4000 Router Series MPLS Configuration Guide

Make-before-break

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

using extra bandwidth.

In cases of tunnel reoptimization, 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 style, where resources are reserved for individual senders and cannot be shared

among senders on the same session.

• SE—Shared-explicit style, where resources are reserved for senders on the same session and shared

among them. SE is mainly used for make-before-break.

Figure 19 Diagram 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 30M bandwidth for

the old CRLSP and 40M 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.

Route pinning

Route pinning enables CRLSPs to always use the original optimal path even if a new optimal route has

been learned.

On a network where route changes frequently occur, you can use route pinning to avoid re-establishing

CRLSPs upon route changes.