Example 411 Decoded Path Message

1. RSVP: version:1 flags:0000 type:PATH cksum:1049 ttl:254 reserved:0 length:200

2. SESSION type 7 length 16:

3. Destination, TunnelId 8, Source

4. HOP type 1 length 12: C0A80304

6. TIME_VALUES type 1 length 8 : 00007530

7. EXPLICIT_ROUTE type 1 length 36:

8. (#1) Strict IPv4 Prefix, 8 bytes,

9. (#2) Strict IPv4 Prefix, 8 bytes,

10. (#3) Strict IPv4 Prefix, 8 bytes,

11. (#4) Strict IPv4 Prefix, 8 bytes,

12. LABEL_REQUEST type 1 length 8 : 00000800

13. SESSION_ATTRIBUTE type 7 length 16:

14. setup_pri: 7, reservation_pri: 7 MAY REROUTE

15. SESSION_NAME:gsr4_t8

16. SENDER_TEMPLATE type 7 length 12:

17. Source, tunnel_id 1948

18. SENDER_TSPEC type 2 length 36:

19. version=0, length in words=7

20. service id=1, service length=6

21. parameter id=127, flags=0, parameter length=5

22. average rate=12500000 bytes/sec, burst depth=1000 bytes

23. peak rate = 12500000 bytes/sec

24. min unit=0 bytes, max unit=0 bytes

25. ADSPEC type 2 length 48:

26. version=0 length in words=10

27. General Parameters break bit=0 service length=8

29. Minimum Path Bandwidth (bytes/sec):19375000

30. Path Latency (microseconds):0

31. Path MTU:4470

32. Controlled Load Service break bit=0 service length=0

Table 4-30 analyzes, line by line, the output shown in Example 4-11.

Table 4-30. Discussion of Example 4-11

Line Number(s)



This is the common header, seen in all RSVP messages. The TTL field is the TTL that the original RSVP message was sent with.

2 and 3

This is the SESSION object. It contains the message destination (, the RID of the LSP tail), a Tunnel ID (8), and the tunnel source (, the RID of the LSP headend).

The Tunnel ID is the interface number on the headend tunnel, which is interface Tunnel8.

4 and 5

This is the RSVP_HOP object. It contains the IP address of the interface this message was just transmitted from. In this case, it's C0A80304, or, which is gsr4's interface address between gsr4 and gsr5.


The TIME_VALUES object. This is how often the headend refreshes the tunnel, in milliseconds. 0x7530 is 30,000 milliseconds, or 30 seconds.

7 through 11

These lines are the EXPLICIT_ROUTE object. EXPLICIT_ROUTE (often called the ERO) is the result of the CSPF calculation on the headend. It is a list of hops that this Path message needs to be propagated down. As each hop receives this ERO, it removes itself from the ERO; that's why you don't see any addresses for GSR1 in Figure 4-31.


The LABEL_REQUEST object. This is a signal from the headend that it would like a label returned to it in the RESV that comes back. 00000800 is the Layer 3 PID of the traffic that will be carried in the LSP; 0x0800 is IP, which is what the Cisco IOS Software always signals.

13 through 15

The SESSION_ATTRIBUTE object is optional, but it is always sent by Cisco LERs. The setup and holding priorities are both given here, as are some flags. In this case, the only flag set is MAY REROUTE, which is set by default on all LSPs.

SESSION NAME, although it looks like a separate object, is part of the SESSION_ATTRIBUTE object. It carries a text string describing the tunnel. By default, this string is hostname_tifnum. So you can tell that the tunnel you're looking at is interface Tunnel8 on gsr4. However, if the description command is used on the headend tunnel configuration, that description is carried in this field instead.

16 and 17

The SENDER_TEMPLATE object contains information about the originator of the Path message. Specifically, it contains the source RID and something labeled tunnel_id. This label is incorrect—it should be called the LSP ID. This ID is different from the tunnel ID carried in the SESSION object, but it is used in conjunction with it to do make-before-break. See the "What Is Make-Before-Break?" section for more details. Some show commands also call this number the tunnel instance—same thing, different name.

18 through 24

These are the SENDER_TSPEC. The SENDER_TSPEC is information from the tunnel headend about what kind of traffic it expects to send. Most of the information here is not used by MPLS Traffic Engineering. The only part that is used is the average rate, which is the amount of bandwidth the LSP wants to reserve. It's listed here as 12,500,000 bytes, which is 100,000,000 bits. So this tunnel is making a 100-Mbps bandwidth reservation, which matches the configuration shown for the tunnel.

25 through 32

The ADSPEC is an optional object, but it is always sent by Cisco IOS Software. It contains information about the path that the LSP is taking.

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