Priority-queuing in Action

Too see how priotiy-queuing impacts traffic flows, I set up a small lab. R2, R3 and R4 connect to the same LAN as R5. R5 connects to R6. Each router has a loopback x.x.x.x where x is the router number.

R2
\
R3-------f0/0 R5 s1/1 ----R6
/
R4

On R5 we make 3 ACLs:

R5(config)#access-list 102 permit icmp host 2.2.2.2 host 6.6.6.6
R5(config)#access-list 103 permit icmp host 3.3.3.3 host 6.6.6.6
R5(config)#access-list 104 permit icmp host 4.4.4.4 host 6.6.6.6

Now we assign each ACL to a priority-list

R5(config)#priority-list 1 protocol ip high list 102
R5(config)#priority-list 1 protocol ip medium list 103
R5(config)#priority-list 1 protocol ip normal list 104

Assign the priority-list to the interface with the priority-group command:

R5(config)#int s1/1
R5(config-if)#priority-group 1

Now let's verify the priority queuing is working.

Below we get a baseline and see that R3 normally gets an average round trip time of 34-38 ms for 200 pings. When R2 starts sending traffic, we can see the R3 round trip average (and max time) increase about 50%.

R3 pings, R2 is silent:

R3#ping 6.6.6.6 source 3.3.3.3 repeat 200

Type escape sequence to abort.
Sending 200, 100-byte ICMP Echos to 6.6.6.6, timeout is 2 seconds:
Packet sent with a source address of 3.3.3.3
Success rate is 100 percent (200/200), round-trip min/avg/max = 4/34/188 ms

R3#ping 6.6.6.6 source 3.3.3.3 repeat 200

Type escape sequence to abort.
Sending 200, 100-byte ICMP Echos to 6.6.6.6, timeout is 2 seconds:
Packet sent with a source address of 3.3.3.3
Success rate is 100 percent (200/200), round-trip min/avg/max = 4/38/104 ms

R3 and R2 both send pings (I started sending 250 1200-byte packets from R2 first, then quickly hopped over to R3 and started the ping)

R3#ping 6.6.6.6 source 3.3.3.3 repeat 200

Type escape sequence to abort.
Sending 200, 100-byte ICMP Echos to 6.6.6.6, timeout is 2 seconds:
Packet sent with a source address of 3.3.3.3
Success rate is 100 percent (200/200), round-trip min/avg/max = 4/57/220 ms

R3#ping 6.6.6.6 source 3.3.3.3 repeat 200

Type escape sequence to abort.
Sending 200, 100-byte ICMP Echos to 6.6.6.6, timeout is 2 seconds:
Packet sent with a source address of 3.3.3.3
Success rate is 100 percent (200/200), round-trip min/avg/max = 8/66/368 ms

Notice how much higher the avg and max times are for R3 when R2 is pinging R6. We can see directly how priority-queuing impacts traffic flows.