03-29-2002 12:47 AM - edited 03-01-2019 09:05 PM
hello-
first question -
When the router of isp recieve an ip address - does
the first thing it do is to check to which AS it belongs ?
is it has a cache where there is map of AS numbers and scopes of ip's ?
how it knows to which AS the ip address belongs?
second queastion -
does any isp has uniq AS, or maybe any isp who connect to the backbone has a uniq AS ?
thanks you -
03-29-2002 06:08 AM
The as which originated the prefix is carried in the packet, along with the as you are receiving the prefix from. There is no cache or other table which tells the router which ip address belongs to which as--that's what BGP's job is, to provide this information. :-)
Each ISP has a unique as, assigned by a set of central numbering authorities. Organizations can also get their own as, by applying to these same numbering authorities.
Russ
03-29-2002 07:57 AM
thanks for your answer -
but i didn't actually understand it.
how bgp knows to which as the ip belong ?
is this information is find in the packet ?
what you mean by saying "prefix" ?
hmm-
thanks again -
03-29-2002 01:57 PM
Russ, answered the question.
Again, when an ip address block is announced it carries the BGP AS number with it.
Prefix is another term used when describing ip address blocks. eg: 10.0.0.0/16
JK
03-30-2002 04:42 AM
thank you for the answer -
i understand that when you create block you add to it
as number.
but when a packet is arriving to arouter.
does in the packet itself there is the as number ?
i know that a packet contain - source address destination address, data , and more.
does it contain also as number ?
when "the bits that describe" the as number are added ?
sorry for disturbing you .
but i have to unferstand once and for all.
--bye--
03-31-2002 04:02 AM
AS NUMBERS and BGP ROUTING is how routers in the internet decide where to route IP packets. BGP is used to building the routing table. Thats all.
Every packet that comes into the router is forwarded
to the entry for that prefix (range of ip's) in the routing table. AS numbers have nothing to do the routing process. Each packet is not inspected, and each packet dosent contain an AS number. only BGP
packets used between routers running BGP contain
AS information, These are special packets passed
by bgp peers to as part of the bgp process.
Bottom Line: Do Not confuse the BGP route deciding
process (or any routing protcol, or static routes)
from the routing routers do once they have learned
where to route.
BGP - Decide where to forward packets
Routing - Forwarding packets based on entries in
the routing table.
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