I don't understand the logic behind these prefixes

[hfakoor222]

Slide 64 - 67 is conveying outbound load balancinf/multi-homing using prefixes

However to me the prefix in slide 3seems redundant, but I don't believe I understand it properly 

Here 

ip prefix-list ISP-B permit 100.64.0.0/19
ip prefix-list ISP-B permit 100.64.0.0/21
!
ip prefix-list ISP-C permit 100.64.0.0/19
ip prefix-list ISP-C permit 100.64.28.0/22

doesn't   ip prefix-list ISP-B permit 100.64.0.0/19 aggregate already include ip prefix 100.64.0.0/21

so why is it being define both  /19   and   /21  in the

 prefix-list

command?

wouldn't setting it once with    100.64.0.0/19  achieve the same objective   as 21 is longer and included in /19?

[Jon Marshall · Accepted solution]

No because they are exact matches so 100.64.0.0/19 only matches 100.64.0.0 255.255.224.0 and nothing else. 

If you want to match both you have to have two lines or you could use - 

100.64.0.0/19 le 21 

but that would also include 100.64.0.0/20 which you may or may not want. 

Jon

[MHM Cisco World]

simply 
we do load sharing between two ISP
so we split the original subnet in two 
we advertise one via /20 ISP1 and other via /21 ISP2 
and to get failover we also advertise the original /19 Subnet to both ISP.

[MHM Cisco World]

keep notice that the longest match is wining when Router want to FORWARD TRAFFIC 
and because the split is longest match it win.

[hfakoor222]

Thanks. I am rereading the slides for a second time so I will look out for that when I get further

[MHM Cisco World]

when check next slide of advanced BGP and see my comment can be  solution please mention that.
great job friend 
keep study