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RSVP DSBM (Designated Subnetwork Bandwidth Manager)

en January 03 , 2018
RSVP will work fine when you need to make a reservation on the link between two routers, but what if you have a shared segment? An example could be a couple of routers that is connected to the same half-duplex Ethernet network. These routers will share the bandwidth so when multiple routers make an RSVP reservation it’s possible that we oversubscribe.
The routers should know about all RSVP reservations that are made on this shared segment and that’s exactly why we have the DSBM (Designated Subnetwork Bandwidth Manager).
One of the routers on the shared segment will be elected as the DSBM and all other RSVP routers will proxy their RSVP PATH and RESV messages through the DSBM. This way we will have centralized admission control and we won’t risk oversubscribing the shared segment.
Besides being in charge of admission control, the DSBM can also distribute other information to RSVP routers, for example the amount of non-reservable traffic that is allowed in the shared segment or the average/peak rate and burst size for non-RSVP traffic.
The election to become the RSVP DSBM uses the following rules:
Multiple DSBMs can be configured for a shared segment but DSBM is non-preemptive. This means that once the election is over, the router that was elected will stay as the DSBM even if you configure another router later with a higher priority.
Configuration-wise it’s easy to implement DSBM, let’s use the following topology to see how it works:
Just 3 routers connected to the same switch. First we will enable RSVP on all interfaces:

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If you want, you can configure the DSBM to tell other RSVP routers to limit the reservations:

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I'll set the maximum bandwidth to 2048 kbit. We can also set a number of parameters for non-RSVP traffic:

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With R3 as the DSBM it will be in the middle of all RSVP messages. We can test this by configuring a reservation between R1 and R2:

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When we check R3 you can see that it knows about the reservation that we just configured:

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