By default, BGP sends those capabilities in its OPEN message. In addition, BGP supports graceful restart.
When BGP receives a notification 2/4 (Unsupported optional parameters) in response to an OPEN, it assumes that the peer does not support capability negotiation and MBGP and sends an OPEN message without any capability.
If the peer speaker sends no capabilities, but the local speaker is configured for the IPv4 unicast capability, the assumption is that the peer speaker is operating in legacy mode, and the session defaults to the exchange of IPv4 unicast NLRIs (a non MBGP session).
If the local speaker is configured explicitly with the IPv4 unicast family disabled, it cannot peer with legacy peers, and it will send the Optional Attribute Error whenever it receives an update packet. Because the IPv4 address family is enabled for Extreme Networks switches by default, it is recommended that you explicitly disable this address family when you desire non-standard behavior.