Available Bandwidth Information

Perhaps the most appealing attribute of MPLS Traffic Engineering is the capability to reserve bandwidth across your network. You configure the amount of reservable bandwidth on a link using the following per-interface command:

router(config-if)#ip rsvp bandwidth [<1-10000000 total-reservable-bandwidth> [per-flow-bandwidth]]

This command can take two parameters. The first is the amount of total reservable bandwidth on the interface, in kbps. The second is the maximum amount of bandwidth that can be reserved per flow on the interface. The per-flow maximum is irrelevant to MPLS Traffic Engineering and is ignored. However, the ip rsvp bandwidth command is used for more than just MPLS Traffic Engineering, and the per-flow-bandwidth parameter has relevance to non-MPLS-related RSVP.

NOTE

Currently, RSVP for MPLS TE and "classic RSVP" (for IP microflows) do not play nicely together. You cannot enable an RSVP pool and have it reserved by both RSVP for MPLS TE and classic RSVP. The heart of the issue is that if you configure ip rsvp bandwidth 100 100, both MPLS TE and classic RSVP (as used for Voice over IP, DLSW+, and so on) each think they have 100 kbps of bandwidth available. This behavior is not a feature. If you're going to use RSVP, use it for MPLS TE or IP

signalling; don't use both in your network at the same time.

If you don't configure the ip rsvp bandwidth command, the default reservable bandwidth advertised for that interface is 0. A common cause of tunnels not coming up during testing or initial deployment of MPLS Traffic Engineering is configuring a TE tunnel to require a certain amount of bandwidth but forgetting to configure available bandwidth on any link. See Chapter 11 for more details on how to detect this.

You don't have to specify a value for the total-reservable-bandwidth value in the ip rsvp bandwidth command. If you don't specify a value, the default is 75 percent of the link bandwidth. The link bandwidth is determined by the interface type or the per-interface bandwidth command.

The per-flow maximum defaults to being equal to the total-reservable-bandwidth parameter, but as previously mentioned, it's irrelevant anyway. So fuggetaboutit.

How much bandwidth do you allocate to the interface? That's a larger question than it might seem at first. It has to do with your oversubscription policies and how you enforce them; see Chapter 10, "MPLS TE Deployment Tips," for more information.

You can double-check your configuration using the command show ip rsvp interface, as demonstrated in Example 3-4.

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