from here: http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_s3g.html#wp1041409
synchronization
To enable the synchronization between BGP and your Interior Gateway Protocol (IGP) system, use the synchronization
command in address family or router configuration mode. To enable the
Cisco IOS software to advertise a network route without waiting for the
IGP, use the no form of this command.
synchronization
no synchronization
Command History
Release
|
Modification
|
|---|---|
10.0
|
This command was introduced.
|
12.0(7)T
|
Address family configuration mode was added.
|
12.2(8)T
|
Command default behavior changed to disabled.
|
Usage Guidelines
Usually, a BGP speaker does not advertise a route to an external
neighbor unless that route is local or exists in the IGP. By default,
synchronization between BGP and the IGP is turned off to allow the Cisco
IOS software to advertise a network route without waiting for route
validation from the IGP. This feature allows routers and access servers
within an autonomous system to have the route before BGP makes it
available to other autonomous systems.
Use the synchronization command if routers in the autonomous system do not speak BGP.
and from yap:
http://www.itcertnotes.com/2012/01/bgp-synchronization.html
The BGP synchronization rule states that a BGP router cannot use an IBGP-learned transit route nor advertise it to an EBGP peer; unless it is synchronized in both BGP and IGP routing tables – IGP has redistributed and propagated it across the AS and installed it in the local IP routing table. BGP running in a transit AS that is passing traffic between ASes should not advertise a route before all routers in the AS have learned about the route via IGP. The BGP synchronization rule ensures consistency information throughout an AS to avoid routing black holes within the AS, eg: advertising a destination to an EBGP neighbor when not all routers within the AS can route to the destination.
BGP synchronization should only be used when there are routers in the AS that do not run BGP. It is safe to disable BGP synchronization only if all routers in the transit path in the AS – the path between the BGP border routers, are running BGP; or when the AS is not a transit AS. BGP synchronization is enabled by default in earlier version of Cisco IOS releases. Most current Cisco IOS releases (Cisco IOS Release 12.2(8)T and later) has disabled BGP synchronization by default, as most ISPs run BGP on all routers within their ASes.
and this from:
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDQQFjAA&url=http%3A%2F%2Fwww.cisco.com%2Fimage%2Fgif%2Fpaws%2F13753%2F25.pdf&ei=ZTLaUJDuHbC70QGH5oGoBg&usg=AFQjCNFedx4tm6q2qoiGrJaayKP43xsE_Q&sig2=x_0yP-0qBR0zaP-0HXQUvw
Why Routers Ignore Paths
Assume that all paths that a router receives for a particular prefix are arranged in a list. The list is similar to
the output of the show ip bgp longer−prefixes command. In this case, some paths are not considered as
candidates for the best path. Such paths typically do not have the valid flag in the output of the show ip bgp
longer−prefixes command. Routers ignore paths in these circumstances:
• Paths that are marked as not synchronized in the show ip bgp longer−prefixes output
If BGP synchronization is enabled, there must be a match for the prefix in the IP routing table in order for an internal BGP (iBGP) path to be considered a valid path. BGP synchronization is enabled by
default in Cisco IOS® Software. If the matching route is learned from an Open Shortest Path First (OSPF) neighbor, its OSPF router ID must match the BGP router ID of the iBGP neighbor. Most users prefer to disable synchronization with use of the no synchronization BGP subcommand.
Note: Synchronization is disabled by default in Cisco IOS Software Release 12.2(8)T and later.
• Paths for which the NEXT_HOP is inaccessible
Be sure that there is an Interior Gateway Protocol (IGP) route to the NEXT_HOP that is associated with the path.
and from yap:
http://www.itcertnotes.com/2012/01/bgp-synchronization.html
The BGP synchronization rule states that a BGP router cannot use an IBGP-learned transit route nor advertise it to an EBGP peer; unless it is synchronized in both BGP and IGP routing tables – IGP has redistributed and propagated it across the AS and installed it in the local IP routing table. BGP running in a transit AS that is passing traffic between ASes should not advertise a route before all routers in the AS have learned about the route via IGP. The BGP synchronization rule ensures consistency information throughout an AS to avoid routing black holes within the AS, eg: advertising a destination to an EBGP neighbor when not all routers within the AS can route to the destination.
BGP synchronization should only be used when there are routers in the AS that do not run BGP. It is safe to disable BGP synchronization only if all routers in the transit path in the AS – the path between the BGP border routers, are running BGP; or when the AS is not a transit AS. BGP synchronization is enabled by default in earlier version of Cisco IOS releases. Most current Cisco IOS releases (Cisco IOS Release 12.2(8)T and later) has disabled BGP synchronization by default, as most ISPs run BGP on all routers within their ASes.
and this from:
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDQQFjAA&url=http%3A%2F%2Fwww.cisco.com%2Fimage%2Fgif%2Fpaws%2F13753%2F25.pdf&ei=ZTLaUJDuHbC70QGH5oGoBg&usg=AFQjCNFedx4tm6q2qoiGrJaayKP43xsE_Q&sig2=x_0yP-0qBR0zaP-0HXQUvw
Why Routers Ignore Paths
Assume that all paths that a router receives for a particular prefix are arranged in a list. The list is similar to
the output of the show ip bgp longer−prefixes command. In this case, some paths are not considered as
candidates for the best path. Such paths typically do not have the valid flag in the output of the show ip bgp
longer−prefixes command. Routers ignore paths in these circumstances:
• Paths that are marked as not synchronized in the show ip bgp longer−prefixes output
If BGP synchronization is enabled, there must be a match for the prefix in the IP routing table in order for an internal BGP (iBGP) path to be considered a valid path. BGP synchronization is enabled by
default in Cisco IOS® Software. If the matching route is learned from an Open Shortest Path First (OSPF) neighbor, its OSPF router ID must match the BGP router ID of the iBGP neighbor. Most users prefer to disable synchronization with use of the no synchronization BGP subcommand.
Note: Synchronization is disabled by default in Cisco IOS Software Release 12.2(8)T and later.
• Paths for which the NEXT_HOP is inaccessible
Be sure that there is an Interior Gateway Protocol (IGP) route to the NEXT_HOP that is associated with the path.
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