H3C SecPath F1800-A Operation Manual page 319

Operation Manual - Network and Routing Protocol
H3C SecPath F1800-A Firewall
It is not recommended to use this configuration unless you can make sure that the
ASs adopt the same IGP and routing method.
6.2.17 Configuring BGP Load Balancing
I. Iteration-Based BGP Load Balancing
It has been mentioned in introducing routing table that next hop address is a key entry
in the routing table. The router forwards packets according to the next hop address.
Because of the protocol particularity, the route's next hop address generated through
BGP may not be the directly connected adjacent neighbor of the current router (one of
the common reasons is that IBGPs do not modify next hop information during
advertising route information). In that case, the router must find a directly reachable
address, through which the packet can be successfully forwarded to the next hop
address specified in the routing table.
The route to the directly reachable address is called dependent route. The process of
finding the dependent route based on the next hop address is route iteration.
VRP supports iteration-based BGP load balancing, that is, if the dependent route has
implemented load balancing (given there are three next hop addresses), BGP will
generate the same number of next hop addresses to guide packet forwarding. Note
that iteration-based BGP load balancing is not configured through any command. It is
always enabled on VRP. The only limit to BGP load balancing is that whether the
dependent route performs load balancing.
II. EBGP Load Balancing
EBGP load balancing is usually called BGP load balancing, which differs from IGP
load balancing in implement.
IGP calculates metrics for different routes to the same destination address
according to its route algorithm and performs load balancing between routes with
identical metrics.
BGP cannot determine whether to perform load balancing based on an explicit
metric since it has no route algorithm. However, it can select routes to implement
load balancing depending on its many routing rules. That is, it adds the load
balancing into its routing rules.
VRP supports BGP load balancing. After BGP load balancing is configured, upon
BGP route selection, a rule is added between the last two routing rules "Select routes
learned from EBGP first" and "Select routes advertised by the routers with lowest
BGP ID first": if load balancing is configured and there are multiple external routes
heading for the same AS or AS confederation, select multiple routes based on the
configured routes number for load balancing.
BGP load balancing is only performed on the routes whose AS_PATH attributes are
totally the same.
Chapter 6 BGP/MBGP Configuration
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