BGP Attribute : AIGP-BGP Accumulative IGP

Today I am going to talk about the BGP attribute called as BGP AIGP or so called Accumulated IGP which is now a days used to find the best path in the BGP protocol. 

Now the question is why it is introduced in the BGP after so long ?
The Reason is facing day to day issues when enterprises or service providers come up with the multiple, contiguous BGP AS's in their network and want to handle by single administrator. The expectation is to have the best path from two different AS which is controlled by Single administrator domain. 

IEFT introduces the BGP attribute named as AIGP by which the BGP speakers can perform a best path-selection for a prefix based on IGP metric even if the nodes are in two different AS's.

How BGP can select best path from their attributes ?

AIGP attribute impacts the BGP best path selection process by considering the AIGP metric after the local-preference rule. So, if local-preference for a prefix is same, AIGP metric is considered as a tie-breaker. The interior cost rule is also impacted. Without AIGP enabled, the rule is to prefer the path with lowest IGP metric to the BGP next-hop. However, with AIGP enabled, the resolving AIGP metric is added to the interior cost to the BGP next-hop.

hen BGP installs AIGP attribute routes into the routing information base (RIB), it adds the AIGP cost with the next-hop cost. If the next-hop is a non-recursive IGP route, BGP sets the AIGP metric to the received AIGP value and the first hop IGP metric to the next-hop. If the next-hop is a recursive route with the AIGP metric, BGP adds the received AIGP metric to the next-hop AIGP metric. 

Below is just a topology showing the BGP AIGP attribute. The Topology and the IPs used here are only for the demo purposes and has no relevance with any of the enterprise networks

Fig 1.1-BGP AIGP attribute

Basic Configurations to have the BGP AIGP attribute is as follows : 

Enabling Send and Receive for an AIGP Attribute
Below is showing how to enable AIGP send and receive capability in address family configuration mode: 

Configuring BGP Accumulated IGP
Here now the NDNA router belongs to autonomous system 200 and is configured to send the cost-community attribute to its neighbor at IP address 

Below showing the NDNA router belongs to autonomous system 200 and is configured to send the MED attribute to its neighbor at IP address 

What kind of BGP attribute AIGP is then ?
As you know BGP have attributes like optional, transitive, non-transitive. So here in the case of the AIGP BGP protocol, AIGP is an optional, non-transitive attribute and the type code is 26.

In the Attribute Flags (1 byte long), the high-order bit indicates Optional bit (set to 1) and the second high-order bit indicates Non-Transitive bit (set to 0). The AIGP attribute is encoded in the value field of the TLV. The AIGP TLV type is set to 1 and the IGP metric value is 4 bytes.