H3C Technologies H3C S12500 Series Switches User Manual
Page 214
198
The next hop of a BGP route might 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.
•
Implement BGP load balancing by modifying route selection rules.
BGP differs from IGP in the implementation of load balancing in the following ways:
{
IGP routing protocols, such as RIP and OSPF, compute metrics of routes, and then implement
load balancing over 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 implement load balancing according to
metrics of routes. However, BGP has abundant route selection rules, through which, it selects
available routes for load balancing and adds load balancing to route selection rules.
BGP implements load balancing only on routes that have the same AS_PATH, ORIGIN,
LOCAL_PREF and MED, rather than using the route selection rules as described in "
."
Figure 77 Network diagram
, 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 is configured on Router C, and the two routes have the same
AS_PATH attribute, ORIGIN attribute, 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 the route that has
AS_PATH unchanged but has NEXT_HOP changed to Router C; other attributes are those of the best
route.
NOTE:
BGP load balancing is applicable between EBGP peers, between IBGP peers, and between
confederations.
Router A
Router B
Router C
Router D
Router E
9.0.0.0/24
AS 200
AS 100