Examples of FIB and LFIB Entries
As mentioned beforehand in the chapter, the absolute packet-forwarding activity does not use the IP
routing table (RIB) or the LIB—instead, the FIB is acclimated to advanced packets that accustomed unlabeled,
and the LFIB is acclimated to advanced packets that accustomed already labeled. This area correlates the
information in appearance commands to the conceptual appearance of the FIB and LFIB abstracts structures shown
back in Figure 19-3.
First, afresh absorption on PE1, PE1 artlessly adds advice to the FIB advertence that PE1 should
impose an MPLS header, with characterization amount 22. PE1 additionally populates the LFIB, with an access for
10.3.3.0/24, application that aforementioned characterization amount of 22 and an approachable interface of S0/1/0. Archetype 19-3
shows the capacity of the two tables.
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 Figure 19-10.
Example 19-3 FIB and LFIB Entries for 10.3.3.0/24 on PE1
! This abutting command shows the FIB entry, which includes the bounded tag (24), the
! tags (label) imposed, and approachable interface.
PE1# appearance ip cef 10.3.3.0
10.3.3.0/24, adaptation 65, aeon 0, buried adjacency to Serial0/0/1
0 packets, 0 bytes
tag advice set
local tag: 24
fast tag carbon with Se0/0/1, point2point, tags imposed: {22}
via 192.168.12.2, Serial0/0/1, 0 dependencies
next hop 192.168.12.2, Serial0/0/1
valid buried adjacency
tag carbon with Se0/0/1, point2point, tags imposed: {22}
! The abutting command lists the LFIB access for 10.3.3.0/24, advertisement the aforementioned basic
! information—the bounded tag, the approachable tag (label), and approachable interface.
PE1# appearance mpls forwarding-table 10.3.3.0 24
Local Approachable Prefix Bytes tag Approachable Abutting Hop
tag tag or VC or Tunnel Id switched interface
24 22 10.3.3.0/24 0 Se0/0/1 point2point
712 Affiliate 19: Multiprotocol Characterization Switching
In the abstracts even archetype of Figure 19-3, PE1 accustomed an unlabeled packet and forwarded the
packet to P1, with characterization 22. The advice in the top allotment of Archetype 19-3, assuming the FIB,
matches that aforementioned logic, advertence that a tag (label) amount of 22 will be imposed by PE1.
Next, appraise the LFIB at P1 as apparent in Archetype 19-4. As apparent in Figure 19-3, P1 swaps the
incoming characterization of 22 with approachable characterization 39. For perspective, the archetype additionally includes the LIB
entries for 10.3.3.0/24.
The accent band in the achievement of the appearance mpls forwarding-table command lists the
incoming characterization (22 in this case) and the approachable characterization (39). Note that the admission characterization is shown
under the branch “local tag,” acceptation that characterization (tag) 22 was locally allocated by this router (P1)
and advertised to added routers application LDP, as apparent in Figure 19-8. P1 originally allocated and
advertised characterization 22 to acquaint adjoining routers to advanced packets destined to 10.3.3.0/24 to P1,
with a characterization of 22. P1 knows that if it receives a packet with characterization 22, P1 should absolutely bandy the
labels, forwarding the packet out S0/1/0 with a characterization of 39.
The LIB entries in Archetype 19-4 additionally reinforce the abstraction that (frame-mode) MPLS LSRs retain
all abstruse labels in their LIBs, but alone the currently acclimated labels in the LFIB. The LIB lists P1’s
local characterization (22), and the three alien labels abstruse from P1’s three LDP neighbors. To actualize the
LFIB entry, P1 acclimated the aforementioned affectionate of argumentation apparent in Figure 19-10 to associate the advice in
the acquisition table and LIB and accept a characterization amount of 39 and approachable interface S0/1/0 to forward
packets to 10.3.3.0/24.
To see an archetype of the pop action, accede the LFIB for PE2, as apparent in Archetype 19-5. When
PE2 receives a labeled packet from P1 (label 39), PE2 will try to use its LFIB to advanced the
packet. Back clearing the LFIB, PE2 can calmly apprehend that PE2 should pop the characterization and
forward an unlabeled packet out its Fa0/1 interface. Those affidavit accommodate the actuality that PE2 did
Example 19-4 FIB and LFIB Entries for 10.3.3.0/24 on P1
P1# appearance mpls forwarding-table 10.3.3.0 24
Local Approachable Prefix Bytes tag Approachable Abutting Hop
tag tag or VC or Tunnel Id switched interface
22 39 10.3.3.0/24 0 Se0/1/0 point2point
P1# appearance mpls ldp bindings 10.3.3.0 24
tib entry: 10.3.3.0/24, rev 30
local binding: tag: 22
remote binding: tsr: 1.1.1.1:0, tag: 24
remote binding: tsr: 4.4.4.4:0, tag: 86
remote binding: tsr: 3.3.3.3:0, tag: 39
MPLS Unicast IP Forwarding 713
not accredit MPLS on Fa0/1 and that PE2 has not abstruse any labels from CE2. Archetype 19-5
shows the approachable tag as “untagged.”
Note that while the argument in Archetype 19-5 alone showed LFIB entries, every LSR builds the
appropriate FIB and LFIB entries for anniversary prefix, in apprehension of accepting both unlabeled and
labeled packets.
Label Distribution Protocol Reference
Before wrapping up the advantage of basal MPLS unicast IP forwarding, you should apperceive a few
more capacity about LDP itself. So far, this affiliate has apparent what LDP does, but it has not
provided abundant advice about how LDP accomplishes its tasks. This area hits the main
concepts and summarizes the rest.
LDP uses a Hello affection to ascertain LDP neighbors and to actuate to what IP abode the
ensuing TCP affiliation should be made. LDP multicasts the Hellos to IP abode 224.0.0.2, using
UDP anchorage cardinal 646 for LDP (TDP uses UDP anchorage 711). The Hellos account anniversary LSR’s LDP ID
(LID), which consists of a 32-bit dotted-decimal cardinal and a 2-byte characterization amplitude number. (For
frame-based MPLS, the characterization amplitude cardinal is 0.) An LSR can optionally account a carriage address
in the Hello message, which is the IP abode that the LSR wants to use for any LDP TCP
connections. If a router does not acquaint a carriage address, added routers will use the IP address
that is the aboriginal 4 bytes of the LDP ID for the TCP connections.
After advertent neighbors via an LDP Hello message, LDP neighbors anatomy a TCP connection
to anniversary neighbor, afresh application anchorage 646 (TDP 711). Because the TCP affiliation uses unicast
addresses—either the neighbor’s advertised carriage abode or the abode in the LID—these
addresses charge be attainable according to the IP acquisition table. Once the TCP affiliation is up,
each router advertises all of its bindings of bounded labels and prefixes.
Cisco routers accept the IP abode in the LDP ID aloof like the OSPF router ID. LDP chooses the
IP abode to use as allotment of its LID based on the exact aforementioned argumentation as OSPF, as abbreviated in Table
19-4, forth with added details.
Example 19-5 FIB and LFIB Entries for 10.3.3.0/24 on PE2
PE2# appearance mpls forwarding-table 10.3.3.0 24
Local Approachable Prefix Bytes tag Approachable Abutting Hop
tag tag or VC or Tunnel Id switched interface
39 Untagged 10.3.3.0/24 0 Fa0/1 192.168.36.6
Table 19-4 LDP Reference
LDP Affection LDP Implementation
Transport protocols UDP (Hellos), TCP (updates)
Port numbers 646 (LDP), 711 (TDP)
Hello destination abode 224.0.0.2
Who initiates TCP affiliation Highest LDP ID
TCP affiliation uses this abode Carriage IP abode (if configured), or LDP ID if no
transport abode is configured
LDP ID bent by these rules,
in adjustment or precedence
Configuration
Highest IP abode of an up/up loopback back LDP comes up
Highest IP abode of an up/up non-loopback back LDP
comes up
This concludes the advantage of MPLS unicast IP forwarding for this chapter. Next, the chapter
examines one of the added accepted uses of MPLS, which happens to use unicast IP forwarding:
MPLS VPNs.