propagated outside the next-hop AS. Not all ISPs give out their communities to the public. BGP Extended Community Attribute The BGP Extended Community Attribute was added in 2006, in order to extend the range of such attributes and to provide a community attribute structuring by means of a type field. The extended format consists of one or two octets for the type field followed by seven or six octets for the respective community attribute content. The definition of this Extended Community Attribute is documented in RFC 4360. The IANA administers the registry for BGP Extended Communities Types. The Extended Communities Attribute itself is a transitive optional BGP attribute. A bit in the type field within the attribute decides whether the encoded extended community is of a transitive or non-transitive nature. The IANA registry therefore provides different number ranges for the attribute types. Due to the extended attribute range, its usage can be manifold. RFC 4360 exemplarily defines the "Two-Octet AS Specific Extended Community", the "IPv4 Address Specific Extended Community", the "Opaque Extended Community", the "Route Target Community", and the "Route Origin Community". A number of BGP QoS drafts also use this Extended Community Attribute structure for inter-domain QoS signalling. With the introduction of 32-bit AS numbers, some issues were immediately obvious with the community attribute that only defines a 16-bit ASN field, which prevents the matching between this field and the real ASN value. Since RFC 7153, extended communities are compatible with 32-bit ASNs. RFC 8092 and RFC 8195 introduce a Large Community attribute of 12 bytes, divided in three field of 4 bytes each (AS:function:parameter). Multi-exit discriminators MEDs, defined in the main BGP standard, were originally intended to show to another neighbor AS the advertising AS's preference as to which of several links are preferred for inbound traffic. Another application of MEDs is to advertise the value, typically based on delay, of multiple ASs that have a presence at an IXP, that they impose to send traffic to some destination. Some routers (like Juniper) will used the Metric from OSPF to set MED. Examples of MED used with BGP when exported to BGP on juniper SRX # run show ospf route Topology default Route Table: Prefix Path Route NH Metric NextHop Nexthop Type Type Type Interface Address/LSP 10.32.37.0/24 Inter Discard IP 16777215 10.32.37.0/26 Intra Network IP 101 ge-0/0/1.0 10.32.37.241 10.32.37.64/26 Intra Network IP 102 ge-0/0/1.0 10.32.37.241 10.32.37.128/26 Intra Network