185
•
After establishing a session with a new BGP peer, BGP advertises all the routes matching the above
rules to the peer. After that, BGP advertises only incremental updates to the peer.
BGP load balancing
BGP implements load balancing through route recursion and route selection.
•
BGP load balancing through route recursion.
The next hop of a BGP route may not be directly connected. One of the reasons is next hops in
routing information exchanged between IBGP peers are not modified. The BGP router must find the
directly-connected next hop through IGP. The matching route with the direct next hop is called the
"recursive route." The process of finding a recursive route is route recursion.
The system supports BGP load balancing based on route recursion. If multiple recursive routes to
the same destination are load balanced (suppose three direct next hop addresses), BGP generates
the same number of next hops to forward packets. BGP load balancing based on route recursion
is always enabled by the system rather than configured by using commands.
•
BGP load balancing through route selection.
IGP routing protocols, such as RIP and OSPF, compute the metrics of routes, and implement load
balancing over the routes with the same metric and to the same destination. The route selection
criterion is metric.
BGP has no route computation algorithm, so it cannot perform load balancing according to the
metrics of routes. BGP implements load balancing over the routes that meet the following
requirements:
{
The routes have the same AS_PATH, ORIGIN, LOCAL_PREF, and MED attributes.
{
The routes are all reflected or not reflected by the route reflector.
BGP does not use the route selection rules described in "
BGP route selection
" for load balancing.
Figure 51
Network diagram
In
Figure 51
, Router A and Router B are IBGP peers of Router C. Router D and Router E both advertise a
route 9.0.0.0 to Router C. If load balancing with a maximum number of two routes is configured on
Router C, and the two routes have the same AS_PATH, ORIGIN, LOCAL_PREF, and MED, Router C
installs both the two routes to its routing table for load balancing. After that, Router C forwards to Router
A and Router B a single route that has NEXT_HOP changed to Router C and other attributes changed to
those of the best route.
