How the Correct MTU Is Signaled in RSVP

How the correct MTU is signaled in RSVP varies depending on whether the network devices (for example, routers) explicitly support MTU signaling in RSVP or not.

If the network devices support MTU signaling in RSVP, the following occur when you enable MTU signaling:

• The MTU is signaled from the ingress router to the egress router by means of the Adspec object. Before forwarding this object, the ingress router enters the MTU value associated with the interface over which the path message is sent. At each hop in the path, the MTU value in the Adspec object is updated to the minimum of the received value and the value of the outgoing interface.
• The ingress router uses the traffic specification (Tspec) object to specify the parameters for the traffic it is going to send. The MTU value signaled for the Tspec object at the ingress router is the maximum MTU value (9192 bytes). This value does not change en route to the egress router.
• When the Adspec object arrives at the egress router, the MTU value is correct for the path (meaning it is the smallest MTU value discovered). The egress router compares the MTU value in the Adspec object to the MTU value in the Tspec object. It signals the smaller MTU using the Flowspec object in the Resv message.
• When the Resv object arrives at the ingress router, the MTU value in this object is used as the MTU for the next hops that use the LSP.

In a network where there are devices that do not support MTU signaling in RSVP, you might have the following behaviors:

• If the egress router does not support MTU signaling in RSVP, the MTU is set to the value of the outgoing interface on the ingress router. Setting the MTU to the value of the outgoing interface is the same as the default behavior when MTU signaling is not configured.
• A Juniper Networks transit router that does not support MTU signaling in RSVP always propagates an MTU value of 1500 in the Adspec object.

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