BGP Well-Known Attributes

[arun]

Why orgin attribute is called as well-known mandatory. I understand well-known mandatory attributes must be sent with each and every updates.

As-Path is crucial for loop prevention system and Next-hop address is mandatory for forwarding packets.

But what is the necessity of sending orgin attributes with each and every updates? Please help me to understand. Thanks…

[Satheshkumar Nallasamy]

Its an interesting questions.

To my knowledge, its simply used for route selection. Its mainly helpful for ISP to set routing policy based on origin code. To filter based on the origin and segerate the routes at the ISP edge router.

[Giuseppe Larosa]

Hello Arun,

the origin code provide information on how the BGP advertisement has been generated in the source AS.

if using a network command the origin is internal 'i 'and means origin AS has full knowledge of the IP prefix.

if the prefix is generated as a result of a redistribution the origin code is '?' and it is seen as less accurate.

To be honest if we look at routes in a looking glass almost all prefixes have origin code 'i' as the use of network command provides more stability to prefixes (no flapping during IGP changes)

Hope to help

Giuseppe

[arun]

hi giuslar,

Thanks for ur response. i just want to understand the necessity of sending orgin attribute with each and every updates..

[Giuseppe Larosa]

Hello Arun,

when the protocol was defined the authors felt the need for this attribute and so it is there.

They thought that taking trace of how the prefix had been generated was of value for internet routing and BGP peering

see

http://www.ietf.org/rfc/rfc1771.txt

origin 'i' means I declare the IP prefix is internal to my AS and in my use and this is useful information.

I agree that nowdays almost all routes have origin code 'i' so it can look like not used.

Also another important aspect is that consistency means a public IP prefix should be originated by the same source AS that is the leftmost AS number should be always the same in all possible BGP AS paths in the internet (unless somebody else is advertising the prefix for a mistake)

Hope to help

Giuseppe

[Giuseppe Larosa]

Hello Arun,

please take some time to understand how rating works in netpro

a rating of 1 is usually not considered fair in these forums

if you want to give a rating more then 1 you need to click on one of the stars in the right

*****

depending on which star you select you give a different rating

Best Regards

Giuseppe

[Jon Marshall]

Giuseppe

Can't see why that deserved a 1 so i've +5 to try and balance it out.

Jon