MPLS IP Forwarding: Ascendancy Plane
For authentic IP acquisition to assignment appliance the FIB, routers charge use ascendancy even protocols, like routing
protocols, to aboriginal abide the IP acquisition table and again abide the CEF FIB. Similarly, for
MPLS forwarding to work, MPLS relies on ascendancy even protocols to apprentice which MPLS labels
to use to ability anniversary IP prefix, and again abide both the FIB and the LFIB with the actual labels.
MPLS supports abounding altered ascendancy even protocols. However, an engineer’s best of which
control even agreement to use is mainly accompanying to the MPLS appliance used, rather than any
detailed allegory of the appearance of anniversary ascendancy even protocol. For example, MPLS VPNs use
two ascendancy even protocols: LDP and multiprotocol BGP (MP-BGP).
While assorted ascendancy even protocols may be acclimated for some MPLS applications, MPLS unicast
IP forwarding uses an IGP and one MPLS-specific ascendancy even protocol: LDP. This section, still
focused on unicast IP forwarding, examines the capacity of characterization administration appliance LDP.
MPLS LDP Basics
For unicast IP routing, LDP artlessly advertises labels for anniversary prefix listed in the IP acquisition table.
To do so, LSRs use LDP to accelerate letters to their neighbors, with the letters advertisement an IP prefix
and agnate label. By announcement an IP prefix and label, the LSR is about saying, “If
you appetite to accelerate packets to this IP prefix, accelerate them to me with the MPLS characterization listed in the LDP
update.”
The LDP advertisement is triggered by a new IP avenue actualization in the unicast IP acquisition table.
Upon acquirements a new route, the LSR allocates a characterization alleged a bounded label. The bounded characterization is the
label that, on this one LSR, is acclimated to represent the IP prefix aloof added to the acquisition table. An
example makes the abstraction abundant clearer. Amount 19-7 shows a hardly broadcast adaptation of the
MPLS arrangement apparent beforehand in this chapter. The amount shows the basal action of what occurs
when an LSR (PE2) learns about a new avenue (10.3.3.0/24), triggering the action of advertising
a new bounded characterization (39) appliance LDP.
NOTE The ancient pre-standard adaptation of LDP was alleged Tag Administration Agreement (TDP).
The appellation tag switching was additionally generally acclimated instead of characterization switching.
MPLS Unicast IP Forwarding 705
Figure 19-7 LDP Action Triggered by New Unicast IP Route
The amount shows the afterward simple three-step action on PE2:
1. PE2 learns a new unicast IP route, which appears in the IP acquisition table.
2. PE2 allocates a new bounded label, which is a characterization not currently advertised by that LSR.
3. PE2 uses LDP to acquaint to neighbors the mapping amid the IP prefix and characterization to all
LDP neighbors.
Although the action itself is simple, it is important to agenda that PE2 charge now be accessible to process
labeled packets that access with the new bounded characterization amount in it. For example, in Amount 19-7, PE2
needs to be accessible to advanced packets accustomed with characterization 39; PE2 will advanced the packets with the
same next-hop and approachable interface advice abstruse in the IGP Update at footfall 1 in the figure.
Although interesting, the action apparent in Amount 19-7 shows alone the advertisement of one
segment of the abounding characterization switched aisle (LSP). An MPLS LSP is the accumulated set of labels that
can be acclimated to advanced the packets accurately to the destination. For example, Abstracts 19-2 and 19-
3 appearance a abbreviate LSP with characterization ethics 22 and 39, over which packets to subnet 10.3.3.0/24 were
sent. Amount 19-7 shows the advertisement of one part, or segment, of the LSP.
The routers in the MPLS billow charge use some IP acquisition agreement to apprentice IP routes in adjustment to
trigger the LDP action of announcement labels. Typically, for MPLS unicast IP routing, you would
use an IGP to apprentice all the IP routes, triggering the action of announcement the agnate labels.
For example, Amount 19-8 picks up the action area Amount 19-7 ended, with PE2 announcement a
route for 10.3.3.0/24 appliance EIGRP, causing added routers to again use LDP to acquaint labels.
NOTE LSPs are unidirectional.
P1
P2
CE1 PE1 PE2 PE2
3
3
2
1
Hmmm…I’m not using
label 39 already — advertise
label 39 with 10.3.3.0/24
LDP:
10.3.3.0/24, 39
LDP:
10.3.3.0/24, 39
IGP Update:
Route to 10.3.3.0/24
B
10.3.3.0/24
706 Affiliate 19: Multiprotocol Characterization Switching
Figure 19-8 Completed Action of Announcement an Entire LSP
The accomplish in the amount are as follows, appliance calculation that continues the calculation from
Figure 19-7:
4. PE2 uses EIGRP to acquaint the avenue for 10.3.3.0/24 to both P1 and P2.
5. P1 reacts to the anew abstruse avenue by allocating a new bounded characterization (22) and appliance LDP to
advertise the new prefix (10.3.3.0/24) to characterization (20) mapping. Agenda that P1 advertises this label
to all its neighbors.
6. P2 additionally reacts to the anew abstruse avenue by allocating a new bounded characterization (86) and appliance LDP
to acquaint the new prefix (10.3.3.0/24) to characterization (86) mapping. P2 advertises this characterization to all
its neighbors.
This aforementioned action occurs on anniversary LSR, for anniversary avenue in the LSR’s acquisition table: anniversary time an
LSR learns a new route, the LSR allocates a new bounded characterization and again advertises the characterization and prefix
mapping to all its neighbors—even aback it is accessible that announcement the characterization may not be useful.
For example, in Amount 19-8, P2 advertises a characterization for 10.3.3.0/24 aback to router PE2—not terribly
useful, but it is how frame-mode MPLS LSRs work.
P1
P2
PE1 PE2 CE2
5
5
5
5
6
6
6
4
4
Hmmm…I’m not using
label 22 already — advertise
label 22 with 10.3.3.0/24
Hmmm…I’m not using
label 86 already — advertise
label 86 with 10.3.3.0/24
LDP:
10.3.3.0/24, 86
B
10.3.3.0/24
IGP Update:
Route to 10.3.3.0/24
IGP Update:
Route to 10.3.3.0/24
LDP Update at Footfall 6:
LDP:
10.3.3.0/24, 22
LDP Update at Footfall 5:
S0/1/1
S0/1/1
Once the routers accept all abstruse about a prefix appliance the IGP protocol, and LDP has advertised
label/prefix mappings (bindings) to all added adjoining LSRs, anniversary LSR has abundant information
with which to characterization about-face packets from admission E-LSR to departure E-LSR. For example, the same
data even action apparent in Abstracts 19-2 and 19-3 could action aback PE1 receives an unlabeled
packet destined to an abode in 10.3.3.0/24. In fact, the labels advertised in Abstracts 19-7 and
19-8 agilely bout the beforehand MPLS abstracts even abstracts (19-2 and 19-3). However, to
complete the abounding process, you charge to accept a bit added about what occurs central an
individual router, in particular, a abstracts anatomy alleged the MPLS Characterization Advice Base (LIB).