Router Alert Label

When an MPLS LSP ping or traceroute is issued, the echo request has the IP Router Alert option on it. The echo reply does not have it, by default. When you use the command ping mpls or traceroute mpls with the keyword reply mode router-alert, the echo reply has the IP Router Alert option. If the return path is an LSP, the echo reply is labeled and has the top label 1 (the Router Alert label. The label 1 ensures that the packet is inspected at each LSR. The LSR at each hop removes the Router Alert label, the LSR makes a forwarding decision on the label underneath, the LSR performs a label operation, the LSR pushes the Router Alert label onto the label stack again, and the LSR forwards the packet. Figure 14-8 shows an example where a P router has a hardware forwarding error. The LFIB entry in hardware is wrong for the echo reply packet on the router P2. The echo reply packet is erroneously forwarded to router P4 instead of router P1. As such, the echo reply packet does not make it back to router PE1. The user is unsure whether it was the echo request packet or the echo reply packet that did not make it. If the echo reply packet has the Router Alert label in the label stack, the hardware on P2 does not forward the packet; it is forwarded correctly to router P1 and reaches PE1.

Figure 14-8 Echo Reply with Router Alert Label

Hardware Forwarding Error

Figure 14-8 Echo Reply with Router Alert Label

Hardware Forwarding Error

Example 14-9 shows the output of debug mpls packet for the echo Reply packet, with the Router Alert Label as the top label.

Example 14-9 MPLS LSP Ping with Reply Mode Router Alert

00:01

39:

MPLS:

Se4/0:

recvd: CoS=6, TTL=254

Label(s)=1/24

00:01

39:

MPLS:

Et3/1:

xmit: CoS=6, TTL=253,

Label(s)=1/26

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