Ibgp And Igp Synchronization; Settlements For Problems In Large Scale Bgp Networks - 3Com S7906E Configuration Manual

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
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 1-11 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:
The local AS is not a transitive AS (AS20 is a transitive AS in the above figure).
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.
Route dampening
BGP route dampening is used to solve the issue of route instability such as route flaps, that is, a route
comes up and disappears in the routing table frequently.
When a route flap occurs, the routing protocol sends an update to its neighbor, and then the neighbor
needs to recalculate routes and modify the routing table. Therefore, frequent route flaps consume large
bandwidth and CPU resources and even affect network normal operation.
1-11, Router E has learned a route of 8.0.0.0/8 from Router D via BGP. Then Router E
1-11