Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

two questions about bgp -

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 -

  • Other Network Infrastructure Subjects
5 REPLIES
Gold

Re: two questions about bgp -

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

New Member

Re: two questions about bgp -

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 -

New Member

Re: two questions about bgp -

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

New Member

Re: two questions about bgp -

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--

noc
New Member

Re: two questions about bgp -

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.

121
Views
0
Helpful
5
Replies
This widget could not be displayed.