Reserved Labels
Labels 0 through 15 are reserved labels. An LSR cannot use them in the normal case for forwarding packets. An LSR assigns a specific function to each of these labels. Label 0 is the explicit NULL label, whereas label 3 is the implicit NULL label. Label 1 is the router alert label, whereas label 14 is the OAM alert label. The other reserved labels between 0 and 15 have not been assigned yet.
Implicit NULL Label
The implicit NULL label is the label that has a value of 3. An egress LSR assigns the implicit NULL label to a FEC if it does not want to assign a label to that FEC, thus requesting the upstream LSR to perform a pop operation. In the case of a plain IPv4-over-MPLS network, such as an IPv4 network in which LDP distributes labels between the LSRs, the egress LSR—running Cisco IOS—assigns the implicit NULL label to its connected and summarized prefixes. The benefit of this is that if the egress LSR were to assign a label for these FECs, it would receive the packets with one label on top of it. It would then have to do two lookups. First, it would have to look up the label in the LFIB, just to figure out that the label needs to be removed; then it would have to perform an IP lookup. These are two lookups, and the first is unnecessary.
The solution for this double lookup is to have the egress LSR signal the last but one (or penultimate) LSR in the label switched path (LSP) to send the packets without a label. The egress LSR signals the penultimate LSR to use implicit NULL by not sending a regular label, but by sending the special label with value 3. The result is that the egress LSR receives an IP packet and only needs to perform an IP lookup to be able to forward the packet. This enhances the performance on the egress LSR.
The use of implicit NULL at the end of an LSP is called penultimate hop popping (PHP). The LFIB entry for the LSP on the PHP router shows a "Pop Label" as the outgoing label. Figure 3-3 shows penultimate hop popping.
)
Figure 3-3 Penultimate Hop Popping
The use of implicit NULL is widespread and not confined only to the example in Figure 3-3. It could be that the packets have two or three or more labels in the label stack. Then the implicit NULL label used at the egress LSR would signal the penultimate hop router to pop one label and send the labeled packet with one label less to the egress LSR. Then the egress LSR does not have to perform two label lookups. The use of the implicit NULL label does not mean that all labels of the label stack must be removed. Only one label is popped off. In any case, the use of the implicit NULL label prevents the egress LSR from having to perform two lookups. Although the label value 3 signals the use of the implicit NULL label, the label 3 will never be seen as a label in the label stack of an MPLS packet. That is why it is called the implicit NULL label.
Explicit NULL Label
The use of implicit NULL adds efficiency when forwarding packets. However, it has one downside: The packet is forwarded with one label less than it was received by the penultimate LSR or unlabeled if it was received with only one label. Besides the label value, the label also holds the Experimental (EXP) bits. When a label is removed, the EXP bits are also removed. Because the EXP bits are exclusively used for quality of service (QoS), the QoS part of the packet is lost when the top label is removed. In some cases, you might want to keep this QoS information and have it delivered to the egress LSR. Implicit NULL cannot be used in that case.
The explicit NULL label is the solution to this problem, because the egress LSR signals the IPv4 explicit NULL label (value 0) to the penultimate hop router. The egress LSR then receives labeled packets with a label of value 0 as the top label. The LSR cannot forward the packet by looking up the value 0 in the LFIB because it can be assigned to multiple FECs. The LSR just removes the explicit NULL label. After the LSR removes the explicit NULL label, another lookup has to occur, but the advantage is that the router can derive the QoS information of the received packet by looking at the EXP bits of the explicit NULL label.
You can copy the EXP bits value to the precedence or DiffServ bits when performing PHP and thus preserve the QoS information. Or, if the label stack has multiple labels and the top label is popped off, you can copy the EXP bits value to the EXP field of the new top label. However, Chapter 12, "MPLS and Quality of Service," gives you two examples where this is not wanted; thus, the use of the explicit NULL label is warranted.
Router Alert Label
The Router Alert label is the one with value 1. This label can be present anywhere in the label stack except at the bottom. When the Router Alert label is the top label, it alerts the LSR that the packet needs a closer look. Therefore, the packet is not forwarded in hardware, but it is looked at by a software process. When the packet is forwarded, the label 1 is removed. Then a lookup of the next label in the label stack is performed in the LFIB to decide where the packet needs to be switched to. Next, a label action (pop, swap, push) is performed, the label 1 is pushed back on top of the label stack, and the packet is forwarded. Refer to Chapter 14, "MPLS Operation and Maintenance," for more details on the Router Alert label.
Example 3-8 shows the output of debug mpls packet on a router for a labeled packet with the Router Alert label on it.
Example 3-8. Debug MPLS Packet Showing Label 1
00:39:14: MPLS: Et1/1: recvd: CoS=6, TTL=255, Label(s)=1/21
00:39:14: MPLS: Et1/3: xmit: CoS=6, TTL=254, Label(s)=1/18
00:38:13: MPLS turbo: Se4/0: rx: Len 76 Stack {1 6 255} {20 6 255} - ipv4 data
00:38:13: MPLS les: Se4/0: rx: Len 76 Stack {1 6 255} {20 6 255} - ipv4 data
Example 3-8 shows two possible formats in the output. Both formats have the labels sorted from left to right or topmost label to bottommost label. The first format is the old format, with the slash separating the labels. The second format is the new format with the {label EXP TTL} format.
OAM Alert Label
The label with value 14 is the Operation and Maintenance (OAM) Alert label as described by the ITU-T Recommendation Y.1711 and RFC 3429. OAM is basically used for failure detection, localization, and performance monitoring. This label differentiates OAM packets from normal user data packets. Cisco IOS does not use label 14. It does perform MPLS OAM, but not by using label 14. Chapter 14 covers MPLS OAM in greater detail.