4000 Router Series MPLS Configuration Guide

New objects added to the Resv message include:

• LABEL—Advertises the label allocated by the downstream node to the upstream node.

• RECORD_ROUTE—Records the path that the CRLSP actually traverses and the label allocated by

each node on the path.

CRLSP setup procedure

Figure 28 Setting up a CRLSP

As shown in Figure 28, a CRLSP is set up using the following steps:

1. The ingress LSR generates a Path message that carries LABEL_REQUEST, and then forwards the

message along the path calculated by CSPF hop-by-hop towards the egress LSR.

2. After receiving the Path message, the egress LSR generates a Resv message carrying the

reservation information and the LABEL object, and forwards the Resv message to the ingress LSR

along the reverse direction of the path that the Path message traveled. The Resv message

advertises labels, reserves resources, and creates a reserve state on each LSR it passes, so QoS

can be guaranteed for services transmitted on the CRLSP.

3. When the ingress LSR receives the Resv message, the CRLSP is established.

RSVP refresh mechanism

Refresh messages

RSVP maintains resource reservation states on a node by periodically sending messages.

The resource reservation states include path states and reservation states. A path state is saved in a path

state block (PSB), and a reservation state is saved in a reservation state block (RSB). A PSB is created by

a Path message and saves the LABEL_REQUEST object. A RSB is created by a Resv message and saves

the LABEL object.

The path states and reservation states are refreshed periodically by Path and Resv messages. A state is

removed if no refresh messages for the state are received in a certain interval, and the CRLSP established

based on this state is also removed.

The Path and Resv messages for refreshing the resource reservation states are collectively referred to as

refresh messages. Refresh messages can also be used to recover from lost RSVP messages.

When multiple RSVP sessions exist on a network, the periodically sent refresh messages can cause

network degradation. In this case, the refreshing interval of Path and Resv messages should not be too

short. However, delay sensitive applications want to recover from lost RSVP messages through the refresh

messages as soon as possible. In this case, the refreshing interval should not be too long. You can use the

summary refresh (Srefresh) and the reliable RSVP message delivery functions to find the appropriate

balance.

Ingress Egress

Sender Receiver

Path

Resv