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:
- The router with the highest priority becomes the DSBM.
- If the priority is the same, the highest IP address is the tie-breaker.
Multiple DSBMs can be configured for a shared segment, but DSBM is non-preemptive. This means that once the election is over, the elected router 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 three routers connected to the same switch. First, we will enable RSVP on all interfaces:
R1(config)#interface FastEthernet 0/0 R1(config-if)#ip rsvp bandwidth
R2(config)#interface FastEthernet 0/0 R2(config-if)#ip rsvp bandwidth
R3(config)#interface FastEthernet 0/0 R3(config-if)#ip rsvp bandwidth
Now we’ll configure R3 as the DSBM for this segment:
R3(config)#interface FastEthernet 0/0 R3(config-if)#ip rsvp dsbm candidate
If you want, you can configure the DSBM to tell other RSVP routers to limit the reservations: