I dont entirely agree , the command just disables ipv4 unicast address family peering capability , the router is multiprotocol whether you enable or disable this command . you may have a scenario where you dont want to receive full internet or ipv4 bgp routes e.g MPLS PE node
IPv4 address family routing information is advertised by default for each BGP routing session configured with the neighbor remote-as command, unless you first configure the no bgp default ipv4-unicast command before configuring the neighbor remote-as command.
we need to go a step further sometimes then command reference
I understand this is a really old post, but I see that the question is still now answered. This post also happens to be on the top of google searces so I thought I can probably add to the existing discussion.
I found an explanation for this command, have a look.
By default, the IPv4 address family is advertised to every peer of every address family.
Lets say you are running IPv4 and IPv6. Lets say you have native IPv6 transit. Do you really want to advertise IPv4 prefixes over a native IPv6 link? No.
So by saying "no bgp default ipv4-unicast" you are disabling this behaviour. After doing this command the router will only advertise IPv4 prefixes to IPv4 neighbours, and will not attempt to advertise IPv4 prefixes to IPv6 neighbours.
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