Accumulated IGP Metric Attribute (AIGP)

[hlias_giannio (Ilias G)]

Hello Laz,

many thanks for your reply. Based on that, I would like your feedback on the below as well.

@lagapidis

However, that accumulated metric from within the AS is then advertised to neighboring ASes using eBGP. This does not violate the non-transitive nature of the attribute.

I had the impression that non-transitive attributes are not advertised to eBGP peers. What is the difference between transitive and non-transitive then?

@lagapidis

Similarly, if you look at the MED attribute lesson, you will find that the MED attribute is exchanged between autonomous systems.

That’s true but only if MED is applied via a route-map directly to the BGP neighbor (as shown in the examples). Correct me if I am wrong, but the order of operations here is important. If for example I use the network command and apply a route-map, then MED won’t be advertised to eBGP peers. The local router would have already processed the MED value in its BGP table and since the attribute is non-transitive it won’t be advertised to neighboring ASes.

@lagapidis

AIGP is non-transitive in the sense that the accumulated metric that is determined using AIGP occurs only within the AS. You can’t accumulate the metric along the path of R1, R2, R3, and R4 using AIGP simply because that path traverses two ASes.

From the above I understand that if AIGP has been advertised, let’s say, 3 ASes away, then it will still show the value that originally created in the first AS. The intermediate ASes will never touch/add on that value?