Hi Lokesh,
There are a couple of potential issues.
- You can blackhole traffic. For example let’s say you have these 4 networks behind a router:
192.168.0.0 /24
192.168.1.0 /24
192.168.2.0 /24
192.168.3.0 /24
If you would create a summary like 192.168.0.0 /20 then basically you are advertising the 192.168.0.0 - 192.168.15.0 range to other routers. When your router receives a packet with destination 192.168.6.6 or something it will drop it.
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Asymmetric routing, This one is harder to explain with text only. When you advertise a summary it’s possible that other routers will select a different (more specific) path as the next hop.
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Traffic engineering: If you use a routing protocol then you can play with the different metrics for each prefix. If you advertise a summary instead of all the different prefixes then you won’t be able to change metric (or BGP attributes) for a single prefix.
Rene