Alteon Application Switch Operating System Application Guide
Border Gateway Protocol
Document ID: RDWR-ALOS-V2900_AG1302
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Unless otherwise specified, the router compares metric attributes for paths from external neighbors
that are in the same AS.
Selecting Route Paths in BGP
BGP selects only one path as the best path. It does not rely on metrics attributes to determine the
best path. When the same network is learned via more than one BGP peer, BGP uses its policy for
selecting the best route to that network. The BGP implementation in Alteon uses the following
criteria to select a path when the same route is received from multiple peers:
1. Local fixed and static routes are preferred over learned routes.
2. With iBGP peers, routes with higher local preference values are selected.
3. In the case of multiple routes of equal preference, the route with lower AS path weight is
selected, using the following algorithm:
AS path weight = 128 x AS path length (number of autonomous systems transversed)
4. In the case of equal weight and routes learned from peers that reside in the same AS, the lower
metric is selected.
A route with a metric is preferred over a route without a metric.
5. The lower cost to the next hop of routes is selected.
6. In the case of equal cost, the eBGP route is preferred over iBGP.
7. If all routes are from eBGP, the route with the lower router ID is selected.
When the path is selected, BGP puts the selected path in its routing table and propagates the
path to its neighbors.
BGP Failover Configuration
This section describes an example configuration to create redundant default gateways for Alteons at
a Web Host/ISP site, eliminating the possibility, should one gateway go down, that requests are
forwarded to an upstream router unknown to Alteon.
As shown in
Figure 15 - Example BGP Failover Configuration, page 132
, Alteon is connected to ISP
1 and ISP 2. The customer negotiates with both ISPs to allow Alteon to use the ISPs’ peer routers as
default gateways. The ISP peer routers announce themselves as default gateways to Alteon.
