As the size of the N/w increases the entries in the routers routing table increases, this increase causes ‘2’ things to increase
- Memory consumed by Routing Tables.
- CPU processing time required to scan the entries in Routing Tables also increases.
and also the Band width needed also increases.
at particular point it is not possible almost all for a router to maintain routing tables still the size increases.
So the possible solutions for this is to use Hierarchial routing.
In this Hierarchial routing there are regions. The regions consists of no.of routers and he routers in a region are aware of how to route packets in it’s own regional routers but nothing about internal structure of other regions.
Hierarchial routing may be ‘2’ level Hierarchy as shown in the given figure.
Initially assume we don’t have Hierarchial routing that there exists 17 routers 1A, 1B, 2A, 2B, 2C, 2D……………….5E and the routing table for all these 17 routers by choosing no.of hops and destination line as parameters is given in the figure.
from router 1A to reach router 1B, it uses line 1B itself and the no.of hops are ‘1’. Similarly, from route 1A to router 4B it uses line 1C and no.of hops are ‘4’ as 1A to 1C to 3B to 4A to 4B.
if we use Hierarchial routing the no.of entries previously 17 are reduced to ‘7’. only.
The 17 outers are divided into 5 regions with region having some no.of routers.
If we observe the table , the table consists of 7 entries , the destination as region 2,3,4,5 but not routers 2A, 2B,…..etc.
but for it’s own region it is aware of other routers 1A, 1B, 1C from 1A to reach region 4 it uses line 1C and no.of hops are 3……..
even there is a problem with Hierarchial routing is choosing the best path based on path lengths.
the best from 1A to 5C is Via region 2 rather than Via region 3. If N/w size increases we go for other levels of Hierarchy that is a 3 level Hierarchy.
Kamour and Kleinrock (1979) discovered that the no.of levels for a N router subnet is entries for a router .