"no bgp default ipv4-unicast" just means that you won't automatically negotiate regular ipv4 route exchange with your bgp neighbors. The command is simply there for convenience depending on how many neighbors you have and what capabilities you are running with them.
For example, suppose you have 99 BGP peers you are running IPv6 routing with, and 1 you are running IPv4 routing with. With the default option of "bgp default ipv4-unicast", you would have to go to the ipv4 unicast address family and *deactivate* the 99 peers you run only IPv6 with. If you say "no default ipv4-unicast", all 100 peers will have ipv4 off, then you can *activate* just the single one that you want.
Unless the lab question asks you, there's never a case where you have to issue this command. You can always selectively disable the ipv4 address family for peers that don't use it. The command is there just to save lines of syntax in your config.
Think of it like the "area authentication" command in OSPF or the "passive-interface default" command. You could accomplish the same thing with other longer syntax options (e.g. OSPF authentication at each interface level or individual passive interface commands), but it may save time to have the option apply to multiple links at the same time.
HTH,
Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan_at_INE.com<mailto:bmcgahan_at_INE.com>
Internetwork Expert, Inc.
http://www.INE.com
On Feb 1, 2011, at 7:47 AM, "Jeferson Guardia" <jefersonf_at_gmail.com<mailto:jefersonf_at_gmail.com>> wrote:
'no bgp default ipv4-unicast
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Received on Tue Feb 01 2011 - 07:58:43 ART
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