Configuration Guide User guide

FastIron Configuration Guide 1345
53-1002494-02
BGP4 overview
8. Prefer routes in the following order:
• Routes received through EBGP from a BGP4 neighbor outside of the confederation
• Routes received through EBGP from a BGP4 router within the confederation
• Routes received through IBGP
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, prefer the route that comes from the BGP4 router with the lowest router
ID.
NOTE
Brocade Layer 3 switches support BGP4 load sharing among multiple equal-cost paths. BGP4
load sharing enables the Layer 3 switch 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 autonomous systems are not compared.
BGP4 message types
BGP4 routers communicate with their neighbors (other BGP4 routers) using the following types of
messages:
• OPEN
• UPDATE
• KEEPALIVE
• NOTIFICATION
OPEN messages exchanged with BGP4 routers
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 Brocade Layer 3 switches.
• 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