Motivation of MPLS Label Distribution Protocol
Lab: Setup and Overview
LDP concept, configuration and Troubleshooting
Lab: MPLS basic configuration

Agenda
IP Routing /Forwarding Limitations Limitations of IP over ATM Limitations of IP based Traffic Engineering MPLS concepts MPLS vs IP over ATM MPLS-TE vs IP-TE MPLS Benefits

IP Routing/Forwarding Limitations
IP Routing is based on: Routing protocols, which are used to distribute Layer 3 routing information
Forwarding is based on the Layer 3 destination address
Routing lookups are performed on every hop
IP Limitations (contd)
Destination-based routing lookup is needed on every hop
IP Limitations: IP over ATM
Layer 2 devices have no knowledge of Layer 3 routing informationvirtual circuits must be manually established
Layer 2 topology may be different from Layer 3 topology, resulting in suboptimal paths
Even if the two topologies overlap, the hub-and-spoke topology is usually used because of easier management
IP Limitations: Traffic Engineering
Based on Routing Protocol forwarding, all traffic between Site A and Site B takes theOC-192 link, even if this link is congested The alternate A-C-B link may be underutilized
Destination-based routing does not provide any mechanism for unequal cost balancing(except for variance in EIGRP)
Policy-based routing can be used to forward packets based on other parameters, but thisis not a scalable solution
How MPLS helps
MPLS (Multi Protocol Label Forwarding) uses Labels toforward packets
Labels usually correspond to IP destination networks (equal to traditional IP forwarding)
Labels can also correspond to other parameters, such as ATM VC (in case of ATM over MPLS), VLAN id (in case of Ethernet over MPLS) or even QoS parameters
Now packet forwarding is no longer strictly tied to IPdestination address
MPLS is an IETF standard based on RFC 3031, 3032
How MPLS helps (contd)
Labels assigned to packets can be based on:

Destination prefix

Traffic Engineering tunnel

VPN-ID, ATM VC, VLAN ID

Class of Service
Basic MPLS Concepts:
Label based forwarding
Basic MPLS Concepts:
Label based forwarding (contd)
Only edge routers perform a routing (Layer 3) lookup
Basic MPLS Concepts:
Label based forwarding (contd)

Core routers forward packets based on MPLS label lookups

Core router can be any device capable of doing label forwarding, so we might as well use a switch, if needed
MPLS versus IP over ATM
Layer 2 devices are IP-aware and run a routing protocol
MPLS Versus IP over ATM (contd)
There is no need to manually establish virtual circuits
MPLS Versus IP over ATM

Layer 2 devices are IP-aware and run a routing protocol

There is no need to manually establish virtual circuits

MPLS provides a virtual full-mesh topology
Traffic Engineering with MPLS
Traffic can be forwarded based on labels via the primary and secondary links Load sharing across unequal paths can be achieved We shall cover MPLS Traffic Engineering in detail in later modules
MPLS Benefits

Separates Control plane & the forwarding plane

Only ingress router needs to look up the network layer & make routing decision Other LSRs only swap labels

Source Based routing: eg explicit routes in MPLS-TE

Scalability: Hierarchy of Routing (via label stacking)

AnyThing over MPLS (AToM): Labels are common binding between different Layer 2 technologies like ATM,Ethernet Summary
Forwarding based on IP Routing only requires a layer 3 lookup at each router
MPLS forwards packets based on labels
MPLS separates the control plane and forwarding plane

Agenda
MPLS Architecture MPLS Terminology MPLS Labels Label Switch Routers LFIB and outgoing labels MPLS and BGP Label Distribution Protocols Summary
MPLS Architecture
MPLS has two major components:
Control plane: Exchanges Layer 3 routing information and labels
Data plane: Forwards packets based on labels
Control plane contains complex mechanisms toexchange routing information, such as OSPF, EIGRP,IS-IS, and BGP, and to exchange labels, such as TDP,LDP, BGP, and RSVP
Data plane forwards packets based on CEF (LFIB)
MPLS Architecture
Population of RIB/FIB/LIB/LFIB in an MPLS router
MPLS Architecture (Cont)
Control plane and Data plane in action

FEC ( Forwarding Equivalence Class)
Group of IP packets forwarded in the samemanner (eg over same forwarding path)

A FEC can represent a: Destination IP prefix, VPNID, ATM VC, VLAN ID, Traffic Engineering tunnel, Class of Service
MPLS Terminology:Label Switch Path (LSP)

LSPs are derived from IGP routing information  
LSPs may diverge from IGP shortest path LSP tunnels (explicit routing) with TE  
LSPs are unidirectional
Return traffic takes another LSP
MPLS terminology:LSR, LER

Label Edge Router

LSR (Label Switch Router) is any netwrok router/switchrunning MPLS label switching

LER (Label Edge Router) is an edge LSR Also referred to as PE (Provider Edge) router
MPLS terminology:
Upstream and Downstream LSRs
Rtr-C is the downstream neighbor of Rtr-B for destination1716810/24
Rtr-B is the downstream neighbor of Rtr-A for destination1716810/24
LSRs know their downstream neighbors through the IProuting protocol
Next-hop address is the downstream neighbor
MPLS Labels
MPLS uses a 32-bit label field that is inserted between Layer 2 and Layer 3 headers (frame-mode)
MPLS with ATM uses the VPI, VCI fields of the ATM header as the label (cell-mode)
MPLS Labels:
Label Format (Shim header)
Label = 20 bits COS/EXP = Class of Service, 3 bits S = Bottom of Stack, 1 bit TTL = Time to Live (Loop detection)
MPLS Labels: Special Label values
SPECIAL LABEL VALUES  0  IPv4 Explicit Null  1  Router Alert  2  IPv6 Explicit Null  3  Implicit Null
MPLS Labels:
Frame Mode Label Encapsulation
PPP Header (Packet over SONET/SDH)
LAN MAC Label Header

Shim header is used with Ethernet, 8023, or PPP frames  
Sits between the Layer 2 and Layer 3 header

L2 frame has
ethertype=0x8847 to indicate frame carrying MPLSunicast packet
ethertype=0x8848 to indicate frame carrying MPLSunicast packet
MPLS Labels: Cell mode Label Encapsulation

ATM switches forward cells, not packets  In case of label stack: First level label could be in VPI  
Second level label could be in VCI
MPLS Labels: Label Assignment Labels have local significance Each LSR binds his own label mappings

LSR assigns labels to prefixes learnt in therouting table < Label, prefix, prefix mask > are exchangedbetween adjacent LSRs Label Stack

An MPLS packet may have more than one label

Frame Mode can handle a stack of two or more labels, depending on the platform

Bottom most label has the S-bit set to 1

ATM cells can have a stack of labels in the VPI, VCI fields

LSRs label switch packets based ONLY on the labelat the top of the stack
Label Stack (Cont)
1716810/24
Label = 5  
Label = 21  
IP packet D=171681012  
Label = 7  
Label = 21  
IP packet D=171681012  
Rtr-A forwards the labelled packet based on the label at the top of thelabel stack
MPLS Labels:
Label Stack (Cont)
The following scenarios may produce more than one label:
MPLS VPNs (two labels: The top label points to the egress router and the second label identifies the VPN)
MPLS TE with Fast Reroute (two or more labels: The top label is for the backup tunnel and the second label points to the primary tunnel destination)
MPLS VPNs combined with MPLS TE (three or more
labels)
MPLS Labels:
Label Distribution modes
Unsolicited
Downstream LSR advertises Label Binding to all adjacent LSRs, irrespective of whether they demand the Label binding or not Example: LDP, MP-iBGP
On-demand
Downstream LSR advertises Label Binding to those adjacent LSRs, who demand the Label binding Example: RSVP-TE, ATM
MPLS Labels: Unsolicited example
Rtr-A Rtr-B Rtr-C
LSRs distribute labels to the upstream neighbors
MPLS Labels:
Downstream on-demand example
Use label 40 for destination
1716810/24
Use label 30 for destination
1716810/24
Rtr-A Rtr-B Rtr-C
Request label for Request label for destination 1716810/24 destination 1716810/24

Upstream LSRs request labels to downstream neighbors

Downstream LSRs distribute labels upon request
MPLS Labels:
Control modes
Independent LSP control
LSR binds a Label to a FEC independently, whether or not the LSR has received a Label the next-hop for the FEC The LSR then advertises the Label to its neighbor Example: LDP
Ordered LSP control
LSR only binds and advertise a label for a particular FEC if:
it is the egress LSR for that FEC or
it has already received a label binding from its next-hop
Example: RSVP-TE
MPLS Labels: Retention modes

Liberal retention

LSR retains labels from all neighbors
In case, the next-hop LSR disappears, LSR already has the OutLabel for the next best next-hops Quick convergence Requires more memory and label space Example: LDP

Conservative retention

LSR retains labels only from next-hops neighbors LSR discards all labels for FECs which are not routing next-hops Free memory and label space Example: ATM cell mode
MPLS Labels: Penultimate Hop Popping
The label at the top of the stack is removed (popped) by the upstream neighbor of the egress LSR  The egress LSR requests the “popping” through the label distribution protocol Egress LSR advertises implicit-null label  One lookup is saved in the egress LSR
MPLS Labels:
Penultimate Hop Popping Example
Egress LSR needs to do an IP lookup for finding more specific route Egress LSR need NOT to receive a labelled packet
labelled will have to be popped anyway
MPLS Labels:
Penultimate Hop Popping Example (contd)
Label Switch Routers:
Architecture of LSRs
LSRs, regardless of the type, perform these functions: Exchange routing information Exchange labels Forward packets or cells
The first two functions are part of the controlplane
The last function is part of the data plane
Label Switch Routers: Exchanging Routing updates Label Switch Routers: Exchanging and Assigning Labels
…

In Label is the local label generated by the LSR

Out Label is the remote label advertised by the adjacent LSR,
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which is the IGP next hop
Label Switch Routers: Forwarding Packets
0 12889
Label imposition Label swapping
Label Popping
of 4 4->9
Label Switch Routers: Label functions
An LSR can perform the following functions: Insert (impose) a label or a stack of labels on ingress Swap a label with a next-hop label or a stack of labels in
the core Remove (pop) a label on egress ATM LSRs can swap a label with only one label (VPI/VCI fields change)
Label Switch Routers: Cell Mode
ATM LSR can forward only cells
ATM edge LSR segments packets into cells and forwards them into an MPLS ATM domain, or reassembles cells into packets and forwards them out of an MPLS ATM domain
MPLS Forwarding Plane:
LFIB and Outgoing Labels

I Pop
II Untagged -Untag the incoming MPLS packet
III Aggregate - Untag and then do a FIB lookup
IV 0

MPLS Forwarding Plane:Outgoing label types
MPLS Forwarding Plane:Outgoing label types (cont)
Untagged Convert the incoming MPLS packet to an IP packet and forward it  Pop Pop the top label from the label stack present in an incoming MPLS packet and forward it as an MPLS packet If there was only one label in the stack, then forward it as an IP packetSAME as imp-null label  Aggregate Convert the incoming MPLS packet to an IP packet and then do a FIB lookup for it to find out the outgoing interface  0 (zero) Same as exp-null label Simplify fills 0 in the first 20 bits oflabel; helps to preserve the EXP value of the top label
MPLS and BGP

Labels are assigned to FECs which are derived from IProuting protocols (IGP)

Labels are NOT assigned to BGP routes

BGP routes use recursive routing to find next-hopreachability

Labels are assigned to BGP next-hops

This saves CPU/Memory, label space and stability on
core LSRs
Core LSRs are preserved from BGP instability
We can assign labels to BGP learnt routes based on
RFC 3107
MPLS and BGP (cont)
Ingress LSR receives IP packet
Destination is given by BGP
BGP has next-hop known in the IGP
Label is available for BGP next-hop, through IGP route
Packet will traverse the core using IGP (BGP next-hop) label
Label Distribution Protocols
Several protocols for label exchange

TDP/LDP

RSVP

BGP
Summary
MPLS LSRs have separate control planes and forwarding
planes
Labels can be in Shim header or as part of ATM header Labels have adevrtisement modes, retention modes & control modes
All LSRs perform three functions:
Exchange routing information
Exchange labels
Forward packets or cells (depending on type) based on
labels
There are several label distribution protocols
Agenda
LDP Concepts LDP Identifier LDP PDU LDP Messages LDP Session Establishment LDP Sessions between ATM LSRs Targeted LDP sessions Summary
LDP Concepts

Label Distribution Protocol

LDP works between adjacent/non-adjacent peers

LDP sessions are established between peers

LDP messages sent in the form of TLVs <Type, Length, Value>

Standardized via RFC 3036
TDP/LDP Transport
Uses TCP for reliable transport  Well-known TCP port LDP (port 646) TDP (port 711)  LSR with higher LDP router-id opens aconnection to port 646 of other LSR  Design Choice: One TDP/LDP session per TCP connection
LDP Identifier
Identifies tag space
6 bytes (4 bytes =>IP address, 2 bytes =>Label space ID)
LDP Identifier: Label Space
LSRs establish one LDP session per label space
Per-platform label space requires only one LDP session,even if there are multiple parallel links between a pair of LSRs
Per-platform label space is announced by setting thelabel space ID to 0, for example:
LDP ID = 1001:0
A combination of frame-mode and cell-mode MPLS, or multiple cell-mode links, results in multiple LDPsessions
Label Space and number of LDPsessions
Label Space and number of LDPsessions (Cont)
Label Space and number of LDPsessions (Cont)
Label Space and number of LDPsessions (Cont)
One LDP session is established for each announced LDP identifier (router ID + label space)
The number of LDP sessions is determined by the number of different label spaces
The bottom right example is not common, because ATM LSRs do not use Ethernet for packet forwarding, and frame-mode MPLS across ATM uses per-platform label space
LDP Protocol Data Units


Version => LDP version Current LDP version is 1

PDU Length (excludes Version and PDU Length fields) => total length of PDU in bytes

LDP Identifier => discussed earlier

LDP Messages => one or more LDP messages
LDP Message
Message
header
LDP Message TLVs
U
Value
U bit is the Unknown TLV bit If the received TLV is of unknown type, then if: U=0, send Notification Message to the originator of this message and ignore the entire message U=1, silently ignore the unknown TLV and process the rest of the message
F bit is the Forward unknown TLV bit F bit is only applicable when the U=1 F=0,the unknown TLV is not forwarded with its LDP message F=1,the unknown TLV is forwarded with its LDP message

DISCOVERY messages

ADJACENCY messages deal with initialization,keepalive & shutdown of sessions

LABEL ADVERTISEMENT messages deal with labelbinding, requests, withdrawal & release

NOTIFICATION messages provide advisory information & signal errors

Used to discover and maintain the presence of newpeers using HELLO messages

Hello packets (UDP) sent to all-routers multicastaddress (224002)

Direct unicast hello is sent to non-adjacent neighbors

Once session is established, HELLO messages serveas link integrity messages

Session is bi-directional
INITIALIZATION
Two LSRs negotiate on various parameters & options
These include keepalive timer values, Label ranges, Unsolicited vs On-demand label advertisement, Ordered vs Independent mode, Liberal vsConservative Label retention
KEEPALIVE
LDP message that indicates that neighbor is alive
LABEL RELEASE
An LSR releases a Label Binding that it previously got from it’s LDP peer Used in Conservative Label Retention mode
LABEL REQUEST
Used by an upstream LSR to request a Label binding for a prefix from the downstream LDP peer Used in downstream on-demand mode
LABEL ABORT REQUEST
Send to abort the LABEL REQUEST message
LABEL MAPPING
Are the TLV object containing <Label, prefix> information
LABEL WITHDRAWAL
Used to revoke a previously advertised label binding
NOTIFICATION
Used for Error Notification and Advisory
LDP Session Establishment
LDP establishes a session by performing the following:
Hello messages are periodically sent on all interfaces that are enabled for MPLS
If there is another router connected to that interface, that it also has MPLS enabled, it will respond by trying to establish a session with the source of the hello messages
UDP is used for hello messages It is targeted at “all
routers on this subnet” multicast address (224002)
TCP is used to establish the session
Both TCP and UDP use well-known LDP port number 646 (711 for TDP)
LDP Hello Message
Hello messages are targeted at all routers reachable through 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 hello message
A 6-byte LDP identifier (TLV) identifies the router (first four bytes) and label space (last two bytes)
LDP Neighbor Discovery
LDP Neighbor Discovery
LDP Session: Transport Connection
Once LDP peers receive hellos, they establish a TCPconnection
Peer with higher LDP router-id is active LSR and thepeer with lower LDP router-id is the passive LSR
Active LSR tries to open a TCP connection to the well-known LDP port number 646 of the passive LSR, whilethe passive LSR waits for the active LSR to initiate theconnection
LDP Session: Session Initialization
Active LDP peer (1002) sends Initialization message to passive LDP peer Initialization message contains important parameters: Session keepalive time (default=180 sec) Label distribution method: Downstream unsolicited Max PDU length Receiver’s LDP Identifier Whether Loop Detection is enabled Some optional parameters
LDP Session:
Session Initialization (cont)

Passive LDP peer sends Initialization message and/or keepalive message to active LDP peer if Initialization message parameters are acceptable

Passive LDP peer could also send Error Notification & close the LDP connection if something was unacceptable
LDP Session:
Session Initialization (cont)
Active LDP peer sends keepalive to passive LDP peer & the LDP session is up
The session is ready to exchange label mappings after receiving the first keepalive
LDP Sessions Between ATM LSRs
An IP adjacency between ATM LSRs is established through the control virtual circuit (0/32)
The control virtual circuit is used for LDP as well as for IP routing protocols
VSI protocol is used to populate the ATM switching matrix (LFIB) in the data plane of some ATM switches (Cisco implementation)
Targeted LDP Sessions
LDP neighbor discovery of nonadjacent neighbors differs from normal discovery only in the addressing of hello packets:
Hello packets use unicast IP addresses instead of
multicast addresses
When a neighbor is discovered, the mechanism to establish a session is the same
Summary
TCP is used to establish LDP sessions between neighbors
LDP uses PDUs to carry messages
LDP hello messages contain an identifier field that uniquely
identifies the neighbor and the label space
Per-platform label space requires only one LDP session
Routers that have the higher IP address must initiate the TCP
session
LDP session negotiation is a three-step process
LDP sessions between ATM LSRs use the control VPI/VCI, which
by default is 0/32
Nonadjacent neighbor discovery is accomplished by using unicast
IP addresses instead of multicast
Agenda
Configuration  Verifying Your Configuration  Monitoring LDP
Configuring MPLS
Network Topology
7200a  7200a 4444 (loop)  CE1 100200  12008a 10030  12008b  7200b  CE3  
CE1  100200  100100100100(lo0 )  
12008a  10030  5555 (lo0)  10040 (2/0) 10050 (3/0)  
12008b  10050 (3/0) 10040 (2/0)  11111111(lo0)  100170  
7200b  100170  12121212 (lo0)  100220  
CE3  100220  30303030(lo0)  
Configuring LDP
Global
ip cef <distributed>
mpls label protocol <ldp | tdp | both>
tag-switching tdp router-id Loopback0
mpls ldp explicit-null (optional)
no mpls ip propagate-ttl (optional)
Interface
mpls ip or tag-switching ip (enables this interface for MPLS
forwarding)
mpls label protocol ldp
(optional, if you want to run LDP on this interface only, while other
interfaces don’t run LDP or run another label protocol such as TDP)
Configuring Conditional LabelDistribution


Conditional label advertisement only works over frame-mode interfaces  Parameters:

Net-ACL  the IP ACL that selects the destinations for which the labels will be generated

TDP-ACL  the IP ACL that selects the TDP neighbors that willreceive the labels
Conditional Label Distribution Example
The customer is already running IP infrastructure
MPLS is only needed to support MPLS/VPN services

Labels should only be generated for loopback interfaces (BGP next-hops) of all routers

All loopback interfaces are in one contiguous address block (1921682540/24)
Router Configuration
Enable conditional label advertisment
Agenda
Configuration  Verifying Your Configuration  Monitoring LDP
Verifying your configuration
hostname mpls-7200a
!
ip cef
mpls label protocol ldp
tag-switching tdp router-id Loopback0
!
interface Ethernet3/0
tag-switching ip
Agenda
Configuration  Verifying Your Configuration  Monitoring LDP
Monitoring LDP
show mpls interface <x> detail  show mpls ldp discovery  show mpls ldp neighbor  show mpls ip/ldp binding <prefix> <prefix­
length>  show mpls forwarding-table <prefix>
<prefix-length>
sh ip cef <prefix>
show mpls ldp parameters
Show mpls interface
mpls-7200a#sh mpls interface Interface IP Tunnel Operational Ethernet3/0 Yes (ldp) No Yes
mpls-7200a#sh mpls interface ethernet3/0 detail Interface Ethernet3/0:
IP labeling enabled (ldp)
……<snip>……
Fast Switching Vectors:
IP to MPLS Fast Switching Vector
MPLS Turbo Vector
MTU = 1500
Show mpls interface (contd)
“sh mpls interface [detail]”
Lists whether MPLS is enabled and the application that enabled MPLS on the interface
MPLS Enabled
! interface Serial2/0 description To P1 ser2/0
LDP Enabled
ip address 101326/30
mpls label protocol ldp
tag-switching ip
tag-switching mtu 1508 !
MPLS MTU
Show mpls interface (contd)

MPLS is Operational LDP not enabled
LDP not enabled BGP+Label Enabled
MPLS MTU
LDP discovery/adjacency:
commands and debugs
show mpls ldp discovery  debug mpls ldp transport  debug mpls ldp session io
LDP discovery
“debug mpls ldp transport events”

LDP adjacency debugs
LDP discovery, connection setup and shutdown events
mpls-7200a#debug mpls ldp transport events
debugging for LDP discovery and connection setup / shutdown events
2d11h: ldp: Send ldp hello; Ethernet3/0, src/dst 10034/224002, inst_id 0
2d11h: ldp: Rcvd ldp hello; Ethernet3/0, from 10035 (5555:0), intf_id 0, opt
0xC
shutting neighbor
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2d11h: %CLNS-5-ADJCHANGE: ISIS: Adjacency to mpls-12008a (Ethernet3/0) Down, hold
time expired
2d11h: ldp:Discovery hold timer expired for adj 0x17D45A0, 5555:0,will close conn
2d11h: ldp: Discovery hold timer expired for adj 0x17D45A0; 5555:0
2d11h: ldp: adj_addr/adj_xport_addr: 10035/5555
2d11h: ldp: LDP ptcl SM; close xport request for adj 0x0
2d11h: ldp: Close LDP transport conn for adj 0x17D45A0
2d11h: ldp: Closing ldp conn 4444:646 <-> 5555:11012, adj 0x17D45A0
2d11h: ldp: Adj 0x17D45A0; state set to closed
2d11h: ldp: Send ldp hello; Ethernet3/0, src/dst 10034/224002, inst_id 0
LDP session i/o debug
LDP session I/O, excluding periodic Keep Alives
mpls-7200a#debug mpls ldp session io <all>
bringing neighbor down
2d11h: %CLNS-5-ADJCHANGE: ISIS: Adjacency to mpls-12008a (Ethernet3/0) Down, hold
time expired
2d11h: ldp: Sent notif msg to 5555:0 (pp 0x17A0870)

bringing neighbor up
2d11h: %CLNS-5-ADJCHANGE: ISIS: Adjacency to mpls-12008a (Ethernet3/0) Up, new
adjacency
2d11h: ldp: Rcvd init msg from 5555 (pp 0x0)
2d11h: ldp: Sent init msg to 5555:0 (pp 0x0)
2d11h: ldp: Sent keepalive msg to 5555:0 (pp 0x0)
2d11h: ldp: Rcvd keepalive msg from 5555:0 (pp 0x0)
2d11h: ldp: Sent address msg to 5555:0 (pp 0x186CB38)
2d11h: ldp: Sent label mapping msg to 5555:0 (pp 0x186CB38)
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LDP neighbor
mpls-7200a#sh mpls ldp neighbor
Peer LDP Ident: 5555:0; Local LDP Ident 4444:0
TCP connection: 555511000 - 4444646
State: Oper; Msgs sent/rcvd: 268/264; Downstream Up time: 03:41:45
LDP discovery sources:
Ethernet3/0, Src IP addr: 10035
Addresses bound to peer LDP Ident:
10035 10045 10055 5555
LDP neighbor (contd)


LDP binding commands
“sh mpls ip binding detail” Lists all prefixes with labels & LDP neighbors  “sh mpls ip binding <prefix> <mask> det” Lists ACLs (if any), prefix bindings, and LDP neighbors Notice “Advertised to:” field  “sh mpls ip binding advertisement-acls” Lists LDP filter, if there is any, on the first line Prefixes followed by “Advert acl(s):” are advertised via LDP, others are not
LIB information
mpls-7200a#sh mpls ip binding 12121212 32
12121212/32
in label: 21
out label: 19 lsr: 5555:0 in use
mpls-7200a#sh mpls ldp binding 12121212 32
tib entry: 12121212/32, rev 48
local binding: tag: 21
remote binding: tsr: 5555:0, tag: 19
LDP binding related debugs
mpls-7200a#debug mpls ldp bindings
shutting neighbor
2d11h: %CLNS-5-ADJCHANGE: ISIS: Adjacency to mpls-12008a (Ethernet3/0) Down, hold
time expired
2d11h: tagcon: tibent(5555/32): label imp-null from 5555:0 removed
2d11h: tagcon: route_tag_change for: 5555/32
inlabel 16, outlabel withdrwn, nexthop lsr 5555:0, reason response to
find_route_tags
2d11h: tagcon: Deassign peer id; 5555:0: id 0
2d11h: tagcon: tc_iprouting_table_change: 5555/255255255255, event 0x2
2d11h: tagcon: rib change: 5555/255255255255; event 0x2; ndb attrflags
0x1000000;
ndb->pdb_index/pdb->index 0x3/0x3
2d11h: tagcon: rib change: 5555/255255255255; event 0x2; ndb attrflags
0x1000000;
ndb->pdb_index/pdb->index 0x3/undef
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LFIB information
show mpls forwarding-table <prefix>
<prefix-length>
sh ip cef <prefix> internal
Looking at LFIB
Looking at LFIB on 12008a
Local  Outgoing  Prefix  
tag  tag or VC  or Tunnel Id  
19  19  12121212/32
Per-destination load-sharing, slots: 0 2 4 6 8 10 12 14
19 12121212/32 498 Et3/0 100511
MAC/Encaps=14/18, MTU=1500, Tag Stack{19}
AABBCC000503AABBCC0004038847 00013000
No output feature configured
Per-destination load-sharing, slots: 1 3 5 7 9 11 13 15
Monitoring LDP: LDP parameters
mpls-7200a#sh mpls ldp parameters
Protocol version: 1
Downstream label generic region: min label: 16; max label: 100000
Session hold time: 180 sec; keep alive interval: 60 sec
Discovery hello: holdtime: 15 sec; interval: 5 sec
Discovery targeted hello: holdtime: 180 sec; interval: 5 sec
Downstream on Demand max hop count: 255
TDP for targeted sessions
LDP initial/maximum backoff: 15/120 sec
LDP loop detection: off
Forwarding traffic down the LSP
mpls-7200a#sh mpls forwarding-table 12121212
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag
21
Note: Bytes tag switched this will increment if packets are being tag
switched using this entry
mpls-12008a#sh mpls forwarding-table label 19
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched  interface
19
19 12121212/32 1176 Et3/0  100511
mpls-12008b#sh mpls forwarding-table labels 19
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag
19
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LDP binding and advertisemsnt debugs
Be Careful on the production routers  “debug mpls ldp advertisements” Useful to see label bindings that are advertised  “debug mpls ldp binding” Useful to see label bindings that are received  “debug mpls ldp message sent|received” Useful for the protocol understanding purposes
MPLS Configuration Example
MPLS on LAN Configuration Example
Configuring IP TTL Propogation
sh ip cef detail
sh mpls ldp neighbor
sh mpls ldp discovery
sh mpls forwarding table
MPLS LAB Topology
Agenda
Control Plane Troubleshooting Tips
Case Studies  Forwarding Plane
Types of forwarding cases Load sharing MTU issues
Troubleshooting Tips Case Studies
Control Plane  Troubleshooting Tips

“Sh mpls ldp discovery | inc ldp|tdp”

“sh mpls ldp discovery”  2nd line in output

Control Plane  Troubleshooting Tips
Check IP reachability to remote LSR_ID on both LSRs “ping <lsr_id>”  Check for ACL or ICMP unreachable blockages  Untagged outgoing label for /32 routes ie PEs’ loopbacks is almost always alarming  Check the label binding for a prefix on both LSRs “sh mpls ldp bind <prefix> <mask>”
Control Plane  Troubleshooting Tips



Agenda
Control Plane
Troubleshooting Tips
Case Studies
Forwarding Plane
Types of forwarding cases Load sharing MTU issues
Troubleshooting Tips Case Studies
MPLS Control Plane  Protocol mismatch
Prob#1  session establishment (Protocol mismatch)
TIP  Check for the protocol mismatch and fix it
MPLS Control Plane  No route
Prob#2  session establishment (No route to peer)
TIP  Check for IP reachability to LDP_ID Fix it by letting
PE1 advertise 1013161/32 via IGP to P1
Presentation_ID ⓒ 2008 Cisco Systems, Inc All rights reserved Cisco Confidential
MPLS Control Plane  No Specific route
oops Ok
Ok
P1 doesn’t have a specific route to PE1
MPLS Control Plane  No Specific route(contd)
Prob#3 -Session establishment (Contd)
Gig8/0/044 1013141/32 1013148/32
TIP  Check for IP connectivity first Unless Layer3 is up, Layer4 (TCP session for LDP) won’t come up
MPLS Control Plane  Untagged outboundlabel
Prob#4 -“Untagged” problem
Pos4/1/0
Pos0/0
MPLS Control Plane  Untagged outboundlabel (contd)
Prob#4 -“Untagged” problem (contd)
FIB’s local label is different from that
Pos4/1/0
of LIB Unresolved ?
Pos0/0
1110128138
TIP  If local label for a prefix is not same in FIB and LIB,
then issue “clear ip route <prefix>” to fix
Presentation_ID ⓒ 2008 Cisco Systems, Inc All rights reserved Cisco Confidential
MPLS Control Plane  No LFIB entry
Prob#5  LFIB entry disappears  No LFIB entry  This might occur if the RIB owner for an IPv4 routes changes from IGP to BGP


MPLS Control Plane  No LFIB entry (contd)
Agenda
Control Plane Troubleshooting Tips Case Studies
Forwarding Plane Types of forwarding cases Load sharing MTU issues Troubleshooting Tips Case Studies
MPLS Forwarding Plane
Three cases in the MPLS forwarding ­1) Label Imposition -IP to MPLS conversion 2) Label swapping - MPLS to MPLS 3) Label disposition -MPLS to IP conversion
So, depending upon the case, we need to check­1) FIB -  For IP packets that get forwarded as MPLS 2) LFIB - For MPLS packets that get fwded as MPLS 3) LFIB -For MPLS packets that get fwded as IP
MPLS Forwarding Plane
Case 1: IP packets get forwarded as MPLS
PE2 1110/30
PE1 does a FIB lookup for
the incoming IP packet


MPLS Forwarding Plane
Case 2: MPLS packets get forwarded as MPLS
PE2 PE1



MPLS Forwarding Plane
Case 3: MPLS packets get forwarded as IP
PE1

the edge

the PHP router

look at the LFIB (not
FIB)
Agenda
Control Plane Troubleshooting Tips Case Studies
Forwarding Plane
Types of forwarding cases
Load sharing
MTU issues Troubleshooting Tips Case Studies
MPLS Forwarding Plane -Loadsharing
Loadsharing (due to multiple paths to a prefix) in MPLS is no different from that of IP  Hashing-algorithm is still the typical ‘FIB based’ ie per-dest loadsharing by default **  So the below “show command” is still relevant “Sh ip cef exact-route <source> <dest>” etc
But the dest must be known  in the FIB table, otherwise the command won’t work
Won’t work on P routers for the VPN prefixes
Agenda
Control Plane Troubleshooting Tips Case Studies
Forwarding Plane Types of forwarding cases Load sharing MTU issues Troubleshooting Tips Case Studies
MPLS Fwd Plane -Fragmentation



FragmentationMTU Setting in MPLS
Most of the interfaces (depending upon thehardware) support transmitting packets bigger thanthe “interface MTU” size
“mpls mtu <bytes>” can be applied to an interface
to change the MPLS MTU size on the interface
MPLS MTU size is checked by the router
while converting an IP packet into a labeled packet    or transmitting a labeled packet
FragmentationMTU Setting in MPLS
Remember that ­ ‘mpls mtu <bytes>” command has no effect on
“interface or IP MTU” size
By default, MPLS MTU = interface MTU
MPLS MTU setting doesn’t affect MTU handling for
IP-to-IP packet switching
MTU Setting in MPLSConfiguring the MPLS MTU MTU Setting in MPLSBefore setting the MPLS MTU
-MPLS MTU is 1508 bytes (changed):
Agenda
Control Plane Troubleshooting Tips Case Studies
Forwarding Plane Types of forwarding cases Load sharing MTU issues Troubleshooting Tips
Case Studies
MPLS Fwd Plane  Troubleshooting Tips




MPLS Fwd Plane  Troubleshooting Tips

“sh adjacency <interface>”  Check that the LFIB’s outgoing label is same as theincoming label in neighbor’s LFIB

MPLS Forwarding Plane  TAG adj

L2 header for MPLS
L2 header for IP
MPLS Fwd Plane  Show commands
“sh mpls forwarding” Shows all LFIB entries (vpn, non-vpn, TE etc)  “sh mpls forwarding <prefix>” LFIB lookup based on a prefix  “sh mpls forwarding label <label>” LFIB lookup based on an incoming label  “sh mpls forwarding <prefix> detail” Shows detailed info such as L2 encap etc
MPLS Fwd Plane  Debugs
Be Careful on the production routers
“Debug mpls lfib cef” Useful for seeing FIB and LFIB interaction when alabel is missing for a prefix  “debug mpls lfib struct” Shows changes in the LFIB structures when label is allocated/deallocated
Agenda
Control Plane Troubleshooting Tips Case Studies
Forwarding Plane Types of forwarding cases Load sharing MTU issues Troubleshooting Tips
Case Studies
MPLS Forwarding Plane -No entry in LFIB
Prob#1 -No entries in LFIB
Presentation_ID ⓒ 2008 Cisco Systems, Inc All rights reserved Cisco Confidential
TIP  Enable CEF It is must for MPLS
MPLS Forwarding Plane-Out label is Untagged
Prob#2 -“Untagged” problem
LDP session is UP;LIB has correct binding;but LFIB has “Untagged”  Pos0/1
TAG ADJ for pos0/1 is incomplete No good
MPLS Forwarding Plane-Out label is Untagged(contd)
Adj is incomplete;check the interface
MPLS Forwarding Plane-Out label is Untagged(contd)
TIP  If the interface doesn’t have “TAG” adj, then the label
will not get installed in LFIB Fix PPP in this case
Presentation_ID ⓒ 2008 Cisco Systems, Inc All rights reserved Cisco Confidential
MPLS Forwarding Plane-Recursive rewrite
Prob#3 -“Recursive rewrite” problem

MPLS Forwarding Plane-Recursive rewrite(contd)

“Recursive rewrite” usually means that
(a) Either the label to the next-hop is not available
(b) Or there is an internal problem with the CEF recursionresolution process

(a) usually turns out to be a LDP problem, and should be fixed by investigating into LDP
(b) could be fixed by “clear ip route <prefix>” or “clear ip bgp *”

MPLS Forwarding Plane-Recursive rewrite(contd)
In order to troubleshoot (a), check the label availability for the next-hop (in LIB) If it is missing, then fix LDP Because there is no LDP

MPLS Forwarding Plane-Recursive rewrite(contd)
LDP session needs to be established first  It is an LDP (control plane) problem  Troubleshoot for the LDP (as shown in the control plane section)

Conclusion