What information does AS PATH contain in this case?

Question

http://www.cidr-report.org/cgi-bin/as-report?as=AS50999&v=4&view=2.0

In the announced prefixes section, you can see:

Prefix               AS Path                              Aggregation Suggestion
 109.171.128.0/17     6939 6762 48237 35819 50999  
 109.171.128.0/18     4777 2497 3356 50999   
 109.171.128.0/21     6939 1299 3356 50999 + Announce - aggregate of 109.171.128.0/22 (6939 1299 3356 50999) and 109.171.132.0/22 (6939 1299 3356 50999)

So every row is a group of IP addresses from this AS (autonomous system). If someone from outside wants to communicate with an IP from that group, it needs to go enter that AS.

Now, so far I thought every AS consists of BGP routers. If they want to communicate with a different AS, they look in the routing table and check the AS PATH attribute of the specified subnet. This way, they know which AS to talk to to reach the destination AS.

With this in mind, what's the meaning of AS Path attribute from the quote above? It looks like the AS stores information about how others can reach it, and 'broadcasts' that information to others, using BGP protocol. Is that right?

By the way, some of the addresses on that site are marked green or red. Any idea what does that mean?

Answer 1

You seem to have some misconceptions about BGP operation. Routing works the same regardless of the protocol -- the router looks up the destination address, finds the best (longest) match, and then forwards that packet to the next hop router.

The AS PATH is one of BGP's metrics for choosing a particular path over other matching ones. The AS PATH you see is the list of ASes to get to the destination network, from the point of view of the website you linked to (Univ of Delaware). The AS PATH will be different from the point of view of other ASes. You can look at others from this link.

As the website explains, the green routes have been added and the red ones withdrawn since the last report.

Answer 2

The AS_Path attribute shows the hops (ASes through which the traffic must travel) to reach the the prefix. This attribute is one of several which BGP may use to determine the best path to reach that prefix.

The prefixes in red indicate that the prefix was withdrawn, and the ones in green indicate a new prefix announcement.

BGP doesn't broadcast anything, it uses TCP to connect to specifically configured neighbors.

Answer 3

The meaning of AS Path attribute on the BGP protocol is a way to the router to know how far is some network located. If you are studying BGP, i guess you are familiar with RIP, OSPF, ISIS and so on. RIP works by hops, router know how to reach network by hops, OSPF works with bandwith, with someting called cost, on the reference of the link bandwiths that the networks are connected, well, BGP works with the AS-PATH, bigger or shorter AS-PATH, is something like the way to reach some networks.

If some networks are marked with red and another with green, i think this is to show you what is the best i guess, here i dont see thats colors, on routers only see wich is the best, which are valid routes and that kind of stuff.

Here you can fin how BGP makes a selection of the best route to work:

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

On your reading, you will see where it looks the "AS PATH".

And, on reference to "broadcast to his neighbors",.. remember, broadcast is the mechanins to send the same info to all equipments in SOME NETWORK. BGP neighbors are mainly on different network, but if you want to say that the router send learned networks to other BGP neighbors, the you are rigth.