3Com MSR 50 Series Configuration Manual page 833

Select the route with the shortest AS-PATH
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Select ORIGIN IGP, EGP, Incomplete routes in turn
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Select the route with the lowest MED value
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Select routes learned from EBGP, confederation, IBGP in turn
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Select the route with the smallest next hop cost
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Select the route with the shortest CLUSTER_LIST
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Select the route with the smallest ORIGINATOR_ID
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Select the route advertised by the router with the smallest Router ID
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CLUSTER_IDs of route reflectors form a CLUSTER_LIST. If a route reflector
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receives a route that contains its own CLUSTER ID in the CLUSTER_LIST, the
router discards the route to avoid routing loop.
If load balancing is configured, the system selects available routes to
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implement load balancing.
Route selection with BGP load balancing
The next hop of a BGP route may not be a directly connected neighbor. One of the
reasons is next hops in routing information exchanged between IBGPs are not
modified. In this case, the router finds the direct route via IGP route entries to
reach the next hop. The direct route is called reliable route. The process of finding
a reliable route to reach a next hop is route recursion.
Currently, the system supports BGP load balancing based on route recursion,
namely if reliable routes are load balanced (suppose three next hop addresses),
BGP generates the same number of next hops to forward packets. Note that BGP
load balancing based on route recursion is always enabled by the system rather
than configured using command.
BGP differs from IGP in the implementation of load balancing in the following:
IGP routing protocols such as RIP, OSPF compute metrics of routes, and then
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implement load balancing on 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
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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.
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BGP implements load balancing only on routes that have the same AS_PATH
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attribute, ORIGIN attribute, LOCAL_PREF and MED.
BGP load balancing is applicable between EBGPs, IBGPs and between
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confederations.
If multiple routes to the same destination are available, BGP selects routes for
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load balancing according to the configured maximum number of load
balanced routes.
BGP Overview 833