What that means, a provider router does not send this the path_attribute outside of it's AS. Take this simple BGP topology
R1 = AS1
R2 = AS2
R3 = AS3
R1 is announcing the prefix 10.10.10.0/24 & sourced from it's loopback interface, with a metric value of 200.
"What would R2 see ?" and "What would R3 see ?"
Okay let's see the cfg of the 3 players;
Okay now let's look at the bgp table for 10.10.10.0/24;
I want to point out the following; the R1 is the source of the advertisement and the metric are only adjusted via the route-map applied on towards neighbor 126.96.36.199.
Now let's look at R2 bgp table;
Notice the metric it received from 188.8.131.52 ( R1 @ AS1 ) equals 200 ? Okay that's was because of the applied route-map called bgp-med ; and with the set metric 200 statement.
Now moving on to R3.
It's bgp table will not reflect this same metric as seen via R2. And that's the point of this blog and the title "Non-transitive attributes"
You notice we are metric free on R3 for advertisement received from 184.108.40.206 ( R2 @ AS2 ) for the prefix 10.10.10.0/24. R2 drop that attribute, & from it's advertisements towards R3.
I hope this give you an ideal and better understanding on path attributes and how some things are not included in bgp announcements.
Here's some of the common path_attributes that you might run across ;
ORIGIN ( mandatory well known how the route was originated network statement, redistribute, etc.... )
AS_PATH ( mandatory well known changes each time we cross a AS boundary known simple as bgp path or AS hops towards the origination, as we cross AS, we add the AS# to this path_attribute )
AS4_PATH ( mandatory well known 4 byte AS same concept as the above )
NEXT_HOP ( mandatory well known changes, per each AS router )
ATOMIC_AGGREGATOR ( discretionary well-known provides information that part of the attribute might be lost during summarizations )
Local-Preference ( discretionary well-known local within that AS locally )
AGGREGATOR (optional transitive )
AS4_AGGREGATOR (optional transitive )
COMMUNITIES (optional transitive used to enforce bgp policies , 32bit value )
EXTEND_COMMUNITIES (optional transitive used to enforce bgp policies , extended and increase bit size typically represented as AS:VALUE i.e 7018:65001 )
Multiprotocol UN/Reachable ( optional non-transitive )
Multi_Exit_Discriminator ( optional non-transitive )
Cluster_ID ( optional non-transitive using primary by BGP router reflectors )
Now go out and do some BGP :)
Freelance Network / Security Engineer
kfelix ----a----t----socpuppets ---d---o---t---com
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