Recommendation to avoid use of BGP Extended Communities at Internet Exchange Route Servers

The main use-case for Extended Communities are as Route Targets within VPN deployments, but historically Extended Communities also have been used as an operational utility to signal requests to IXP Route Servers such as functionality to reduce propagation scope or request AS_PATH prepending. Use of Extended Communities arose from a lack of support to fit 4-octet Autonomous System Numbers (ASNs) in Classic BGP communities , thus operators improvised a method that could allow BGP signaling from IXP participants with 4-octet ASN. The 6-octet space for the Global and Local administrator part of the BGP Extended Community provides sufficient space for a single 4-octet ASN. However, the 6-octet space is not sufficient enough should a 4-octet ASN participant of an IXP want to send a signal to a 4-octet ASN Route Server or to another 4-octet ASN participant. Moreover, the flexibility to insert a 4-octet ASN either in the Global or the Local Administrator part, proved to bring extra complexity both in the BGP implementations and in the route propagation functions that are being triggered through BGP Extended Communities. Although, this method was widely considered to be an acceptable workaround for a period of time, a more robust and future proof solution was needed that could overcome the aforementioned obstacles. BGP Large communities addressed the operational requirements for working with 4-octet ASNs in a variety of scenarios. With a total space of 12 octets divided into 3 separate fields, signalling between 2-octet ASNs and 4-octet ASNs, or 4-octet ASNs and 4-octet ASNs, making the use of BGP Extended Communities redundant. Since the introduction of BGP Large communities in 2017 - by now - virtually all BGP implementations have adopted this standard, making this feature usable in all public Internet deployments. At the moment of writing this recommendation, there are still IP Network and IXP operators that support BGP Extended Communities for IXP Route Server signaling purposes. However, supporting three flavors of BGP Communities (Classic, Large, and Extended) contribute to increased memory consumption, increased complexity in Routing Policies, and reduced stability of the Internet ecosystem as BGP speakers need to send a BGP UPDATE message every time any type of BGP Community is added, removed, or modified. As each and every BGP UPDATE message propagated and received requires CPU cycles for processing, any efforts that minimize the number of BGP UPDATE messages are advantageous for the routing system. The authors of this document posit that Extended Communities are superfluous in context of the existence of Large Communities.