# What is hierarchical routing?

In hierarchical routing, the routers are divided into regions. Each router has complete details about how to route packets to destinations within its own region. But it does not have any idea about the internal structure of other regions.

As we know, in both LS and DV algorithms, every router needs to save some information about other routers. When network size is growing, the number of routers in the network will increase. Therefore, the size of routing table increases, then routers cannot handle network traffic as efficiently. To overcome this problem we are using hierarchical routing.

In hierarchical routing, routers are classified in groups called regions. Each router has information about the routers in its own region and it has no information about routers in other regions. So, routers save one record in their table for every other region.

For huge networks, a two-level hierarchy may be insufficient hence, it may be necessary to group the regions into clusters, the clusters into zones, the zones into groups and so on.

## Example

Consider an example of two-level hierarchy with five regions as shown in figure −

Let see the full routing table for router 1A which has 17 entries, as shown below −

## Full Table for 1A

Dest.LineHops
1A--
1B1B1
1C1C1
2A1B2
2B1B3
2C1B3
2D1B4
3A1C3
3B1C2
4A1C3
4B1C4
4C1C4
5A1C4
5B1C5
5C1B5
5D1C6
5E1C5

When routing is done hierarchically then there will be only 7 entries as shown below −

## Hierarchical Table for 1A

Dest.LineHops
1A--
1B1B1
1C1C1
21B2
31C2
41C3
51C4

Unfortunately, this reduction in table space comes with the increased path length.

## Explanation

Step 1 − For example, the best path from 1A to 5C is via region 2, but hierarchical routing of all traffic to region 5 goes via region 3 as it is better for most of the other destinations of region 5.

Step 2 − Consider a subnet of 720 routers. If no hierarchy is used, each router will have 720 entries in its routing table.

Step 3 − Now if the subnet is partitioned into 24 regions of 30 routers each, then each router will require 30 local entries and 23 remote entries for a total of 53 entries.

## Example

If the same subnet of 720 routers is partitioned into 8 clusters, each containing 9 regions and each region containing 10 routers. Then what will be the total number of table entries in each router.

## Solution

10 local entries + 8 remote regions + 7 clusters = 25 entries.