RSVP DSBM (Designated Subnetwork Bandwidth Manager)

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

Configuration-wise, it’s easy to implement DSBM. Let’s use the following topology to see how it works:

RSVP DSBM

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:

R3(config-if)#ip rsvp bandwidth 2048

I’ll set the maximum bandwidth to 2048 kbit. We can also set some parameters for non-RSVP traffic:

R3(config-if)#ip rsvp dsbm non-resv-send-limit ?
  burst     Maximum burst (Kbytes)
  max-unit  Maximum packet size (bytes)
  min-unit  Minimum policed unit (bytes)
  peak      Peak rate (Kbytes/sec)
  rate      Average rate (Kbytes/sec)

Let’s verify if R3 has won the election:

R1#show ip rsvp sbm detail 

Interface: FastEthernet0/0
Local Configuration             Current DSBM
  IP Address: 192.168.123.1       IP Address: 192.168.123.3
  DSBM candidate: no              I Am DSBM: no
  Priority: 64                    Priority: 64
  Non Resv Send Limit             Non Resv Send Limit
    Rate: unlimited                 Rate: 2147483 Kbytes/sec
    Burst: unlimited                Burst: 536870 Kbytes
    Peak: unlimited                 Peak: unlimited
    Min Unit: unlimited             Min Unit: unlimited
    Max Unit: unlimited             Max Unit: unlimited
R2#show ip rsvp sbm detail 

Interface: FastEthernet0/0
Local Configuration             Current DSBM
  IP Address: 192.168.123.2       IP Address: 192.168.123.3
  DSBM candidate: no              I Am DSBM: no
  Priority: 64                    Priority: 64
  Non Resv Send Limit             Non Resv Send Limit
    Rate: unlimited                 Rate: 2147483 Kbytes/sec
    Burst: unlimited                Burst: 536870 Kbytes
    Peak: unlimited                 Peak: unlimited
    Min Unit: unlimited             Min Unit: unlimited
    Max Unit: unlimited             Max Unit: unlimited
R3#show ip rsvp sbm detail 

Interface: FastEthernet0/0
Local Configuration             Current DSBM
  IP Address: 192.168.123.3       IP Address: 192.168.123.3
  DSBM candidate: yes             I Am DSBM: yes
  Priority: 64                    Priority: 64
  Non Resv Send Limit             Non Resv Send Limit
    Rate: unlimited                 Rate: 2147483 Kbytes/sec
    Burst: unlimited                Burst: 536870 Kbytes
    Peak: unlimited                 Peak: unlimited
    Min Unit: unlimited             Min Unit: unlimited
    Max Unit: unlimited             Max Unit: unlimited

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:

R1(config)#ip rsvp sender-host 192.168.123.2 192.168.123.1 tcp 23 0 128 64
R2(config)#reservation-host 192.168.123.2 192.168.123.1 tcp 23 0 ff rate 128 64

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

R3#show ip rsvp installed 
RSVP: FastEthernet0/0
BPS    To              From            Protoc DPort  Sport  
128K   192.168.123.2   192.168.123.1   TCP    23     0

That’s all I wanted to share about DSBM for now.