Brocade Multi-Service IronWare Routing Configuration Guide (Supporting R05.6.00) - Manual (Page 35)

Multi-Service IronWare Routing Configuration Guide 7
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Overview of BGP4
NOTE
The device supports BGP4 load sharing among multiple equal-cost paths. BGP4 load sharing
enables the device to balance traffic across the multiple paths instead of choosing just one
path based on device 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. If compare-router ID is enabled, prefer the path that comes from the BGP4 device with the
lowest device ID. If a path contains originator ID attributes, then originator ID is substituted for
the ROUTER ID in the decision.
12. Prefer the path with the minimum cluster list length.
13. If the route is a BGP4 VRF instance, prefer the route with the smallest RD value.
14. Prefer the route that comes from the lowest BGP4 neighbor address.
BGP4 message types
BGP4 devices communicate with neighbors (other BGP4 devices) using the following types of
messages:
• OPEN
• UPDATE
• KEEPALIVE
• NOTIFICATION
• ROUTE REFRESH
OPEN message
After a BGP4 device establishes a TCP connection with a neighboring BGP4 device, the devices
exchange OPEN messages. An open message indicates the following:
• BGP4 version – Indicates the version of the protocol that is in use on the device. BGP4 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 device.
• AS number – An autonomous system number (ASN) identifies the AS to which the BGP4 device
belongs. The number can be up to four bytes.
Hold Time – The number of seconds a BGP4 device will wait for an UPDATE or KEEPALIVE
message (described below) from a BGP4 neighbor before assuming that the neighbor is not
operational. BGP4 devices 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 device
closes the TCP connection to the neighbor and clears any information it has learned and
cached from the neighbor.
You can configure the Hold Time to be 0, in which case a BGP4 device will consider neighbors
to always be up. For directly-attached neighbors, you can configure the device to immediately
close the TCP connection to the neighbor and clear entries learned from an EBGP neighbor if
the interface to that neighbor goes down. This capability is provided by the fast external fail
over feature, which is disabled by default.