PLANET XGS3-24040 - Manual (Page 309)

Chapter 39 BGP
39-3
IBGP are used in the AS. It sends message to all the BGP neighbors in the AS. IBGP exchanges AS routing
information in a big organization. Attention, the switches in the AS needn’t be connected physically. Only if the
switches are in the same AS, they can be neighbors each other. Because BGP can’t detect route, the route
tables of other inner route protocols (such as static route, direct route, OSPF and RIP) need contain neighbor
IP addresses and these routes are used to exchange information among BGPs. In order to avoid routing
loops, when a BGP speaker receives a route notification from inner neighbor, it would not notify this route to
other inner neighbors.
EBGP is used among the AS, and it transmits routing information to the BGP neighbors of outer ASes. EBGP
need physical connection and share the same medium. Because EBGP need physical connection, the
boundary equipments between two AS are usually running EBGP. When a BGP speaker receives routing
information from outer neighbors, it notifies these routes to other inner neighbors.
3． Route attribute
BGP-4 can share and query inner IP route table through relevant mechanisms, but it has its own route table.
In the BGP route table, each route has a network number, AS listing information (also called AS path) that it
passed and some routing attributes (such as origin). The routing attribute that BGP-4 used is very complex,
this attribute can be used as metrics to select path.
4． Route-selecting policy of BGP
When receiving BGP notification about a same route from several neighbors, selecting the best route need to
be take into account after routing filtering. This process is called BGP route selecting process. BGP route
selecting process will start only when the following conditions are fulfilled:
 The switch’s route must be next hop reachable. That is in the route table there is the route that can
reach the next hop.
 BGP must be synchronized with IGP (unless asynchronism is configured; only restricted to IBGP)
BGP route selecting process is based on the BGP attribute. When there are several routes that indicate the
same destination, BGP need select the best route to the destination. The decision-making process is as the
following:
1．Select the route with the most weight first;
2. If the weights are the same, select the route with the most local preference;
3. If the local preferences are the same, select the route generated by local switch.
4. If the local preferences are the same and there is no route generated by local switch, select the route
with the shortest AS path;
5. If the AS paths are the same, select the route with the lowest “origin” type （IGP
6. If the “origin” types are the same, select the route with the lowest MED attribute. Unless activating
command “bgp always-compare-med”, this comparison is only available among the routes from the same
neighbor AS.
7. If the MED attributes are the same, EBGP is preferable to outer confederation and outer confederation is
preferable to IBGP.
8. If it’s still the same by now, BGP router ID （router ID）is used to break the balance. The best route is the
one from the least router ID.
9. If it’s still the same by now, BGP router ID （router ID）is used to break the balance. The best route is the
one from the least router ID.