BGP AIG PA type

[BVC]

Unless I'm misunderstanding AIGP and the types of BGP routes, Cisco and other vendors say that AIGP is a optional non-transitive BGP PA. But from my understanding AIGP is passed from peer to peer in the same AS, and also other peers in different AS (if they support the AIGP feature)? Is this correct?

Am I miss understanding optional non-transitive PAs? I thought optional non-transitive can be transferred between iBGP peers, but when peering with an eBGP router PAs that have are optional non-transitive are not transferred?

[Giuseppe Larosa]

Hello @BVC ,

see the following link

https://networklessons.com/bgp/accumulated-igp-metric-attribute-aigp

the AIGP path attribute can be passed over eBGP sessions to direct eBGP peers.

the requirements is that all the involved ASes use the same IGP protocol otherwise the accumulated IGP metric would make no sense it would be not comparable.

Edit:

see also the following from cisco site

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/119001-configure-aigp-00.html

"This solution is similar to the solution where the Multi Exit Discriminator (MED) is set to the IGP metric. However, in this case, step 6 (the lowest MED) decides the best path. This step comes after step 4, where the shortest path decides the best path. The best path is often already found before step 6 is reached. With the AIGP solution, the normal BGP decision is changed so that the AIGP is checked after step 3 in order to determine whether the route was advertised locally. If different neighbor Autonomous Systems (ASs) peer with the BGP speaker, it means that the always-compare-med value must be enabled.

The AIGP metric attribute is specified in RFC 7311, which is the Accumulated IGP Metric Attribute for BGP. In order to carry the AIGP metric value in the cost community, the procedures specified in draft-retana-idr-aigp-cost-community (Use of the Cost Community to carry the Accumulated IGP Metric) are used."

https://www.rfc-editor.org/rfc/rfc7311

Hope to help

Giuseppe

[MHM Cisco World]

non-transitive and transitive 
AS1-AS2-AS3 
if we config any attribute transitive in AS1 we will see it in AS2 & AS3 
if we config any attribute non-trasitive in AS1 we will see it only in AS2, it will not pass to AS3

[BVC]

Right I think I understand, so you would say it's a mixture of both non-transitive and transitive as the AIGP attribute can be passed from AS1 to AS2, and then to AS3 ?

[MHM Cisco World]

sorry for late reply 
this lab for you 

[BVC]

Thank you very much for this, really appreciate it. Unless I'm mistaken the AIGP value went from AS200 outgoing to AS100, then the value stayed within AS100, but when it came to getting advertised to AS300 it never did? That makes it an optional non-transitive doesn't it?

[MHM Cisco World · Accepted solution]

Exactly'

You are right 

I config aigp in as200 toward as100'

As100 use it for best path but it not re advertise to as300.

MHM