Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
Chapter 5 BGP Configuration
5-11
BGP differs from IGP in the implementation of load balancing in the following:
z
IGP routing protocols such as RIP, OSPF compute metrics of routes, and then
implement load balancing on routes with the same metric and to the same
destination. The route selection criterion is metric.
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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.
Note:
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BGP implements load balancing only on routes that have the same AS_PATH,
ORIGIN, LOCAL_PREF and MED.
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BGP load balancing is applicable between EBGPs, between IBGPs and between
confederations.
z
If multiple routes to the same destination are available, BGP selects routes for load
balancing according to the configured maximum number of load balanced routes.
Figure 5-10
Network diagram for BGP load balancing
In the above figure, Router D and Router E are IBGP peers of Router C. Router A and
Router B both advertise a route destined for the same destination to Router C. If load
balancing is configured 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 route
table for load balancing. After that, Router C forwards routes to Router D and Router E
only once, with AS_PATH unchanged, NEXT_HOP changed to Router C’s address.
Other BGP transitive attributes apply according to route selection rules.
