Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 40)

16 Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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Traffic engineering database
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When the LSP passes admission control, the ingress LER sends a Path message to the
address at the top of the ERO list. This is the address of a physical interface on the next LSR in
the path. As the ingress LER did, this LSR performs admission control to make sure the
outbound interface has enough reservable bandwidth to accommodate the LSP.
When the LSP passes admission control, the LSR then removes its address from the top of the
ERO list and sends the Path message to the address now at the top of the ERO list. This
process repeats until the Path message reaches the last node in the ERO list, which is the
egress LER.
3. The egress LER receives the Path message and sends a Resv message towards the ingress
LER.
Resv messages flow upstream from the receiver of the Path message to the sender (that is,
from the egress LER to the ingress LER), taking the exact reverse of the path specified in the
ERO. In response to the LABEL_REQUEST object in the Path message, the Resv message from
the egress LER includes a LABEL object. The LABEL object is used to associate labels with
interfaces on the LSRs that make up the LSP.
4. As the Resv messages travel upstream, resources are reserved on each LSR.
When an LSR receives a Resv message, it again performs admission control on the interface
where the Resv message was received (that is, the interface that is the outbound interface for
packets travelling through the LSP). When the LSP still passes admission control, bandwidth is
allocated to the LSP. The LSR allocates the amount of bandwidth specified by the LSPs
mean-rate setting, using bandwidth available to its hold priority level. This may cause lower
priority LSPs active on the device to be preempted.
Once bandwidth has been allocated to the LSP, the LABEL object in the Resv message is used
to associate labels with interfaces in the LSRs MPLS forwarding table.
Figure 7 shows an
example of how this works.
FIGURE 7 How the RSVP LABEL object associates a label with an interface in the MPLS
forwarding table
In the example above, the LSR receives a Resv message on interface 3/1 from the
downstream LSR in the ERO. The Resv message has a LABEL object containing label 456. After
performing admission control and bandwidth allocation, the LSR adds an entry to its MPLS
forwarding table for this LSP, associating label 456 with outbound interface 3/1.
The LSR then takes a label from its range of available labels (for example, 123) and places it in
the LABEL object in the Resv message that it sends to the upstream LSR. In this example, the
LSR sends the Resv message out interface 2/1 to the upstream LSR in the ERO. In its MPLS
forwarding table for this LSP, the LSR associates label 123 with inbound interface 2/1.