Why a summary route is needed?

[teru-lei]

Hi all,

I am thinking about why a summary route is needed. The summary route will let some destination to null0, but also the routing protocol advertise the more specific route, although BGP can use "summary-only" parameter to just let the summary route advertise, but like ospf, what I know to cut the more specific route is to use "distribute-list", it's a trouble to config the access-list for the distribute-list and may be need to modify it when the routing config change. Is there another method to make the routing only advertise the summary-only? And Is the aim of the summary route just for "reduce the entry in the routing ta ble, saving the RAM usage?" Thank You!

Best Regards

Teru Lei

[vincent-n]

I'm no expert (yet) but I'll try to answer your question.

1. You're right about having summary routes and they will reduce the amount of entries in the routing table -> better router utilization BUT

2. you can also use summary routes to hide information (more specific) to places that do not need to know the more specific routes. For instance when you have a hub-spoke network. Generally speaking, you can hide the information of more specific routes to the leaf nodes by advertising only default/summary routes. That is to go anywhere from the leaf nodes, they will have to come through the hub site anyway.

3. I cannot verify for you now (for OSPF) but I know that using EIGRP, I can advertise a summarised address to my leaf nodes and the leaf nodes does not know any other routes apart from directlty connected subnets and the default route (to the hub site)

I'm sure that others can give you a better answer or something else that I've not stated.

[lgijssel]

The previous posting is quite correct. The originator of the question stated that there still were more explicit routes in the routing table. Appearently he/she expects the summary to replace all routes that fall in the specified range. This is generally true, BUT routing updates can come from different sources. Normally every route that traverses the summarizing router is not forwarded but summarized. Routes pointing to another direction still need to be in the table, otherwise routing loops would easily occur.

The effects of summarization are not observed on the whole network, but only on devices connected through a summarizing interface.

Also remember that summarization works PER protocol. An OSPF or RIP originated route will not be summarized (or processed othwerwise) by EIGRP.