Huawei Quidway NetEngine80E Configuration Manual page 769

HUAWEI NetEngine80E/40E Router
Configuration Guide - IP Routing
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Routing Selection Principles for Load Balancing
In BGP, the next-hop address of a generated route may not be the address of the peer that is
directly connected to the local device. One common scenario is that the next hop is not changed
when a route is advertised between IBGP peers. Therefore, before forwarding a packet, the
device must find a directly reachable address, through which the packet can reach the next hop
specified in the routing table. In this process, the route to the directly reachable address is called
a dependent route. BGP routes depend on these dependent routes for packet forwarding. The
process of finding a dependent route based on the next-hop address is called route iteration.
The NE80E/40E supports iteration-based BGP load balancing. If load balancing is configured
for a dependent route (assume that there are three next-hop addresses), BGP generates the same
number of next-hop addresses to forward packets. BGP load balancing based on iteration does
not need to be configured by using commands. This feature is permanently enabled on the
NE80E/40E.
BGP load balancing is different from IGP load balancing in the following implementation
methods:
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Configuring and Managing a Large-Scale BGP Network
The configuration and maintenance of a large-scale BGP network is difficult. To facilitate BGP
network maintenance and management, you can configure peer groups, community, route
reflector, or confederation as required.
Issue 02 (2014-09-30)
The BGP speaker advertises the routes learned from EBGP peers to all BGP peers
(including EBGP and IBGP peers) except the peers that advertise these routes.
The BGP speaker does not advertise the routes learned from IBGP peers to its IBGP peers.
The BGP speaker advertises the routes learned from IBGP peers to its EBGP peers.
The BGP speaker advertises all preferred BGP routes to the new peers when peer
relationships are established.
In IGPs, if there are different routes to the same destination address, an IGP calculates
metrics of these routes based on its own routing algorithm and performs load balancing
among the routes with the same metric.
BGP does not have a routing algorithm, and therefore cannot determine whether to perform
load balancing among routes based on explicit metrics. However, BGP contains many route
attributes, which have different priorities in route selection principles. As a result, BGP
performs load balancing according to route selection principles. Specifically, load
balancing is performed according to the configured maximum number of equal-cost routes
only when all the routes have the same high preference.
NOTE
l By default, BGP performs load balancing only among the routes with the same AS_Path attribute. You
can use the bestroute as-path-ignore
command to configure BGP not to compare the AS_Path attribute of routes when performing load
balancing.
l BGP load balancing is also applicable between ASs in a confederation.
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Table 8-3 lists these solutions.
8 BGP Configuration
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