EIGRP Pacing - What's it ?

I don't know how many of you have ever noticed/queried about eigrp pacing when it comes to EIGRP.  Further questions like

1. Why do we need it ? In which case it's essential ?
2. How does it prevent WAN link overload ?

When we do "show ip eigrp interface" we see a column Pacing Time Un/Reliable as shown below
router#show ip eigrp interface
IP-EIGRP interfaces for process 1
                             Xmit Queue                Mean             Pacing Time           Multicast
Interface   Peers     Un/Reliable                SRTT             Un/Reliable           Flow Timer
Se0            3             0/0                           288                  10/380               2832
Se1            0             0/0                            0                       0/10                   0

What's this Pacing time ?
Alright i got it , it means how it paces the hello , update and queries across the links. Right ? (with little grin on my face )

Partially right!! Lets see what it is ?

Why do we need EIGRP pacing ?

EIGRP can easily overload the WAN links in scenarios where several neighbors are connected to the same router using a serial link over a single low speed frame-relay network. Alternatively, the central router with a high speed link can overload the link between the switched WAN network and the access router  with routing updates causing link congestion and packet drops.EIGRP pacing was introduced in IOS maintenance releases 10.3(11), 11.0(8),and 11.1(3) to prevent these phenomena and to guarantee that EIGRP never uses more
than an operator-specified percentage of the WAN link bandwidth.

In which case it's essential ?

EIGRP pacing is really important when we have large number of neighbors are reachable over the same physical interface. Otherwise, inbuilt Transport mechanism in EIGRP prevents congestion because it uses a window size of 1.

How does it prevent WAN link overload ?

-By emitting the EIGRP packets on to the WAN link in predefined time intervals. Ofcourse, you need to configure the correct bandwidth over the interfaces using bandwidth command & the amount of bandwidth available for eigrp using ip bandwidth percentage.

The pacing time intervals vary for Reliable packets(Update,query,reply) and unreliable packets(hello,ack).The pacing intervals are computed from the interface bandwidth.

The minimum value for reliable pacing interval is 10 msec and there is no minimum value for unreliable pacing interval.
It's computed separately for every interface and subinterface so, its very importanct to set appropriate bandwidth over the physical and logical interface otherwise the combined EIGRP traffic sent over a physical interface might overload the WAN link even though the traffic on each subinterface created over the
physical interface is properly paced.