aggregate-address in BGP

Taken from:

http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_a1g.html#wp1037074

Usage Guidelines

You can implement aggregate routing in BGP and multiprotocol BGP either by redistributing an aggregate route into BGP or multiprotocol BGP, or by using this conditional aggregate routing feature.

Using the aggregate-address command with no keywords will create an aggregate entry in the BGP or multiprotocol BGP routing table if any more-specific BGP or multiprotocol BGP routes are available that fall in the specified range. The aggregate route will be advertised as coming from your autonomous system and will have the atomic aggregate attribute set to show that information might be missing. (By default, the atomic aggregate attribute is set unless you specify the as-set keyword.)

Using the as-set keyword creates an aggregate entry using the same rules that the command follows without this keyword, but the path advertised for this route will be an AS_SET consisting of all elements contained in all paths that are being summarized. Do not use this form of the aggregate-address command when aggregating many paths, because this route must be continually withdrawn and updated as autonomous system path reachability information for the summarized routes changes.

Using the summary-only keyword not only creates the aggregate route (for example, 193.*.*.*) but also suppresses advertisements of more-specific routes to all neighbors. If you want to suppress only advertisements to certain neighbors, you may use the neighbor distribute-list command, with caution. If a more-specific route leaks out, all BGP or multiprotocol BGP routers will prefer that route over the less-specific aggregate you are generating (using longest-match routing).

Using the suppress-map keyword creates the aggregate route but suppresses advertisement of specified routes. You can use the match clauses of route maps to selectively suppress some more-specific routes of the aggregate and leave others unsuppressed. IP access lists and autonomous system path access lists match clauses are supported.

Using the advertise-map keyword selects specific routes that will be used to build different components of the aggregate route, such as AS_SET or community. This form of the aggregate-address command is useful when the components of an aggregate are in separate autonomous systems and you want to create an aggregate with AS_SET, and advertise it back to some of the same autonomous systems. You must remember to omit the specific autonomous system numbers from the AS_SET to prevent the aggregate from being dropped by the BGP loop detection mechanism at the receiving router. IP access lists and autonomous system path access lists match clauses are supported.

Using the attribute-map keyword allows attributes of the aggregate route to be changed. This form of the aggregate-address command is useful when one of the routes forming the AS_SET is configured with an attribute such as the community no-export attribute, which would prevent the aggregate route from being exported. An attribute map route map can be created to change the aggregate attributes.

BGP Lab

Interesting things I discovered during the BGP Lab:

- when configuring "bgp router-id x.x.x.x" command on a BGP speaker that has a session with another BGP speaker, thet BGP session is torn down, and reestablished. In OSPF there was a need for "clear ip ospf process"

- when aggregating in BGP with "aggregate-address x.x.x.x mask y.y.y.y" the prefix that was generated has weight 32768, and has the ATOMIC-AGGREGATE (well-known discretionary) attribute present. This attribute states that during the aggregation some information was lost (e.g. AS_PATH information)

- when aggregating in BGP with "aggregate-address x.x.x.x mask y.y.y.y as-set" the prefix has an AS_SET made of all pe AS_PATH values (UNORDERED) in all the prefixes aggregated and, because no information was lost during aggregation, the ATOMIC-AGGREGATE attribute is no longer present.

The Sphinx

For the moment I will post only some of the more representative pictures from one of the trips.

















The Sphinx

The church at "Pestera Ialomicioara"

It's finally here - Introduction

Hello everyone!
Finally! My first blog. I hardly believe it myself, but I see that it is time for my blog to see the light of day.

Maybe the timing isn't perfect, it may be too soon, or a little bit too late for me to start this blog!
It doesn't really matter now! I said I would start it and here it goes....

Before I dive into things, I should probably explain my reasons for doing this. I believe it has always been a hidden dream of mine of writing something, anything, and probably this "desire" or "instinct" is in everyone of us - as a way of making ourselves better known, more familiar to people who surround us.

Well, in my own attempt of trying to make me better understood by me and my fellow readers I have come up with this idea of having a blog with the main purpose of describing the trips I have taken and will take around the country and maybe outside its borders. Also, there might be times when I will deem appropriate to bring into focus other topics, non-related to this sort of "expeditions".

And finally, another reason for creating this blog is to have an account of the places I have visited, so that after many, many years from now I will be able to remember the feelings and sensations I have experienced during my attempts to know my own country and have a good time in the meanwhile.