NOTE:
• BGP implements load sharing only on routes that have the same WEIGHT, LOCAL_PREF,
ORIGIN, AS_PATH, MED and IGP COST.
• BGP load sharing is applicable between eBGP peers and between iBGP peers.
• If multiple routes to the same destination are available, BGP selects the configured number of
routes for load sharing. The maximum number of routes for load sharing is currently 4. Load
sharing is enabled by default.
Figure 62: Network diagram for BGP load sharing
In
Figure 62: Network diagram for BGP load sharing
on page 377, 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
sharing 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 sharing. After that, Router C forwards to
Router D and Router E the route that has AS_PATH unchanged but has NEXT_HOP changed to Router C; other
BGP transitive attributes are those of the best route.
BGP route advertisement rules
The current BGP implementation supports the following route advertisement rules:
• When multiple feasible routes to a destination exist, the BGP speaker advertises only the best route to its
peers.
• A BGP speaker advertises only routes used by itself.
• A BGP speaker advertises routes learned from an eBGP peer to all its peers, both eBGP and iBGP.
• A BGP speaker does not advertise routes learnt from an iBGP peer to its other iBGP peers.
• A BGP speaker advertises routes learnt from iBGP to eBGP peers. Note that BGP and IGP synchronization is
disabled always and those routes are advertised to eBGP peers directly.
Chapter 17 Border Gateway Protocol (BGP)
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