Lab
MPLS Basic Configuration
Last Update 2011.06.01
1.0.0
Copyright 2011 Kenneth M. Chipps Ph.D.
www.chipps.com
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What is MPLS
• MPLS is the next development in privately
managed shared networks
• Large networks have migrated from
dedicated T carrier circuits directly
connecting sites to each other, to the first
shared service provider network based on
Frame Relay, to now MPLS
• MPLS uses tunnels between sites to carry
traffic needing different types of QoS
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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What is MPLS
• The source work on MPLS is defined in
RFC 3031
• MPLS flows are connection-oriented and
packets are routed along paths preconfigured by service providers called LSP
- Label Switched Paths
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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What is MPLS
• Basic MPLS has been extended with a set
of Generalized MPLS extensions that
allow it to handle different types of traffic
between the same general end points
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Operation of MPLS
• Metaswitch provides an excellent
discussion and graphic to show how
MPLS works
• They say
– MPLS works by tagging the traffic, in this
example packets, with an identifier, a label, to
distinguish the LSPs
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Operation of MPLS
– When a packet is received, the router uses
this label and sometimes also the link over
which it was received to identify the LSP
– It then looks up the LSP in its own forwarding
table to determine the best link over which to
forward the packet, and the label to use on
this next hop
– A different label is used for each hop, and it is
chosen by the router or switch performing the
forwarding operation
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Operation of MPLS
– This allows the use of very fast and simple
forwarding engines, which are often
implemented in hardware
– Ingress routers at the edge of the MPLS
network classify each packet potentially using
a range of attributes, not just the packet's
destination address, to determine which LSP
to use
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Operation of MPLS
– Inside the network, the MPLS routers use only
the LSP labels to forward the packet to the
egress router
– Here is their graphic on this
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Operation of MPLS
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Operation of MPLS
• The MPLS label is a four-byte, fixedlength, locally-significant identifier
• According to Cisco it looks like this
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Operation of MPLS
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Operation of MPLS
• Cisco also says
• This label is placed between the data link
layer header and network layer
• The top of the label stack appears first in the
packet, and the bottom appears last
• The network layer packet immediately follows
the last label in the label stack
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Operation of MPLS
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Types of MPLS Links
• There are many types of MPLS
connections to a service provider’s MPLS
network
• For example, ATT supports MPLS
connections through what they call IP
VPNs
• As they say
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Types of MPLS Links
– Network-based VPN
• Deployed on the AT&T global network, which uses
multiprotocol level switching (MPLS) to provide
high-performance, any-to-any communication
around the globe
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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What is Traffic Engineering
• Metaswitch says this about traffic
engineering in MPLS networks
– Traffic Engineering is the process where data
is routed through the network according to a
management view of the availability of
resources and the current and expected traffic
– The class of service and quality of service
required for the data can also be factored into
this process
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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What is Traffic Engineering
– Traffic Engineering may be under the control
of manual operators
– They monitor the state of the network and
route the traffic or provision additional
resources to compensate for problems as
they arise
– Alternatively, Traffic Engineering may be
driven by automated processes reacting to
information fed back through routing protocols
or other means
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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What is Traffic Engineering
– Traffic Engineering helps the network provider
make the best use of available resources,
spreading the load over the layer 2 links, and
allowing some links to be reserved for certain
classes of traffic or for particular customers
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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What is Traffic Engineering
– There are currently two label distribution
protocols that provide support for Traffic
Engineering
• RSVP - Resource ReSerVation Protocol
• CR-LDP - Constraint-based Routed Label
Distribution Protocol
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS and Traffic Engineering
• MPLS and Traffic Engineering work
together to provide tunnels that carry a
certain class of service from one point to
another
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS in a Lab Environment
• As it is difficult to create a real MPLS
network in a lab environment and since
simulation programs do not offer a MPLS
cloud as they do for Frame Relay, this lab
will use the GNS3 emulation program
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Lab
• This lab is mostly copied from a Cisco
Networking Academy CCNP level lab titled
– Lab 4.1 Configuring Frame Mode MPLS
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Lab
• Here is the topology to create in GNS3
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Lab
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Lab
• In this lab, you will configure a network
using EIGRP as the routing protocol
• Then run MPLS over the IP internetwork to
fast-switch Layer 2 frames
• Here is the configuration for each router
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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R1
•
•
•
•
•
•
•
•
enable
config t
interface loopback 0
ip address 172.16.1.1 255.255.255.0
interface fastethernet 0/0
ip address 172.16.12.1 255.255.255.0
no shutdown
exit
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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R1
•
•
•
•
•
•
•
router eigrp 1
no auto-summary
network 172.16.0.0
interface fastethernet 0/0
mpls ip
exit
end
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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R2
•
•
•
•
•
•
•
enable
config t
interface loopback 0
ip address 172.16.2.1 255.255.255.0
interface fastethernet 0/0
ip address 172.16.12.2 255.255.255.0
no shutdown
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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R2
•
•
•
•
•
interface serial 1/0
ip address 172.16.23.2 255.255.255.0
clockrate 64000
no shutdown
exit
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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R2
•
•
•
•
•
•
•
router eigrp 1
no auto-summary
network 172.16.0.0
interface fastethernet 0/0
mpls ip
exit
end
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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R3
•
•
•
•
•
•
•
•
enable
config t
interface loopback 0
ip address 172.16.3.1 255.255.255.0
interface serial 1/0
ip address 172.16.23.3 255.255.255.0
no shutdown
exit
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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R3
•
•
•
•
•
•
•
router eigrp 1
no auto-summary
network 172.16.0.0
interface fastethernet 0/0
mpls ip
exit
end
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Check Connectivity
• When everything is configured, ping from
R1 to R3
– ping 172.16.3.1
• Check the routing table
– show ip route
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Check Connectivity
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Check Connectivity
• On R1, if you perform a traceroute to the
R3’s loopback, you see the path the
packet follows
• Observe this
• This output changes slightly once we
configure MPLS
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Check Connectivity
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Configuration
• As discussed earlier MPLS is a
standardized protocol that allows routers
to switch packets based on labels, rather
than route switch packets based on
standards in the protocol’s routing formula
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Configuration
• Under normal IP routing, every
intermediate system looks up the
destination prefix of an IP packet in the
Routing Information Base of a router or in
the Forwarding Information Base of a fast
switch at every Layer 3 node
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Configuration
• Instead of switching that is based on
prefix, the first router running MPLS can
encapsulate the IP packet in an MPLS
frame and then further encapsulate the
packet in the Layer 2 frame before
sending it across one of many supported
Layer 2 media
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Configuration
• At the next MPLS-enabled LSR - Label
Switch Router, the MPLS frame is read
and the IP packet is switched as an MPLS
frame from router to router with little
rewrite at each node
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Configuration
• This allows routers to switch multiple
protocols - hence the name - using the
same switching mechanism, as well as
perform some other functionality not
available in traditional destination-based
forwarding, including Layer 2 VPNs - ATM,
Layer 3 VPNs, and traffic engineering
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Configuration
• Configuring the interface-level command
mpls ip on an interface tells the router to
switch MPLS packets inbound and
outbound on that interface as well as
attempt to bring up MPLS adjacencies with
the LDP - Label Distribution Protocol out
that egress interface
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Configuration
• LDP facilitates communication between
MPLS peers by allowing them to inform
each other of labels to assign packets to
particular destinations based on Layer 2,
Layer 3, or other significant information
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
• MPLS has many show commands that you
can use to verify proper MPLS operation
• Issue the
– show mpls interfaces
• command to see a quick summary of
interfaces configured with MPLS
• Keep in mind that you will see this output
because you applied the mpls ip command
to these interfaces
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Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
Verify MPLS Configuration
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
• Issue the
– show mpls ldp discovery
• command to find out local sources for LDP
exchanges and the show mpls ldp
neighbor command to show LDP
adjacencies
• Notice that MPLS chooses its IDs based
on loopback interfaces, similar to other
protocols such as OSPF and BGP
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
• In the configuration you set up, all routers
are acting as Label Switch Routers and
running LDP
• On LSRs, each forwarding equivalence
class - in this case, each routable IP prefix
- is assigned an MPLS label
• LDP automatically distributes labels to
peers to be used when sending traffic to
specific destinations through the LSR
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
• Once labels have been distributed,
switching for MPLS packets is done
through the LIB - Label Information Base
• Display the contents of the LIB using
– show mpls ldp bindings
• There is a binding for every routed prefix;
however, the bindings may vary from
router to router since they can get
swapped at each hop
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
• In a larger network, the way labels are
swapped is easier to see
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
• As mentioned earlier, traceroute would
differ slightly once MPLS was set up
• The output now includes labels for each
hop
• Unfortunately, because of the size of this
network, you only see one label
• In a larger network, you would see more
hops, and therefore more labels
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
• Because you are adding in extra header
information to packets, the MTU of
packets can change
• Remember that each MPLS header is 4
bytes
• The default MTU size of MPLS packets is
taken from the interface it is running on,
which in the case of Ethernet is 1500
bytes
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
• For this lab, we will change the Ethernet
connection between R1 and R2 to support
2 MPLS headers, so we will change the
MPLS MTU to 1508 on their Fast Ethernet
interfaces
• Verify the change using the
– show mpls interfaces interface detail
• command used
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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Verify MPLS Configuration
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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MPLS Lab
• As you can see, GNS3 running Dynagen
and Dynamips is a very useful tool
• As you can also see, MPLS is easy to
setup
Copyright 2011 Kenneth M. Chipps Ph.D. www.chipps.com
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