BGP Attributes
BGP attributes are defined as the characteristics of a routes which is often used by the selection process.
This selection process has 10 criteria which are:
1. Largest weight
2. Highest local preference
3. Locally originated
4. Shortest AS path
5. Lowest origin type (i < e < ?)
6. Lowest MED (metric)
7. eBGP over iBGP
8. Lowest IGP metric to neighbor
9. Older route
10. Lowest router ID
BGP attributes contains two (2) categories which are standard and proprietary.
Well known attributes
Standard attributes are what we call the well known attributes and these are the locally originated, shortest AS path, next hop, highest local preference and summarization (aggregate). highest local preference and summarization (aggregate) are attributes which are optional only for configuration.
Locally originated is by default implying the router where did the route came from on the BGP process. We did some example on this on our previous documents. There are two (2) attribute type to identify an Origin and those are i, internal and ?, unknown. Internal means that the BGP topology for that route is located/created locally. On the other hand, ? is a redistribution of route which is originating from the other network from where the your router is connecting to.
Shortest AS Path is simply defined as the location of the route referenced by the AS number. This AS path somehow address the issue on loop wherein AS path should not have the same number on its entry.
We'll not discuss much on the next hop since it is quite straightforward.
Local preference is defined as to which BGP route you are preferred to use. This attributes almost functions just the same as the weight wherein the value being set by the administrator will stay locally on the router but on the other hand, the local preference will propagate the value to its neighbor but only within its own AS. The higher the value of the local preference, the better it will be.
Summarization (Aggregate) are defined as the suppression of the BGP route for simplification.
Optional Attributes
Lowest MED (Multi Exit Discriminator) metric is used by default to influence the neighbor router's decision regarding on which best path to take.
Aggregator is like the summarization.
And lastly the Community on which is like a BGP system within a BGP system. Details of this will be explained in the other discussion.