MPLS defines protocols that actualize a altered archetype for how routers advanced packets. Instead
of forwarding packets based on the packets’ destination IP address, MPLS defines how routers can
forward packets based on an MPLS label. By disassociating the forwarding accommodation from the
destination IP address, MPLS allows forwarding decisions based on added factors, such as traffic
engineering, QoS requirements, and the aloofness requirements for assorted barter affiliated to
the aforementioned MPLS network, while still because the acceptable advice abstruse appliance routing
protocols.
MPLS includes a advanced array of applications, with anniversary appliance because one or added of
the accessible factors that access the MPLS forwarding decisions. For the purposes of the CCIE
Routing and Switching accounting exam, this book covers two such applications in the aboriginal two major
sections of this chapter:
■ MPLS unicast IP
■ MPLS VPNs
This affiliate ends with a abrupt addition to abounding of the added MPLS applications. Also, as usual,
please booty the time to analysis http://www.ciscopress.com/title/9781587201967 for the latest
version of Appendix C, “CCIE Acquisition and Switching Assay Updates,” to acquisition out if you should
read added about any of the MPLS topics.
MPLS Unicast IP Forwarding
MPLS can be acclimated for simple unicast IP forwarding. With MPLS unicast IP forwarding, the MPLS
forwarding argumentation assiduously packets based on labels. However, back allotment the interfaces out
which to advanced the packets, MPLS considers alone the routes in the unicast IP acquisition table, so
the end aftereffect of appliance MPLS is that the packet flows over the aforementioned aisle as it would accept if MPLS
were not used, but all added factors were unchanged.
MPLS unicast IP forwarding does not accommodate any cogent advantages by itself; however, many
of the added accessible MPLS applications, such as MPLS VPNs and MPLS cartage engineering (TE),
use MPLS unicast IP forwarding as one allotment of the MPLS network. So to accept MPLS as you
NOTE MPLS includes frame-mode MPLS and cell-mode MPLS, while this affiliate only
covers frame-mode MPLS. The ambiguous comments in this affiliate may not administer to cellmode
MPLS.
would typically implement it, you need a solid understanding of MPLS in its most basic form:
MPLS unicast IP forwarding.
MPLS requires the use of control plane protocols (for example, OSPF and LDP) to learn labels,
correlate those labels to particular destination prefixes, and build the correct forwarding tables.
MPLS also requires a fundamental change to the data plane’s core forwarding logic. This section
begins by examining the data plane, which defines the packet-forwarding logic. Following that,
this section examines the control plane protocols, particularly the Label Distribution Protocol
(LDP), which MPLS routers use to exchange labels for unicast IP prefixes.
MPLS IP Forwarding: Data Plane
MPLS defines a completely different packet-forwarding paradigm. However, hosts do not and
should not send and receive labeled packets, so at some point, some router will need to add a label
to the packet and, later, another router will remove the label. The MPLS routers—the routers that
inject (push), remove (pop), or forward packets based on their labels—use MPLS forwarding
logic.
MPLS relies on the underlying structure and logic of Cisco Express Forwarding (CEF) while
expanding the logic and data structures as well. First, a review of CEF is in order, followed by
details about a new data structure called the MPLS Label Forwarding Information Base (LFIB).
CEF Review
A router’s unicast IP forwarding control plane uses routing protocols, static routes, and connected
routes to create a Routing Information Base (RIB). With CEF enabled, a router’s control plane
processing goes a step further, creating the CEF Forwarding Information Base (FIB), adding a FIB
entry for each destination IP prefix in the routing table. The FIB entry details the information
needed for forwarding: the next-hop router and the outgoing interface. Additionally, the CEF
adjacency table lists the new data-link header that the router will then copy in front of the packet
before forwarding.
For the data plane, a CEF router compares the packet’s destination IP address to the CEF FIB,
ignoring the IP routing table. CEF optimizes the organization of the FIB so that the router spends
very little time to find the correct FIB entry, resulting in a smaller forwarding delay and a higher
volume of packets per second through a router. For each packet, the router finds the matching FIB
entry, then finds the adjacency table entry referenced by the matching FIB entry, and forwards the
packet. Figure 19-1 shows the overall process.
698 Chapter 19: Multiprotocol Characterization Switching
Figure 19-1 IP Routing Table and CEF FIB—No MPLS
With this accomplishments in mind, the argument abutting looks at how MPLS changes the forwarding process
using labels.
Overview of MPLS Unicast IP Forwarding
The MPLS forwarding archetype assumes that hosts accomplish packets after an MPLS label;
then, some router imposes an MPLS label, added routers advanced the packet based on that label,
and again added routers abolish the label. The end aftereffect is that the host computers accept no
awareness of the actuality of MPLS. To acknowledge this all-embracing forwarding process, Figure 19-2
shows an example, with accomplish assuming how a packet is forwarded application MPLS.
Figure 19-2 MPLS Packet Forwarding—End to End
Router
Routing Table
Next-Hop
192.168.11.1
Out Int.
S0/0/1
Prefix
10.3.3.0/24
CEF FIB
Adj. IP Address
192.168.11.1
Prefix
10.3.3.0/24
Packet: Dest = 10.3.3.1
CEF Adjacency Table
Out Int.
S0/0/1
New L2 Header
(Details Omitted)
Next-Hop
192.168.11.1
Connected, Static,
Routing Protocols
Best
Routes
Add 1 FIB
Entry Per
Prefix
Encapsulation
and
Forwarding
Details
A
CE1 PE1 P1 PE2
LSR LSR LSR
CE2
1 B
6
3 4 5
2
10.3.3.3
IP IP
IP 22 IP 39 IP IP
MPLS Unicast IP Forwarding 699
The steps from the figure are explained as follows:
1. Host A generates and sends an unlabeled packet destined to host 10.3.3.3.
2. Router CE1, with no MPLS features configured, forwards the unlabeled packet based on the
destination IP address, as normal, without any labels. (Router CE1 may or may not use CEF.)
3. MPLS router PE1 receives the unlabeled packet and decides, as part of the MPLS forwarding
process, to impose (push) a new label (value 22) into the packet and forwards the packet.
4. MPLS router P1 receives the labeled packet. P1 swaps the label for a new label value (39) and
then forwards the packet.
5. MPLS router PE2 receives the labeled packet, removes (pops) the label, and forwards the
packet toward CE2.
6. Non-MPLS router CE2 forwards the unlabeled packet based on the destination IP address, as
normal. (CE2 may or may not use CEF.)
The steps in Figure 19-2 show a relatively simple process and provide a great backdrop from
which to introduce a few terms. The term Label Switch Router (LSR) refers to any router that has
awareness of MPLS labels, for example, routers PE1, P1, and PE2 in Figure 19-2. Table 19-2 lists
the variations of the term LSR, and a few comments about the meaning of each term.
Table 19-2 MPLS LSR Terminology Reference
LSR Type Actions Performed by This LSR Type
Label Switch Router
(LSR)
Any router that pushes labels onto packets, pops labels from packets, or
simply forwards labeled packets.
Edge LSR (E-LSR) An LSR at the edge of the MPLS network, meaning that this router
processes both labeled and unlabeled packets.
Ingress E-LSR For a particular packet, the router that receives an unlabeled packet and then
inserts a label stack in front of the IP header.
Egress E-LSR For a particular packet, the router that receives a labeled packet and then
removes all MPLS labels, forwarding an unlabeled packet.
ATM-LSR An LSR that runs MPLS protocols in the control plane to set up ATM
virtual circuits. Forwards labeled packets as ATM cells.
ATM E-LSR An E-edge LSR that also performs the ATM Segmentation and Reassembly
(SAR) function.