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BSCN question about selection of route summaries

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xyyz

Diamond Member

man what I wouldn't do to get a BSCN book that treats the reader like a total idiot... instead of the two i've gone through that assumes the reader understands the 70% of the explanation that is omitted.

alright this is from the cisco press book: "Building Scalable Cisco Networks"

pg.80 top



<< Selecting routes from route summaries - If more than one entry in the routing tale matches a particular destination, the longest prefix match in the routing table is used. Seeral roures might match one destination, but the logest matching prefix is used.

For example, if a routing tables has the paths shown in Figure 2-8, packets addressed to destination 172.16.5.99 would be routed through the 172.16.5.0/24 path because that address has the longest match with the destination address."

Figure 2-8 Routers will use the longest match when selecting a route

172.16.5.33 - /32 - Host
172.16.5.32 - /27 - Subnet
172.16.5.0 - /24 - Network
172.16.0.0 - /16 - Block of networks
0.0.0.0 - /0 - Default
 >>



what the hell is going on here... I'm lost... okay I get that the route that has the longest prefix is chosen... does anyone care to tell me why?

more importantly though, how does this example work? wouldn't the longest subnet be the subnet that the host belongs to... umm what would it be... errr 172.16.5.96/27?

I'm lost as to why 172.16.5.0/24 is used when you can use the subnet that the host belongs to... that subnet has 3 extra prefix bits than what they've used right now.
 
If you look more closely you'll see that the /27 and /32 route entries do not encompass that IP address. The /24 will be chosen.

172.16.5.36 would follow the /27 route.

make sense? if not the write out the network and broadcast addresses for the /27. then you'll see why that route would not be used for that host.
 
Try to think of it as the "most specific route". 172.16.0.0 - /16 includes the destination 172.16.5.99, but a /16 is a ton of IP's. We could narrow it down quite a bit to 172.16.5.0 - /24, which also includes the destination 172.16.5.99 (includes up to 172.16.5.255)
172.16.5.33 - /32 is only that single IP
172.16.5.32 - /27 - Subnet is 172.16.5.32 - 172.16.5.64, so 172.16.5.99 isn't included in either one of those
0.0.0.0 -/0 Default is used when there is no other route available
 
Ok and how can u have those 2 subnets working together?(172.16.5.32 - /27 - Subnet, 172.16.5.0 - /24 - Network)
 
Anything that doesn't match the 27 bit subnet exactly will be on the 24 bit subnet.

This is a very ugly way of doing the routing because it gets confusing but I think they are just saying that Cisco has the logic to route if there is overlap in subnets. I am sure this is not the way any network should be setup.

Think of it as pattern matching and which pattern is closer matching in an overlap wins.
 
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