Dell PowerConnect B-RX BigIron RX Series Configuration Guide v02.7.02 - Page 814

26 Overview of BGP4 • 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 device. • 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 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 fallover feature, which is disabled by default. • BGP Identifier - The router ID. The BGP Identifier (router ID) identifies the BGP4 router to other BGP4 routers. The device use the same router ID for OSPF and BGP4. If you do not set a router ID, the software uses the IP address on the lowest numbered loopback interface configured on the router. If the device does not have a loopback interface, the default router ID is the lowest numbered IP address configured on the device. For more information or to change the router ID, refer to "Changing the router ID" on page 788. • Parameter list - An optional list of additional parameters used in peer negotiation with BGP4 neighbors. UPDATE message After BGP4 neighbors establish a BGP4 connection over TCP and exchange their BGP4 routing tables, they do not send periodic routing updates. Instead, a BGP4 neighbor sends an update to its neighbor when it has a new route to advertise or routes have changed or become unfeasible. An UPDATE message can contain the following information: • Network Layer Reachability Information (NLRI) - The mechanism by which BGP4 supports Classless Interdomain Routing (CIDR). An NLRI entry consists of an IP prefix that indicates a network being advertised by the UPDATE message. The prefix consists of an IP network number and the length of the network portion of the number. For example, an UPDATE message with the NLRI entry 192.215.129.0/18 indicates a route to IP network 192.215.129.0 with network mask 255.255.192.0. The binary equivalent of this mask is 18 consecutive one bits, thus "18" in the NLRI entry. 742 BigIron RX Series Configuration Guide 53-1001810-01