Router Alert Option and Router Alert Label
IP packets can have a Router Alert option appended to the IP header. This option is an IP option indicating that the router should inspect the packet further when forwarding the packet, even though the packet is not directly addressed to that router. The transit router for the packet should not just forward the packet by doing an IP lookup, but the router should inspect it further before forwarding it. What this inspection means is not defined and is up to the software implementation on the router. The Router Alert option is an IP option like the Timestamp, Loose Source Route, and Strict Source Route options are. The Router Alert option is defined in RFC 2113. An IP option is encoded as a Type Length Value (TLV). Look at Figure 14-1 to see the Type definition of an IP option.
Router Alert Option and Router Alert Label 527
Figure 14-1 IP Option Type Definition
IP Option Type
1 Bit
2 Bits
Copy
Option Class
Option Number
0 Do Not Copy
1 Copy To Fragment
• Option Class
0 Datagram Or Network Control
1 Reserved
2 Debugging and Measurement
3 Reserved
Figure 14-2 shows the IP option with the values for the IP Router Alert option. Figure 14-2 IP Router Alert Option
Type
Length
Value
10010100
00000100
2-Octet Value
IP Option Is Copied To Fragments (COPY =1)
Option Class = 0
Option Number = 20
Length = 4
Value
- 0 Router Shall Examine Packet
- 1-65535 Reserved
This Router Alert option works only if the packet is an IP packet. If the packet were labeled and as such forwarded by the LFIB on the LSR, the LSR would not even know that the packet had the Router Alert option present. Of course, you could program the LSR to perform deep packet inspection and always look at the IP header information of labeled packets to determine whether the Router Alert option were present. However, that could lead to a serious forwarding performance impact on the LSR, so it is not the best solution. It might not even be possible to do this in hardware forwarding engines, or it might be too costly. A better solution is to use a special MPLS label as the top label in the label stack of the packets that the LSRs need to examine. This special label is MPLS label 1, which is called the Router Alert label.
Continue reading here: Router Alert Label
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