techlanguageblog

Please follow the link below for a quick reference to refresh our knowledge on the presence of different LSA types in all types of OSPF area.

https://techlanguageblog.files.wordpress.com/2012/02/ospf-lsas-and-area-types.jpg

Route Summarization aggregates multiple individual IP routes into a single summary route. For example 172.16.1.0/22 is a summary route which includes following individual routes.

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

or any other possible subnets within the range 172.16.1.0 – to 172.16.3.255

Some of the major facts on route summarization are as follows.

1)  Summary route advertises only aggregate or summarized route of the subnets. It does not advertise individual subnets.

2)  The summary route will not get advertised by the router when its routing table does not find any of the individual subnets belongs to the summary route.

3)  The router that does summarization creates a local route to the summary, with destination null0, certainly to prevent routing loops.

4)  Route summarization reduces the size of topology database and routing table. Therefore, indirectly improving convergence in the network.

5)  Route summarization decreases the amount of specific information of routing tables. Therefore, suboptimal paths for some destination networks can be resulted.

6)  The route metric assigned to a summary route is the lowest metric taken from its all individual routes.

7)  The ultimate summary address is the default address 0.0.0.0/0.

Routing protocols has their specific set of methods to configure and deal with route summarization. Let’s understand some points here.

 

Network Route Summarization with EIGRP

1)  EIGRP uses “ip summary-address eigrp <as-number> <network address> <subnet mask>” interface level command to configure route summarization.

2)  EIGRP router advertises summary route out of the interface configured for route summarization when any of the individual routes belongs to that summary is found in its routing table.

3)  The router doesn’t advertise AD with the route. The configured AD is used by the summarizing router to determine whether the null route for the summary should be placed into the routing table.

4)  The EIGRP uses default AD 5 for summary routes.

 

Network Route Summarization with RIP

1)  Route summarization in RIP is almost similar to EIGRP with some limitations due to the RIP defined boundaries.

2)  The command “ip summary-address rip <ip-address> <ip-network-mask>” is used on interface level to summarize the routes in RIP.

3)  RIP router advertises summary route out of the interface configured for route summarization when any of the individual routes belongs to that summary is found in its routing table.

4)  When summary route gets advertised, individual routes belongs to that summary route doesn’t gets advertised by the interface configured for route summarization.

5)  RIPv1 does not support VLSM. Therefore, RIP route summarization works only with RIPv2.

6)  RIP does not support supernetting. The command “ip summary-address rip 172.20.0.0 255.254.0.0 which would combine two class B networks into one summary network is not supported by RIP.

7)  RIP never allows a router to create two summary routes out of its interface. It means on any particular router’s interface, only one ip summary-address rip command is allowed per classful network.

 

Network Route Summarization with OSPF

1)  OSPF uses following two command to configure summary routes depending on whether the summary is for inter-area or external routes.

For inter-area summary “area <area-id> range <ip-address mask> <advertise|not-advertise> cost<cost> command is used.

For external summary “summary-address <address mask> [not advertise] tag <tag>” command is used.

2)  The area specified in “area range” command is the area where the individual subnet resides.

3)  Instead of using the lowest cost of all individual routes used in the summary, area range command can manually set the cost for the summary route.

4)  The “not-advertise” keyword in area range command can essentially be used to filter the subnets implied by the summary.

5)  Similarly “not-advertise” keyword is used in summary-address command is to filter the individual routes when required.