Table of Contents
BGP Origin Attribute is the crucial factor when BGP is employing algorithm to determine the best path to a destination network. This attribute is important in resolving conflicts between various alternative paths available in the network. This attribute comes under mandatory attribute hence receives significant attention in our updates. In our previous blogs, we have discussed BGP local reference attribute, in this article, we will discuss the BGP originate attribute.
What is BGP Origin Attribute?
BGP Origin Attribute is a well-known mandatory attribute used in BGP routing to indicate how a particular route was originated into BGP. The purpose of the origin attribute in BGP is to keep track of the source of a prefix. This attribute can have one of three values: IGP, EGP, or Incomplete.
Origin Attribute Types
- i (IGP) – Originated from an interior gateway protocol (such as EIGRP, RIP etc.).This indicates the route was injected into BGP via the network command under the BGP process.
- e (EGP) – Originated from an external gateway protocol.
- ? (incomplete) – Unknown origin. This usually indicates the route was redistributed into BGP (from either connected, static, or IGP routes). An origin code of “?” is the least preferred.
From amongst the above 3 origin codes, “i” is the most preferred while “?” is the least preferred.
| Origin Code | Type | Description |
|---|---|---|
| 0 | IGP (i) | The route was learned from an Interior Gateway Protocol (e.g., OSPF, EIGRP) and then injected into BGP using a network statement. Considered Most preferred |
| 1 | EGP (e) | The route was learned via the Exterior Gateway Protocol (EGP)—the predecessor of BGP. Rarely seen now |
| 2 | Incomplete (?) | The route was redistributed into BGP from another routing protocol (like static routes or connected routes). Considered Least Preferred |
Related- iBGP vs eBGP
Scenario Example
Below is the scenario where 3 Routers R1, R2 and R3 are connected in serial. With each in different Autonomous system no i.e. 1, 2 and 3 respectively.
In the BGP routing table of R2, the BGP origin code is listed at the end of each line in the table:
The “i” at the end of the first routing entry indicates the 1.1.1.0/24 network was originated via an IGP, via the BGP network command. The 3.3.3.0/24 network was redistributed into BGP in AS 3, since it shows “?” at the end of that routing entry.
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ABOUT THE AUTHOR
You can learn more about her on her linkedin profile – Rashmi Bhardwaj
