need for bgp and AS

[rajeshk200_2]

Hi Dudes,

I am going thro bgp study guides.

i knew that bgp is inter AS ROUTING PROTOCOL. it is used between ISP'S for policy based routing .

my qn is when corpotates (not ISP) need registered AS numbers and need for bgp.

why they should get AS NUMBER ?

what is benefit of using regd AS NUMBER ?

why ISP'S send their routes to customer's network thro'bgp? whatz the need for it?

i can visualize these things on isp networks but not on big corporate networks?

can anybody give me a explanation or link?

Thanx&regards,

Rajesh

[Harold Ritter]

Enterprise customer will typically request an ASN from ARIN (or other organizations) when they are multihomed to the Internet via different ISPs.

The reason is that you advertise your own IP address space that should be originated by a single ASN (your own). The Internet Routing Registry will not allow two ISP to register the same prefix.

Hope this helps,

[rajeshk200_2]

Hi Hritter,

Thanx for the response , partially understood ur explanation .

if i go for two isp's for multihoming each isp will give us different ip range . then how customer's network will be advertised to isp as a single network .

sorry for layman qn.

regds,

rajesh

[Harold Ritter]

Rajesh,

There is obviously a wealth of possible scenarios. You might connect with two ISP and provide your own IP address space or get the IP address block from one ISP and advertise via both, in which case it would be required to use BGP.

In the scenario you describe, there is no need to use BGP at all but you might want to run BGP between your edge routers and the ISPs just for failure detection if you are planning for redundancy.

Hope this helps,

[rajeshk200_2]

Hi hritter,

suppose we got ip address block from one isp (for eg ISP 1)

we want to connect to ISP 2 as a redundancy .

my qn is

will ISP 2 accept ip address block provided by ISP 1?

if ISP 2 accepts that block , my understanding is

that it will affect their routing advts like summarization technics,( i have no idea how isp advertise )

is there any special address blocks supported by both (or all ) ISP'S

regds,

rajesh

[Harold Ritter]

Rajesh,

ISP 2 would accept the prefix out of ISP 1 block if you ask them to. This is something that is commonly seen.

In addition, you would also have to ask ISP 1 to "punch a hole" in their CIDR block. What this means is that ISP 1 will have to advertise this more specific prefix as well as the CIDR block that they already advertise. Otherwise, all traffic to your network would be directed to ISP 2, which advertises the more specific prefix.

Hope this helps,

[rajeshk200_2]

Hi hritter,

thanx , i understood

this will be helpful to my further reading on bgp

Thanx&regards,

rajesh