200
•
When multiple feasible routes to a destination exist, BGP advertises only the optimal route to its
peers. If the
advertise-rib-active
command is configured, BGP advertises the optimal route in
the IP routing table. If not, BGP advertises the optimal route in the BGP routing table.
•
BGP advertises only routes that it uses.
•
BGP advertises routes learned from an EBGP peer to all BGP peers, including both EBGP and
IBGP peers.
•
BGP advertises routes learned from an IBGP peer to EBGP peers, rather than other IBGP
peers.
•
After establishing a session to 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 load balancing is applicable between EBGP peers, between IBGP peers, and between
confederations.
BGP implements load balancing through route recursion and route selection.
BGP load balancing through route recursion
The next hop of a BGP route might not be directly connected. One of the reasons is that the next hop
information exchanged between IBGP peers is 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.
If multiple recursive routes to the same destination are load balanced, BGP generates the same
number of next hops to forward packets.
BGP load balancing based on route recursion is always enabled in the system.
BGP load balancing through route selection
IGP routing protocols, such as RIP and OSPF, can use route metrics as criteria to load balance
between routes that have the same metric. BGP cannot load balance between routes by route
metrics as an IGP protocol does, because BGP does not have a route computation algorithm.
BGP uses the following load balancing criteria to determine load balanced routes:
•
The routes have the same ORIGIN, LOCAL_PREF, and MED attributes. If the
balance
as-path-neglect
command is not configured, the routes must also have the same AS_PATH
attribute.
•
The routes have the same MPLS label assignment status (labeled or not labeled).
BGP does not use the route selection rules described in "
" for load balancing.
As shown in
, Router A and Router B are IBGP peers of Router C. Router C allows a
maximum number of two ECMP routes for load balancing.
Router D and Router E both advertise a route 9.0.0.0 to Router C. Router C installs the two routes to
its routing table for load balancing if the routes meet the BGP load balancing criteria. After that,
Router C forwards to Router A and Router B a single route whose attributes are changed as follows:
•
AS_PATH attribute:
If the
balance as-path-neglect
command is not configured, the AS_PATH attribute does
not change.
If the
balance as-path-neglect
command is configured, the AS_PATH attribute is changed
to the attribute of the optimal route.
•
The NEXT_HOP attribute is changed to the IP address of Router C.
•
Other attributes are changed to be the same as the optimal route.
Содержание HPE FlexNetwork 7500 series
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