How bgp4 selects a path for a route – Brocade Communications Systems Layer 3 Routing Configuration ICX 6650 User Manual
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Brocade ICX 6650 Layer 3 Routing Configuration Guide
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BGP4 overview
Although a Layer 3 Switch BGP4 route table can have multiple routes to the same destination, the
BGP4 protocol evaluates the routes and chooses only one of the routes to send to the IP route
table. The route that BGP4 chooses and sends to the IP route table is the preferred route and will
be used by the Brocade Layer 3 switch. 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 comprised of the network mask bits and an IP address
(IP-address/ mask-bits); for example, 192.168.129.0/18 indicates a network mask of 18 bits
applied to the IP address 192.168.129.0. When a BGP4 Layer 3 switch advertises a route to
one of its neighbors, the route is expressed in this format.
•
AS-path – A list of the other autonomous systems through which a route passes. BGP4 routers
can use the AS-path to detect and eliminate routing loops. For example, if a route received by a
BGP4 router contains the AS that the router is in, the router does not add the route to its own
BGP4 table. (The BGP4 RFCs refer to the AS-path as “AS_PATH”.)
•
Additional path attributes – A list of additional parameters that describe the route. The route
origin and next hop are examples of these additional path attributes.
NOTE
The Layer 3 switch re-advertises a learned best BGP4 route to the Layer 3 switch neighbors even
when the software does not select that route for installation in the IP route table. The best BGP4
route is the route that the software selects based on comparison of the BGP4 route path attributes.
After a Brocade Layer 3 switch successfully negotiates a BGP4 session with a neighbor (a BGP4
peer), the Brocade Layer 3 switch exchanges complete BGP4 route tables with the neighbor. After
this initial exchange, the Brocade Layer 3 switch and all other RFC 1771-compliant BGP4 routers
send UPDATE messages to inform neighbors of new, changed, or no longer feasible routes. BGP4
routers do not send regular updates. However, if configured to do so, a BGP4 router does regularly
send KEEPALIVE messages to its peers to maintain BGP4 sessions with them if the router does not
have any route information to send in an UPDATE message.Refer to
page 285 for information about BGP4 messages.
How BGP4 selects a path for a route
When multiple paths for the same route are known to a BGP4 router, the router uses the following
algorithm to weigh the paths and determine the optimal path for the route. The optimal path
depends on various parameters, which can be modified. (Refer to
1. Is the next hop accessible though an Interior Gateway Protocol (IGP) route? If not, ignore the
route.
NOTE
The device does not use the default route to resolve BGP4 next hop. Also refer to
2. Use the path with the largest weight.