H3C Technologies H3C S6800 Series Switches - Manual (Page 69)

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• Forwarding adjacency—Considers the MPLS TE tunnel as a link that directly connects the tunnel
ingress node and the egress node and advertises the link to the network through an IGP, so every
node in the network uses the MPLS TE tunnel during IGP route calculation.
Figure 21 IGP shortcut and forwarding adjacency diagram
As shown in Figure 21 , an MPLS TE tunnel is present from Router D to Router C. IGP shortcut enables only
the ingress node Router D to use the MPLS TE tunnel in the IGP route calculation. Router A cannot use this
tunnel to reach Router C. With forwarding adjacency enabled, Router A can also know the existence of
the MPLS TE tunnel, so it can use this tunnel to transfer traffic to Router C by forwarding the traffic to
Router D.
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 and automatic bandwidth adjustment, 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 wastes bandwidth resources if some links on the old and new
CRLSPs are the same. It is because you need to reserve bandwidth on these links for the old and new
CRLSPs separately. 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.
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