MPLS v1.03-19 2001, Cisco Systems, Inc.
Interim Packet Propagation
Forwarded IP packets are labeled only on thepath segments where the labels have already
been assigned.
IP: X Lab: 25 IP: X
Network Next hop Label
X B 25
FIB on A
IP lookup is performed inFIB: packet is labeled.
Label Action Next hop
25 pop C
LFIB on B
Label lookup is performed
in LFIB: label is removed.
A B C
E
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
20/92MPLS v1.03-20 2001, Cisco Systems, Inc.
Further Label Allocation
Every LSR will eventually assign a label forevery destination.
Network LSR label
X B 25
local 47
LIB on C
Label Action Next hop47 pop D
LFIB on C
A B C D
E
Network X
Router C assigns label
47 to destination X.X=
47
X = 47
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
21/92MPLS v1.03-21 2001, Cisco Systems, Inc.
Receiving LabelAdvertisement
Every LSR stores received information in itsLIB.
LSRs that receive their label from their next-hop LSR will also populate the IP forwarding
Network LSR labelX B 25
C 47
LIB on E
Network LSR label
X local 25
C 47
LIB on B
Network Next hop Label
X C 47
FIB on B
Network Next hop LabelX C 47
FIB on E
A B C D
E
Network XX=
47
X = 47
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
22/92MPLS v1.03-22 2001, Cisco Systems, Inc.
Populating LFIB
Router B has already assigned a label to X andcreated an entry in the LFIB.
The outgoing label is inserted in the LFIB afterthe label is received from the next-hop LSR.
Network LSR label
X local 25
C 47
LIB on B
Network Next hop Label
X C 47
FIB on B
Label Action Next hop
25 47 C
LFIB on B
A B C D
E
X=
47
X = 47
Network X
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
23/92
MPLS v1.03-23 2001, Cisco Systems, Inc.
Packet Propagation Acrossan MPLS Network
IP: X IP: X
Ingress LSR Egress LSR
A B C
E
Lab: 25 Lab: 47
Network Next hop Label
X B 25
FIB on A
IP lookup is performed inthe FIB, packet is labeled.
Label Action Next hop
47 pop D
LFIB on C
Label lookup is performed
in the LFIB, label is removed.
Label Action Next hop
25 47 C
LFIB on B
Label lookup is performed
in the LFIB, label is switched.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
24/92
MPLS v1.03-24 2001, Cisco Systems, Inc.
Per-Platform Label Allocation
Label Action Next hop
25 75 D
LFIB on B
X = 25
X=25
An LFIB on a router usually does not contain
an incoming interface. The same label can be used on any interface
per-platform label allocation.
LSR announces a label to adjacent LSR onlyonce even if there are parallel links betweenthem.
Network X
B
C
DA
B fit d D b k f
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
25/92
MPLS v1.03-25 2001, Cisco Systems, Inc.
Benefits and Drawbacks ofPer-Platform Label Allocation
Benefits:
Smaller LFIB
Quicker label
exchange
Drawbacks: Insecureany
neighbor LSR can sendpackets with any label
in the LFIB
Label Action Next hop
25 75 D
LFIB on B
Network X
B DA
Label for X is announced
only to A.
X = 25
Lab:
25
Lab: 47
A third-party router can still send
packets toward network X, even though
the label was not announced to it.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
26/92
MPLS v1.03-26 2001, Cisco Systems, Inc.
-MPLS Label Allocation and
Distribution
Packet-mode MPLS label allocation anddistribution follows these rules:
Every LSR assigns a label for every destinationin the IP routing table.
Labels are assigned once per LSR (per-platform).
Every LSR advertises its label assignments toall neighbors.
Every LSR stores all advertised labels in theLIB.
Labels received from next-hop LSRs are used to
populate label information in the FIB and theoutgoing label in the LFIB.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
27/92
MPLS v1.03-27 2001, Cisco Systems, Inc.
Review Questions
Where are received labels stored?
Which forwarding table is used toforward a labeled packet?
Which forwarding table is used toforward an unlabeled packet?
What happens if the next-hop label is
not in the LFIB table?
How many labels are usually assignedto one destination network?
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
28/92
Convergence inPacket-mode MPLS
2001, Cisco Systems, Inc. MPLS v1.03-28
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
29/92
MPLS v1.03-29 2001, Cisco Systems, Inc.
Objectives
Upon completion of this section, youwill be able to perform the followingtasks:
Identify TDP/LDP convergence issues.Describe the interaction betweenrouting protocol convergence and LDPconvergence.
Describe the packet-mode MPLSconvergence after link failure and linkrecovery.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
30/92
MPLS v1.03-30 2001, Cisco Systems, Inc.
Steady State Description
After the LSRs have exchanged the labels,LIB, LFIB and FIB data structures arecompletely populated.
Network Next-hop
X C
Routing table of B
Network Next hop Label
X C 47
FIB on B
Network LSR label
X local 25
C 47
E 75
LIB on B
Label Action Next hop
25 47 C
LFIB on B
A B C D
E
Network X
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
31/92
MPLS v1.03-31 2001, Cisco Systems, Inc.
Link Failure Actions
Routing protocolneighbors and LDPneighbors are lost aftera link failure.
Entries are removed
from various datastructures.
Network Next-hop
X C
Routing table of B
Network Next hop Label
X C 47
FIB on B
Network LSR label
X local 25
C 47
E 75
LIB on B
Label Action Next hop
25 47 C
LFIB on B
"A B C D
E
Network X
"
R ti P t l
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
32/92
MPLS v1.03-32 2001, Cisco Systems, Inc.
Routing ProtocolConvergence
Routing protocols rebuildthe IP routing table andthe IP forwarding table.
Network LSR label
X local 25
C 47
E 75
LIB on B
Label Action Next hop
25 47 C
LFIB on B
Network Next hop Label
X E
FIB on B
Network Next-hop
X E
Routing table of B
A B C D
E
Network X
"
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
33/92
MPLS v1.03-33 2001, Cisco Systems, Inc.
MPLS Convergence
The LFIB and labelinginformation in the FIB arerebuilt immediately afterthe routing protocolconvergence, based on
labels stored in the LIB.
Network LSR label
X local 25
C 47
E 75
LIB on B
Network Next-hop
X E
Routing table of B
Label Action Next hop
25 75 E
LFIB on B
Network Next hop Label
X E 75
FIB on B
A B C D
E
Network X
"
MPLS C
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
34/92
MPLS v1.03-34 2001, Cisco Systems, Inc.
MPLS ConvergenceAfter a Link Failure
MPLS convergence in packet-modeMPLS does not affect the overall
convergence time.MPLS convergence occursimmediately after the routing
protocol convergence, based onlabels already stored in the LIB.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
35/92
MPLS v1.03-35 2001, Cisco Systems, Inc.
Link Recovery Actions
Routing protocolneighbors arediscovered after linkrecovery.
Network LSR label
X local 25
C 47
E 75
LIB on B
Network Next-hop
X E
Routing table of B
Label Action Next hop
25 75 E
LFIB on B
Network Next hop Label
X E 75
FIB on B
A B C D
E
Network X
IP R ti C Aft
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
36/92
MPLS v1.03-36 2001, Cisco Systems, Inc.
IP Routing Convergence AfterLink Recovery
IP routing protocolsrebuild the IP routingtable.
The FIB and the LFIB arealso rebuilt, but the label
information might belackin .
Network LSR label
X local 25
C 47
E 75
LIB on B
Label Action Next hop
25 75 E
LFIB on B
Network Next hop Label
X E 75
FIB on B
Network Next-hop
X E
Routing table of B
C C
pop C
A B C D
E
Network X
MPLS Con ergence
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
37/92
MPLS v1.03-37 2001, Cisco Systems, Inc.
MPLS ConvergenceAfter a Link Recovery
Routing protocol convergence optimizesthe forwarding path after a link recovery.
The LIB might not contain the label from
the new next hop by the time the IPconvergence is complete.
End-to-end MPLS connectivity might be
intermittently broken after link recovery.Use MPLS traffic engineering for make-before-break recovery.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
38/92
MPLS v1.03-38 2001, Cisco Systems, Inc.
Summary
After completing this section, youshould be able to perform thefollowing tasks:
Identify TDP/LDP convergence issues.
Describe the interaction betweenrouting protocol convergence and LDP
convergence.
Describe the packet-mode MPLSconvergence after link failure and link
recovery.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
39/92
MPLS v1.03-39 2001, Cisco Systems, Inc.
Review Questions
What is the impact of LDP/TDPconvergence time on overall
convergence when a link is lost?
What is the impact of LDP/TDPconvergence time on overallconvergence when a link is restored?
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
40/92
Typical Label Distributionover LC-ATM Interfaces and
VC-Merge
2001, Cisco Systems, Inc. MPLS v1.03-40
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
41/92
MPLS v1.03-41 2001, Cisco Systems, Inc.
Objectives
Upon completion of this section, youwill be able to perform the followingtasks:
Describe ATM-related issues that dictateadditional needs for label-distributionprocedures overLC-ATM interfaces.
Describe TDP/LDP label distribution in cell-mode MPLS environments.
Identify the need for per-edge-LSR VCallocation over ATM network.
Describe the functions, benefits and
Issues of Cell Mode MPLS
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
42/92
MPLS v1.03-42 2001, Cisco Systems, Inc.
Issues of Cell-Mode MPLSEnvironments
An MPLS label is encoded as the virtualpath identifier/virtual channel identifier(VPI/VCI) value in cell-mode MPLSenvironments.
Each VPI/VCI combination represents avirtual circuit in ATM.
The number of virtual circuits supported
by router and switch hardware isseverely limited.
Conclusion: labels in cell-mode MPLSare a scarce resource.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
43/92
MPLS v1.03-43 2001, Cisco Systems, Inc.
Building the IP Routing Table
IP routing protocols are used to build IProuting tables on all LSRs.
The routing tables are built as if the ATMswitches were regular routers.
Network Next-hop
X C
Routing table of A
Network Next-hop
X C
Routing table of B
Network Next-hop
X D
Routing table of C
Network Next-hop
X E
Routing table of D
Network Next-hop
X conn
Routing table of E
Network X
A
E
B
C D
Building the IP Forwarding
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
44/92
MPLS v1.03-44 2001, Cisco Systems, Inc.
Building the IP ForwardingTable
Unlabeled IP packets cannot be propagatedacross LC-ATM interfaces.
Forwarding tables are not built until the labelsare assigned to destinations in IP routing
tables.
Network Next-hop
X C
Routing table of A
Network Next-hop
X C
Routing table of B
Network Next-hop
X D
Routing table of C
Network Next-hop
X E
Routing table of D
Network Next-hop
X conn
Routing table of E
Network X
A
E
B
C D
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
45/92
MPLS v1.03-45 2001, Cisco Systems, Inc.
Requesting a Label
Labels have to be explicitly requested over LC-ATM interfaces.
A router requests a label for every destinationin the routing table with the next hopreachable over an LC-ATM interface.
Network Next-hop
X C
Routing table of A
Network Next-hop
X D
Routing table of C
Network Next-hop
X E
Routing table of D
Network Next-hop
X conn
Routing table of E
Network X
A
E
An ATM switch can allocate an
incoming label only if it already
has a corresponding outgoing
label.
RQ X RQ X
A switch requests a
label from its next hop.
RQ X
C D
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
46/92
MPLS v1.03-46 2001, Cisco Systems, Inc.
Allocating a Label
Network Next-hop
X C
Routing table of A
Network Next-hop
X D
Routing table of C
Network Next-hop
X E
Routing table of D
Network Next-hop
X conn
Routing table of E
The egress ATM edge
LSR allocates a label
and replies to the
request.
An ATM LSR can allocate an incoming
label after receiving an outgoing label. It
replies with the allocated label to the
incoming request.
Label Action Next hop
1/56 2/82 D
LFIB on C
Label Action Next hop
2/82 1/37
LFIB on DThe LFIB is actuallythe ATM switching
matrix.
Network LSR label
X E 1/37
local 2/82
LIB on D
Network X
A
EC D
RQ X RQ X RQ X
X=1/37X=2/82X=1/56
Label Action Next hop
1/37 pop
LFIB on E
Network LSR label
X local 1/37
LIB on E
Processing Label
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
47/92
MPLS v1.03-47 2001, Cisco Systems, Inc.
Processing LabelAllocation Reply
Network Next-hop
X C
Routing table of A
The ingress ATM edge LSR requesting a label
inserts the received label in its LIB, FIB, and
(optionally) LFIB.
Network LSR label
X C 1/56
LIB on A
Network Next hop Label
X C 1/56
FIB on A
Network X
A
EC D
RQ X
X=1/56
Allocation Requests from
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
48/92
MPLS v1.03-48 2001, Cisco Systems, Inc.
A
E
B
C D
Network LSR label
X D 2/82
local 1/56local 1/43
LIB on C
Allocation Requests fromAdditional Upstream LSRs
Network Next-hop
X C
Routing table of A
Network Next-hop
X D
Routing table of C
Network Next-hop
X E
Routing table of D
Network Next-hop
X conn
Routing table of E
Every upstream LSR will
request a label for downstream
destinations from an ATM LSR.
The ATM LSR could reuse an
already allocated downstream
label for the second upstream
label.
Network Next-hop
X C
Routing table of B
X=1/43
RQ X
Label Action Next hop
1/56 2/82 D
1/43 2/82 D
LFIB on C
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
49/92
MPLS v1.03-49 2001, Cisco Systems, Inc.
A
E
B
C D
Cell Interleave Issue
If an ATM LSR reuses a
downstream label, cells from
several upstream LSRs might
become interleaved.
Label Action Next hop
1/56 2/82 D
1/43 2/82 D
LFIB on C
Network Next hop Label
X C 1/43
FIB on B
Network Next hop Label
X C 1/56
FIB on A
1/431/43
1/561/56 2/82 2/82
Solution #1allocate a separate downstream label for everyupstream request.
Solution #2prevent cell interleave by blocking incoming cellsuntil a whole frame is collected.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
50/92
MPLS v1.03-50 2001, Cisco Systems, Inc.
A
E
B
C D
Additional Label Allocation
Network Next-hop
X C
Routing table of A
Network Next-hop
X D
Routing table of C
Network Next-hop
X E
Routing table of D
Network Next-hop
X conn
Routing table of E
The ATM LSR requests a new
label from downstream LSRs
for every upstream request.
The ATM egress router has to
allocate a unique label for every
ATM ingress router for every
destination.
Network Next-hop
X C
Routing table of B
RQ X
X=1/43 X=1/38X=2/87
RQ X (2) RQ X (2)
Network LSR label
X local 1/37
local 1/38
LIB on E
Label Action Next hop
1/37 pop
1/38 pop
LFIB on E
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
51/92
MPLS v1.03-51 2001, Cisco Systems, Inc.
A
B
C DA
Virtual Circuit Merge
VC merge is a solution in which incoming cellsare blocked until the last cell in a framearrives.
All buffered cells are then forwarded to thenext-hop ATM LSR.
1/431/43
1/561/56 2/822/822/822/82
Label Action Next hop
1/56 2/82 D1/43 2/82 D
LFIB on C
Benefits and Drawbacks
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
52/92
MPLS v1.03-52 2001, Cisco Systems, Inc.
Benefits and Drawbacksof VC Merge
Benefits of VC mergeThe Merging ATM LSR can reuse thesame downstream label for multipleupstream LSRs.
Drawbacks of VC merge
Buffering requirements increase onthe ATM LSR.
Jitter and delay across the ATMnetwork increase.
The ATM network is effectivelytransformed into a frame-based
network.
Per-Interface Label
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
53/92
MPLS v1.03-53 2001, Cisco Systems, Inc.
Per-Interface LabelAllocation
The LFIB on an ATM switch (ATM switching
matrix) always contains the incoming interface. Labels have to be assigned for individual
interfacesper-interface label allocation.
The same label can be reused (with a differentmeaning) on different interfaces.
Incoming I/F VPI/VCI Outgoing I/F VPI/VCI
ATM 0/0 1/73 ATM 1/3 1/39
LFIB on C
Network X
RQ X
X=1/73
RQ X
X=1/69
The ATM edge
LSR has to
request a label
over every
interface.
A
E
C
Security of Per-Interface
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
54/92
MPLS v1.03-54 2001, Cisco Systems, Inc.
Network X
AE
C
Security of Per-InterfaceLabel Allocation
Incoming I/F VPI/VCI Outgoing I/F VPI/VCI
ATM 0/0 1/73 ATM 1/3 1/39
LFIB on C
1/73
Per-interface label allocation is securelabeledpackets (or ATM cells) are accepted only fromthe interface where the label was actually
assigned.
1/391/73
The packet or cell arriving
through the proper
interface is forwarded.
A labeled packet or cell coming through
a wrong interface is dropped.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
55/92
MPLS v1.03-55 2001, Cisco Systems, Inc.
Summary
After completing this section, youshould be able to perform thefollowing tasks:
Describe ATM-related issues that
dictate additional needs for label-distribution procedures over LC-ATMinterfaces.
Describe TDP/LDP label distribution incell-mode MPLS environments.
Identify the need for per-edge-LSR VCallocation over ATM network.
Describe the functions benefits and
i Q i
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
56/92
MPLS v1.03-56 2001, Cisco Systems, Inc.
Review Questions
What is ATM switching matrix called inMPLS terminology?
Why is it necessary to have the next-hop label before propagating the locallabel?
What are the benefits of VC-Merge?
What are the drawbacks of VC-Merge?
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
57/92
MPLS Label Allocation,Distribution and Retention
Modes
2001, Cisco Systems, Inc. MPLS v1.03-57
Obj i
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
58/92
MPLS v1.03-58 2001, Cisco Systems, Inc.
Objectives
Upon completion of this section, youwill be able to perform the followingtasks:
Describe the difference between per-
interface and per-platform label space.
Describe TDP/LDP unsolicited downstreamand downstream-on-demand labeldistribution.
Describe the difference between orderedand independent label allocation control.
Describe the difference betweenconservative and liberal retention mode.
Identify parameter sets used in Cisco IOS
Label Distribution
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
59/92
MPLS v1.03-59 2001, Cisco Systems, Inc.
Label DistributionParameters
MPLS architecture defines severallabel allocation and distributionparameters:
Per-interface or per-platform labelspace
Unsolicited downstream and
downstream-on-demand labeldistribution
Ordered or independent LSP tunnelcontrol
L b l S P I t f
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
60/92
MPLS v1.03-60 2001, Cisco Systems, Inc.
Network X
AE
C
Label Space: Per Interface
The LFIB on an LSR contains incoming aninterface.
Labels have to be assigned for individualinterfaces.
The same label can be reused (with a differentmeaning) on different interfaces.
Label allocation is secureLSRs cannot send
Incoming I/F VPI/VCI Outgoing I/F VPI/VCI
ATM 0/0 1/73 ATM 1/3 1/39
LFIB on C
L b l S P Pl tf
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
61/92
MPLS v1.03-61 2001, Cisco Systems, Inc.
Label Space: Per Platform
Label Action Next hop
25 75 D
LFIB on B
X = 25
X=25
The LFIB on an LSR does not contain an incominginterface.
The same label can be used on any interface and isannounced to all adjacent LSRs.
The label is announced to adjacent LSRs only onceand can be used on any link.
Per-platforms label space is less secure than per-interface label space.
X = 25
X=25 Network X
B
C
DA
Label Distribution:
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
62/92
MPLS v1.03-62 2001, Cisco Systems, Inc.
A B C D
E
Network X
Label Distribution:Unsolicited Downstream
The Label for a prefix is allocated andadvertised to all neighbor LSRs, regardless ofwhether the neighbors are upstream or
downstream LSRs for the destination.
Network LSR label
X local 25
LIB on B
X = 25X = 25
X=25
Label Distribution:
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
63/92
MPLS v1.03-63 2001, Cisco Systems, Inc.
Network X
A
EC D
Downstream-on-Demand
An LSR will assign a label to a prefix onlywhen asked for a label by an upstream LSR.
Label distribution is a hop-by-hop parameterdifferent label distribution mechanisms cancoexist in an MPLS network.
Network Next-hop
X C
Routing table of A
Network Next-hop
X D
Routing table of C
Network Next-hop
X E
Routing table of D
Network Next-hop
X conn
Routing table of E
RQ X
LSP Control: Independent
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
64/92
MPLS v1.03-64 2001, Cisco Systems, Inc.
Network X
A
EC D
LSP Control: IndependentControl
Network Next-hop
X C
Routing table of A
Network Next-hop
X D
Routing table of C
Network Next-hop
X E
Routing table of D
Network Next-hop
X conn
Routing table of E
RQ X
X=1/37
Label Action Next hop
1/37 pop
LFIB on E
An LSR can always assign a label for a prefix,even if it has no downstream label.
Independent control can be used only forLSRs with Layer 3 capabilities.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
65/92
MPLS v1.03-65 2001, Cisco Systems, Inc.
Network X
A
EC D
LSP Control: Ordered Control
Network Next-hop
X C
Routing table of A
Network Next-hop
X D
Routing table of C
Network Next-hop
X E
Routing table of D
Network Next-hop
X conn
Routing table of E
RQ X RQ X RQ X
X=1/37X=2/82X=1/56
Label Action Next hop
1/56 2/82 D
LFIB on C
Label Action Next hop
2/82 1/37
LFIB on D
Label Action Next hop
1/37 pop
LFIB on E
An LSR can assign a label only if it has alreadyreceived a label from the next-hop LSR;otherwise it must request a label from the next-hop LSR.
Label Retention: Liberal
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
66/92
MPLS v1.03-66 2001, Cisco Systems, Inc.
Label Retention: LiberalRetention Mode
Every LSR stores the received label in its LIB,even when the label is not received from anext-hop LSR.
Liberal label retention mode improves
X = 25X = 25
Network LSR label
X B 25
LIB on A
Network LSR label
X B 25
LIB on C
Network LSR label
X B 25
LIB on E
A B C D
E
X=25Network X
Label Retention:
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
67/92
MPLS v1.03-67 2001, Cisco Systems, Inc.
Conservative Retention Mode
An LSR stores only the labels received fromnext-hop LSRs; all other labels are ignored.
Downstream-on-demand distribution isrequired during the convergence phase.
X = 25X = 25
Network LSR label
X B 25
LIB on A
Network LSR label
X B 25
LIB on C
A B C D
E
Network X
X=25
Cisco IOS Platform MPLS
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
68/92
MPLS v1.03-68 2001, Cisco Systems, Inc.
Cisco IOS Platform MPLSImplementation
Routers with packet interfaces: Per-platform label space, unsolicited
distribution, liberal label retention,independent control
Routers with ATM interfaces: Per-interface label space, on-demand
distribution, conservative or liberallabel retention, independent control
ATM switches:
Per-interface label space, on-demanddistribution, conservative labelretention, ordered control
Summary
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
69/92
MPLS v1.03-69 2001, Cisco Systems, Inc.
Summary
After completing this section, youshould be able to perform the followingtasks:
Describe the difference between per-
interface and per-platform label space. Describe TDP/LDP unsolicited downstream
and downstream-on-demand labeldistribution.
Describe the difference between orderedand independent label allocation control.
Describe the difference betweenconservative and liberal retention mode.
Identify parameter sets used in Cisco IOS
Review Questions
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
70/92
MPLS v1.03-70 2001, Cisco Systems, Inc.
Review Questions
Why and where is downstream-on-demand label propagation used?
What are the benefits and drawbacks
of liberal retention mode?
What are the benefits and drawbacksof per-platform label space?
Why is per-interface label space onATM LSRs needed?
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
71/92
LDP NeighborDiscovery
2001, Cisco Systems, Inc. MPLS v1.03-71
Objectives
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
72/92
MPLS v1.03-72 2001, Cisco Systems, Inc.
Objectives
Upon completion of this section,you will be able to perform thefollowing tasks:
Describe LDP/TDP neighbor discovery.
Describe LDP/TDP sessionestablishment process.
LDP Session Establishment
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
73/92
MPLS v1.03-73 2001, Cisco Systems, Inc.
LDP Session Establishment
LDP and TDP use a similar process to establisha session:
Hello messages are periodically sent on allinterfaces enabled for MPLS.
If there is another router on that interface, it
will respond by trying to establish a sessionwith the source of the hello messages.
User Datagram Protocol (UDP) is used for hellomessages. It is targeted at all routers on thissubnet multicast address (224.0.0.2).
Transmission Control Protocol (TCP) is used toestablish the session.
Both TCP and UDP use well-known LDP portnumber 646 (711 for TDP).
LDP Hello Message
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
74/92
MPLS v1.03-74 2001, Cisco Systems, Inc.
LDP Hello Message
Hello messages are targeted at all routers reachablethrough an interface.
LDP uses well-known UDP and TCP port number 646. The source address used for an LDP session can be set
by adding the transport address TLV to the hellomessage.
A 6-byte LDP identifier TLV identifies the router (first
four bytes) and label space (last two bytes).
Source port=1050
Destination port=646
Source address=1.0.0.1
Destination address=224.0.0.2
IP Header UDP Header LDP Hello Message
Well-know multicast
IP addressidentifying all
routers on the
subnet.
Well-know portnumber used for
LDP.
Transport address=1.0.0.1
Optional TLV
used to identifythe source IP
address for LDP
session.
LDP ID=1.0.0.1:0
6-byte TLV
identifying therouter and
label space.
Label Space
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
75/92
MPLS v1.03-75 2001, Cisco Systems, Inc.
Label Space
LSRs establish one LDP session perlabel space.
Per-platform label space requires onlyone LDP session, even if there are
multiple parallel links between a pair ofLSRs.
Per-platform label space is announcedby setting the label space ID to zero
(for example, LDP ID=1.0.0.1:0).
A combination of frame-mode and cell-mode, or multiple cell-mode links,results in multiple LDP sessions.
Label Space Negotiation
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
76/92
MPLS v1.03-76 2001, Cisco Systems, Inc.
Example
One LDP session is established for each announced LDPidentifier (router ID + label space).
The number of LDP sessions is determined by thenumber of different label spaces.
The bottom right example is not common, since ATMLSRs do not use Ethernet for packet forwarding, andframe-mode MPLS across ATM uses per-platform label
space.
ATMATM
ATM
1.0.0.1:10
1.0.0.1:20
1.0.0.1:0
1.0.0.1:0
1.0.0.1:0
1.0.0.1:0
1.0.0.1:10
1.0.0.1:0
LDP Neighbor Discovery
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
77/92
MPLS v1.03-77 2001, Cisco Systems, Inc.
LDP Neighbor Discovery
1.0.0.1 1.0.0.3
1.0.0.4
MPLS_D
1.0.0.2
TCP(1.0.0.2:104
3 1.0.0.1:646)
UDP: Hello(1.0.0.1:1050 224.0.0.2:646)
UDP: Hello(1.0.0.4:1033 224.0.0.2:646)
UDP: Hello(1.0.0.2:1064 224.0.0.2:646)
UDP: Hello
(1.0.0.1:1051 224.0.0.2:646)
UDP: Hello
(1.0.0.4:1034 224.0.0.2:646)
UDP: Hello
(1.0.0.2:1065 224.0.0.2:646)
UDP: Hello
(1.0.0.1:1052 224.0.0.2:646)
UDP: Hello
(1.0.0.4:1035 224.0.0.2:646)
UDP: Hello
(1.0.0.2:1066 224.0.0.2:646)
TCP(1.0.0.4:1065 1.0.0.1:646)
TCP
(1.0.0.4:1066
1.0.0.2:646)
An LDP session is established from the routerwith the higher IP address.
MPLS_B
MPLS_A NO_MPLS_C
LDP Session Negotiation
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
78/92
MPLS v1.03-78 2001, Cisco Systems, Inc.
LDP Session Negotiation
Peers first exchange initializationmessages.
The session is ready to exchange labelmappings after receiving the first
keepalive.
1.0.0.1 1.0.0.2Initialization message
Establish TCP session
Initialization message
Keepalive
Keepalive
MPLS_AMPLS_B
LDP Sessions
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
79/92
MPLS v1.03-79 2001, Cisco Systems, Inc.
Between ATM LSRs
An IP adjacency between ATM LSRs isestablished through the control virtual circuit(0/32).
The control virtual circuit is used for LDP or TDPas well as for IP routing protocols.
Virtual Switch Interface (VSI) protocol is used topopulate the ATM switching matrix (LFIB) in thedata plane of some ATM switches (Cisco-specificimplementation).
OSPF
OSPF OSPF
OSPF
LDP
LDP LDP
LDP
LFIBLFIB LFIB
LFIB
VSI VSI
0/32 0/320/32
LDP or TDP Discoveryf N dj t N i hb
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
80/92
MPLS v1.03-80 2001, Cisco Systems, Inc.
of Nonadjacent Neighbors
LDP or TDP neighbor discovery ofnonadjacent neighbors differs from normaldiscovery only in the addressing of hellopackets.
Hello packets use unicast IP addressesinstead of multicast addresses.
Once a neighbor is discovered, themechanism to establish a session is thesame.
Summary
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
81/92
MPLS v1.03-81 2001, Cisco Systems, Inc.
Summary
After completing this section, youshould be able to perform the
following tasks:
Describe LDP/TDP neighbor discovery.
Describe the LDP/TDP session
establishment process.
Review Questions
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
82/92
MPLS v1.03-82 2001, Cisco Systems, Inc.
Review Questions
How do routers find LDP/TDP peersreachable through an interface?
Which protocol is used to discover
neighboring LSRs?Which protocol is used for the sessionitself?
How do ATM LSRs establish an LDP/TDPsession?
What is different in an LDP sessionestablishment between non-adjacent peers?
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
83/92
Penultimate HopPopping
2001, Cisco Systems, Inc. MPLS v1.03-83
Objectives
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
84/92
MPLS v1.03-84 2001, Cisco Systems, Inc.
Objectives
Upon completion of this section, youwill be able to perform the followingtasks:
Describe penultimate hop popping.
Describe how LSRs request PHP throughTDP/LDP.
Identify when the PHP could be used in
MPLS network.
Describe the benefits and drawbacks ofPHP.
Double Lookup Scenario
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
85/92
MPLS v1.03-85 2001, Cisco Systems, Inc.
MPLS Domain
Double Lookup Scenario
Double lookup is not anoptimal way of forwardinglabeled packets.
A label can be removed one
hop earlier.
10.0.0.0/8
L=19
10.0.0.0/8
L=18
10.0.0.0/8
L=17
LFIB
18 19
FIB10/8 NH, 19
LFIB
17 18
FIB10/8 NH, 18
LFIB
35 17
FIB10/8 NH, 17
LFIB
19 untagged
FIB10/8 NH
10.1.1.117
10.1.1.118
10.1.1.119
10.1.1.1
Double lookup is
needed:
1. LFIB: remove thelabel.
2. FIB: forward the IP
packet based on IP
next-hop address.
10.0.0.0/8
Penultimate Hop Popping
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
86/92
MPLS v1.03-86 2001, Cisco Systems, Inc.
Penultimate Hop Popping
MPLS Domain
A label is removed on the routerbefore the last hop within an MPLSdomain.
10.0.0.0/8
L=pop
10.0.0.0/8
L=18
10.0.0.0/8
L=17
LFIB
18 pop
FIB10/8 NH, 19
LFIB
17 18
FIB10/8 NH, 18
LFIB
35 17
FIB10/8 NH, 17
LFIB
FIB10/8 NH
10.1.1.117
10.1.1.118
10.1.1.1
10.1.1.1
10.0.0.0/8
Pop or implicit
null label is
advertised.
One single lookup.
Penultimate Hop Popping
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
87/92
MPLS v1.03-87 2001, Cisco Systems, Inc.
Penultimate Hop Popping
Penultimate hop popping optimizesMPLS performance (one less LFIB
lookup).PHP does not work on ATM(VPI/VCI cannot be removed).
The pop or implicit null label usesvalue 3 when being advertised to aneighbor.
Objectives
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
88/92
MPLS v1.03-88 2001, Cisco Systems, Inc.
Objectives
After completing this section, youshould be able to perform thefollowing tasks:
Describe penultimate hop popping.Describe how LSRs request PHPthrough TDP/LDP.
Identify when the PHP could be used inMPLS network.
Describe the benefits and drawbacks ofPHP.
Review Questions
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
89/92
MPLS v1.03-89 2001, Cisco Systems, Inc.
Review Questions
What is the main benefit of
penultimate hop popping?How does a router know that it has topop the label?
Summary
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
90/92
MPLS v1.03-90 2001, Cisco Systems, Inc.
Summary
After completing this chapter, you should beable to perform the following tasks:
Describe the need for LDP/TDP in MPLS network.
Describe the LDP/TDP neighbor discovery and
session establishment procedures. Explain the needs for different LDP/TDP label
distribution modes.
Explain the difference between independent andordered control.
Describe LDP/TDP retention modes(conservative and liberal).
Explain the functions and benefits of penultimate-hop-popping.
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
91/92
2000, Cisco Systems, Inc. www.cisco.co
Blank for Pagination
-
8/14/2019 MPLS10S03-Label Assignment and Distribution
92/92
g