Prefix Matching Routing Table
I have to build a smallest routing table using longest prefix matching with an address pool of 1024 addresses. Longest prefix match is an algorithm to lookup the ip prefix which will be the destination of the next hop from the router.
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Prefix matching routing table. The prefix in the routing table defines how many leftmost contiguous bits to use when matching a packets destination address to a destination network prefix. Nowadays in software based routers this is done by creating a prefix tree index over a routing table performing a longest prefix match in this tree and using the located information to forward the packet cisco calls this the cef. What i have so far is this.
The longest prefix match always wins among the routes actually installed in the routing table while the routing protocol with the lowest administrative distance always wins when installing routes into the routing table. Longest prefix is describing how many bits in the destination address match how many bits in the entries in the routing table. The routing table each router stores ip prefix and the corresponding router.
Longest prefix match also called maximum prefix length match refers to an algorithm used by routers in internet protocol ip networking to select an entry from a forwarding table. Routing is primarily concerned with lookups trying to find a matching entry in a routing table. The most specific of the matching table entries the one with the longest subnet mask is called the longest prefix match.
Because each entry in a forwarding table may specify a sub network one destination address may match more than one forwarding table entry. For example a route table entry of. Longest prefix matching compacting routing tables duration.
This algorithm is used to find the prefix matching the given ip address and returns the corresponding router node. So if the routing table has the entries that you describe in your original post 24 22 16 and 8 then 24 is longer than 22 and 22 is longer than 16 and 16 is longer than 8. The rule is to find the entry in table which has the longest prefix matching with incoming packets destination ip and forward the packet to corresponding next hope.
To handle above situation routers use longest prefix matching rule. 32 goes to a 48 to b 48 to c 128 to d.
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