Dell BigIron RX Series, Руководство по настройке

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742 BigIron RX Series Configuration Guide
53-1002253-01
Overview of BGP4
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9. If all the comparisons above are equal, prefer the route with the lowest IGP metric to the BGP4
next hop. This is the closest internal path inside the AS to reach the destination.
10. If the internal paths also are the same and BGP4 load sharing is enabled, load share among
the paths otherwise go to Step 11.
NOTE
The BigIron RX supports BGP4 load sharing among multiple equal-cost paths. BGP4 load
sharing enables the device to balance the traffic across the multiple paths instead of choosing
just one path based on router ID. For EBGP routes, load sharing applies only when the paths
are from neighbors within the same remote AS. EBGP paths from neighbors in different ASs are
not compared, unless multipath multi-as is enabled.
11. Prefer the path that comes from the BGP4 router with the lowest router ID, if compare-router ID
is enabled. If a path contains originator ID attributes, then originator ID is substituted for the
ROUTER ID in the decision process.
12. Prefer the path with the minimum cluster list length.
13. If the route is a BGP VRF instance, prefer the route with the smallest RD value.
BGP4 message types
BGP4 routers communicate with their neighbors (other BGP4 routers) using the following types of
messages:
•
OPEN
•
UPDATE
•
KEEPALIVE
•
NOTIFICATION
•
ROUTE REFRESH
OPEN message
After a BGP4 router establishes a TCP connection with a neighboring BGP4 router, the routers
exchange OPEN messages. An OPEN message indicates the following:
•
BGP version – Indicates the version of the protocol that is in use on the router. BGP version 4
supports Classless Interdomain Routing (CIDR) and is the version most widely used in the
Internet. Version 4 also is the only version supported on the BigIron RX.
•
AS number – A two-byte number that identifies the AS to which the BGP4 router belongs.
•
Hold Time – The number of seconds a BGP4 router will wait for an UPDATE or KEEPALIVE
message (described below) from a BGP4 neighbor before assuming that the neighbor is dead.
BGP4 routers exchange UPDATE and KEEPALIVE messages to update route information and
maintain communication. If BGP4 neighbors are using different Hold Times, the lowest Hold
Time is used by the neighbors. If the Hold Time expires, the BGP4 router closes its TCP
connection to the neighbor and clears any information it has learned from the neighbor and
cached.
You can configure the Hold Time to be 0, in which case a BGP4 router will consider its