All Cisco-Network Study Notes: The MPLS Characterization Advice Base Feeding the FIB and LFIB

The MPLS Characterization Advice Base Feeding the FIB and LFIB

LSRs abundance labels and accompanying advice central a abstracts anatomy alleged LIB. The LIB essentially

holds all the labels and associated advice that could possibly be acclimated to advanced packets.

However, anniversary LSR charge accept the best characterization and approachable interface to absolutely use and then

populate that advice into the FIB and the LFIB. As a result, the FIB and LFIB accommodate labels

only for the currently acclimated best LSP segment, while the LIB contains all labels accepted to the LSR,

whether the characterization is currently acclimated for forwarding or not.

To accomplish a accommodation about the best characterization to use, LSRs await on the acquisition protocol’s accommodation about

the best route. By relying on the acquisition protocol, the LSRs can booty advantage of the routing

protocol’s loop-prevention appearance and acknowledge to the acquisition protocol’s best for new routes when

convergence occurs. In short, an LSR makes the afterward decision:

For anniversary avenue in the acquisition table, acquisition the agnate characterization advice in the LIB,

based on the approachable interface and next-hop router listed in the route. Add the

corresponding characterization advice to the FIB and LIB.

To bigger accept how an LSR adds advice to the FIB and LFIB, this area continues

the aforementioned archetype as acclimated throughout the affiliate so far. At this point, it is advantageous to appraise the

output of some appearance commands, but first, you charge a little added detail about the archetype network

and the configuration. Amount 19-9 repeats the aforementioned archetype arrangement acclimated in beforehand abstracts in this

chapter, with IP abode and interface capacity included. The amount additionally addendum on which interfaces

MPLS has been enabled (dashed lines) and on which interfaces MPLS has not been enabled (solid

lines).

708 Affiliate 19: Multiprotocol Characterization Switching

Figure 19-9 Archetype Arrangement for Seeing the LIB, FIB, and LFIB

The agreement of MPLS unicast IP acquisition is almost simple. In this case, all six routers use

EIGRP, announcement all subnets. The four LSRs accredit MPLS globally and on the links acclaimed with

dashed curve in the figure. To accredit MPLS for simple unicast IP forwarding, as has been described

so far in this chapter, an LSR artlessly needs to accredit CEF, globally accredit MPLS, and enable

MPLS on anniversary adapted interface. Also, because IOS uses TDP instead of LDP by default, this

configuration overrides the absence to use LDP. Archetype 19-1 shows a sample all-encompassing configuration.

Example 19-1 MPLS Agreement on LSRs for Unicast IP Support

! The aboriginal three commands accredit CEF and MPLS globally, and

! use LDP instead of TDP

ip cef

mpls ip

mpls characterization agreement ldp

! Repeat the abutting two curve for anniversary MPLS-enabled interface

interface blazon x/y/z

mpls ip

! Normal EIGRP agreement abutting – would be configured for all interfaces

router eigrp 1

network …

CE1 Fa0/1 PE1 PE2 CE2

S0/1/1 S0/1/1

Fa0/1

36.3 36.6

S0/1/1 S0/1/0

S0/1/1

24.2

24.4

S0/1/0

34.4

34.3

23.3

23.2

14.4

14.1

12.1

12.2

15.5 15.1

All IP addresses activate with

192.168 unless contrarily noted.

LID 1.1.1.1

LID 2.2.2.2

LID 3.3.3.3

LID 4.4.4.4

10.3.3.10/24

10.3.3.1

Legend:

MPLS-Enabled Links

Non-MPLS Enabled Links

MPLS Unicast IP Forwarding 709

To see how LSRs abide the FIB and LFIB, accede subnet 10.3.3.0/24 again, and ahead about

MPLS from router PE1’s perspective. PE1 has abstruse a avenue for 10.3.3.0/24 with EIGRP. PE1

has additionally abstruse (using LDP) about two labels that PE1 can use back forwarding packets destined

for 10.3.3.0/24—one characterization abstruse from adjoining LSR P1, and the added from neighboring

LSR P2. Archetype 19-2 highlights these details. Agenda that the labels do bout the abstracts and

examples acclimated beforehand in this chapter.

Example 19-2 shows some banal advice and a few decidedly absorbing points. First,

the appearance ip avenue command does not account any new or altered advice for MPLS, but it is

useful to agenda that PE1’s best avenue to 10.3.3.0/24 is through P1. The appearance ip mpls bindings

10.3.3.0 24 command lists the LIB entries from 10.3.3.0/24. Agenda that two alien bindings are

listed—one from P1 (LDP ID 2.2.2.2) and one from P2 (LDP ID 4.4.4.4). This command additionally lists

the bounded binding, which is the characterization that PE1 allocated and advertised to its neighbors.

From Archetype 19-2, you could ahead that PE1 will use a characterization amount of 22, and an outgoing

interface of S0/0/1, back forwarding packets to 10.3.3.0/24. To see the capacity of how PE1 arrives

at that conclusion, accede the linkages apparent in Amount 19-10.

Example 19-2 PE1’s LIB and IP Acquisition Table

PE1# appearance ip avenue 10.0.0.0

Routing access for 10.0.0.0/24, 1 accepted subnets

Redistributing via eigrp 1

D 10.3.3.0 [90/2812416] via 192.168.12.2, 00:44:16, Serial0/0/1

PE1# appearance mpls ldp bindings 10.3.3.0 24

tib entry: 10.3.3.0/24, rev 28

local binding: tag: 24

remote binding: tsr: 2.2.2.2:0, tag: 22

remote binding: tsr: 4.4.4.4:0, tag: 86

NOTE The appellation alien bounden refers to a label-prefix bounden abstruse via LDP from some

LDP neighbor.

710 Affiliate 19: Multiprotocol Characterization Switching

Figure 19-10 PE1’s Process to Determine the Approachable Label

The amount shows the afterward steps:

1. The acquisition table access to 10.3.3.0/24 lists a next-hop IP abode of 192.168.12.2. PE1

compares that next-hop advice to the account of interface IP addresses on anniversary LDP associate and

finds the LDP acquaintance who has IP abode 192.168.12.2.

2. That aforementioned arrangement of the appearance mpls ldp acquaintance command achievement identifies the LDP ID

(LID) of this peer, namely 2.2.2.2.

3. PE1 addendum that for that aforementioned prefix (10.3.3.0/24), the LIB contains one bounded characterization and two

remote labels.

PE1#show ip avenue | accommodate 10.3.3.0

D 10.3.3.0 [90/2812416] via 192.168.12.2, 00:04:33, Serial0/0/1

PE1#show mpls Idp bindings 10.3.3.0 24

tib entry: 10.3.3.0/24, rev 28

local binding: tag: 24

remote binding: tsr: 2.2.2.2:0, tag: 22

remote binding: tsr: 4.4.4.4:0, tag: 86

PE1#show mpls Idp neighbor

Peer LDP Ident: 2.2.2.2:0; Bounded LDP Ident 1.1.1.1:0

TCP connection: 2.2.2.2.60635 - 1.1.1.1.646

State: Oper; Msgs sent/rcvd: 75/76; Downstream

Up time: 00;35;20

LDP analysis sources:

Serial0/0/1, Src IP addr: 192.168.12.2

Addresses apprenticed to associate LDP Ident:

192.168.12.2 2.2.2.2 192.168.1.112 192.168.24.2

192.168.23.2

Peer LDP Ident: 4.4.4.4:0; Bounded LDP Ident 1.1.1.1:0

TCP connection: 4.4.4.4.11711 - 1.1.1.1.646

State: Oper; Msgs sent/rcvd: 26/30; Downstream

Up time: 00:06:17

LDP analysis sources:

Serial0/1/1, Src IP addr: 192.168.14.4

Addresses apprenticed to associate LDP Ident:

192.168.14.4 4.4.4.4 192.168.24.4 192.168.34.4

4. Among the accepted labels listed for prefix 10.3.3.0/24, one was abstruse from a acquaintance whose

LID is 2.2.2.2, with characterization (tag) amount of 22.

As a aftereffect of these steps, PE1 knows it should use approachable interface S0/1/0, with characterization 22, when

forwarding packets to subnet 10.3.3.0/24.

NOTE Many IOS commands still use the earlier tag switching terminology—for example, the

term Tag Switching Router (TSR) is listed instead of LSR in Amount 19-10.