Relationship Between The Bgp4 Route Table And The Ip Route Table - Brocade Communications Systems FastIron SX 800 Configuration Manual

BGP4 overview
FIGURE 27 Example BGP4 autonomous systems

Relationship between the BGP4 route table and the IP route table

The device BGP4 route table can have multiple routes or paths to the same destination, which are learned from different BGP4
neighbors. A BGP4 neighbor is another device that also is running BGP4. BGP4 neighbors communicate using Transmission Control
Protocol (TCP) port 179 for BGP4 communication. When you configure the device for BGP4, one of the configuration tasks you
perform is to identify the device's BGP4 neighbors.
Although a device's BGP4 route table can have multiple routes to the same destination, the BGP4 protocol evaluates the routes and
chooses only one to send to the IP route table. The route that BGP4 chooses and sends to the IP route table is the preferred route . This
route is what the device advertises to other BGP4 neighbors. If the preferred route goes down, BGP4 updates the route information in
the IP route table with a new BGP4 preferred route.
NOTE
If IP load sharing is enabled and you enable multiple equal-cost paths for BGP4, BGP4 can select more than one equal-cost
path to a destination.
A BGP4 route consists of the following information:
• Network number (prefix) - A value made up of the network mask bits and an IP address; for example, 10.215.129.0/18
indicates a network mask of 18 bits applied to the IP address 10.215.129.0. When a BGP4 device advertises a route to one of
its neighbors, it uses this format.
• AS-path - A list of the other autonomous systems through which a route passes. BGP4 devices can use the AS-path to detect
and eliminate routing loops. For example, if a route received by a BGP4 device contains the AS that the device is in, the device
does not add the route to its own BGP4 table. (The BGP4 RFCs refer to the AS-path as "AS_PATH", and RFC 4893 uses
"AS4_PATH" in relation to AS4s.)
• Additional path attributes - A list of additional parameters that describe the route. The route MED and next hop are examples of
these additional path attributes.
NOTE
The device re-advertises a learned best BGP4 route to the device's neighbors even when the software does not select that
route for installation in the IP route table. This can happen if a route from another protocol, for example, OSPF, is preferred. The
best BGP4 route is the route that BGP4 selects based on comparison of the BGP4 route path's attributes.
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FastIron Ethernet Switch Layer 3 Routing
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