Make-before-break

Make-before-break is a mechanism to change an MPLS TE tunnel with minimum data loss and without using extra bandwidth.

Traffic forwarding is interrupted if the existing CRLSP is removed before a new CRLSP is established. The make-before-break mechanism makes sure that the existing CRLSP is removed after the new CRLSP is established and the traffic is switched to the new CRLSP. However, this might waste bandwidth resources if some links on the old and new CRLSPs are the same, because you need to reserve bandwidth on these links for both the old and new CRLSPs. The make-before-break mechanism uses the SE resource reservation style to address this problem.

The resource reservation style refers to the style in which RSVP-TE reserves bandwidth resources during CRLSP establishment. The resource reservation style used by an MPLS TE tunnel is determined by the ingress node, and is advertised to other nodes through RSVP.

The device supports the following resource reservation styles:

• FF—Fixed-filter, where resources are reserved for individual senders and cannot be shared among senders on the same session.

• SE—Shared-explicit, where resources are reserved for senders on the same session and shared among them. SE is mainly used for make-before-break.

As shown in Figure 19, a CRLSP with 30 M reserved bandwidth has been set up from Router A to Router D through the path Router A—Router B—Router C—Router D.

To increase the reserved bandwidth to 40 M, a new CRLSP must be set up through the path Router A——Router E—Router C—Router D. To achieve this purpose, RSVP-TE needs to reserve 30 M bandwidth for the old CRLSP and 40 M bandwidth for the new CRLSP on the link Router C—Router D, but the link bandwidth is not enough.

Using the make-before-break mechanism, the new CRLSP can share the bandwidth reserved for the old CRLSP. After the new CRLSP is set up, traffic is switched to the new CRLSP without service interruption, and then the old CRLSP is removed.