Hierarchical Routing:

Problem with previous LS and DV Algorithms-

As you saw, in both LS and DV algorithms, every router has to save some information about other routers.

When the network size grows and number of routers in network and consequently size of routing tables increase, routers can’t handle network traffic as efficient as they should.

Solution: ........hierarchical routing

Hierarchical Routing Algorithm

  • routers are classified to groups known as “Regions”.
  • each router just has information about the routers in its region and has no information about other routers.
  • Instead routers just save one record in their table for every other region.

 

Example


Consider the following network graph. If we use a DV algorithm, each node of the network has to have a routing table with 17 entries

Original Network and DV routing Table for node A.

Destination Line Weight
A ----- -----
B B 1
C C 1
D B 2
E B 3
F B 3
G B 4
H B 5
I C 5
J C 6
K C 5
L C 4
M C 4
N C 3
O C 4
P C 2
Q C 3

1) Group Network into Regions, here we do 5.

2)From Routing Table for each node in each Region. Example for node A. We only record info about other routers in the same region and also the best single hop to the
"bridge" to other regions in the network.

Destination Line Weight
A ---- ----
B B 1
C C 1
Region 2 B 2
Region 3 C 2
Region 4 C 3
Region 5 C 4

 

Multi-level Hierarchical Routing

This example shows a 2 level hierarchical routing. We can also use 3 or 4 level hierarchical routing. In 3 level hierarchical routing, network classified to number of “Clusters”. Each cluster is made up number of regions (and each region contains a number of routers).

Internet Routing

Hierarchical routing (variations of) is widely used along with other routing protocols.

 

 

© Lynne Grewe