H3C Technologies H3C S7500E Series Switches - Manual (Page 214)

6-11
z A BGP speaker advertises routes learned through iBGP to eBGP peers. Note that if BGP and IGP
synchronization is disabled, those routes are advertised to eBGP peers directly. If the feature is
enabled, only after IGP advertises those routes, can BGP advertise the routes to eBGP peers.
z A BGP speaker advertises all routes to a newly connected peer.
iBGP and IGP Synchronization
Routing information synchronization between iBGP and IGP avoids giving wrong directions to routers
outside of the local AS.
If a non-BGP router works in an AS, it may discard a packet due to an unreachable destination. As
shown in Figure 6-11 , Router E has learned a route of 8.0.0.0/8 from Router D via BGP. Then Router E
sends a packet to 8.0.0.0/8 through Router D, which finds from its routing table that Router B is the
next hop (configured using the peer next-hop-local command). Because Router D has learned the
route to Router B via IGP, it forwards the packet to Router C through route recursion. Router C has no
idea about the route 8.0.0.0/8, so it discards the packet.
Figure 6-11 iBGP and IGP synchronization
If synchronization is enabled in this example, only when the route 8.0.0.0/24 received from Router B is
available in its IGP routing table, can Router D add the route into its BGP routing table and advertise
the route to the eBGP peer.
You can disable the synchronization feature in the following cases:
z The local AS is not a transitive AS (AS20 is a transitive AS in the above figure).
z Routers in the local AS are iBGP fully meshed.
Settlements for Problems in Large Scale BGP Networks
Route summarization
Route summarization can reduce the routing table size on a large network, and allow BGP routers to
advertise only summary routes rather than more specific routes.
Currently, the system supports both manual and automatic route summarization. Manual route
summarization allows you to determine the attribute of a summary route and whether to advertise the
route.