Dell PowerConnect B-RX BigIron RX Series Configuration Guide v02.8.00 - Page 819

Overview of BGP4 26 neighbors to always be up. For directly-attached neighbors, you can configure the BigIron RX 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 BigIron RX 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 790. • 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. • Path attributes - Parameters that indicate route-specific information such as path information, route preference, next hop values, and aggregation information. BGP4 uses the path attributes to make filtering and routing decisions. • Unreachable routes - A list of routes that have been in the sending router's BGP4 table but are no longer feasible. The UPDATE message lists unreachable routes in the same format as new routes. /. KEEPALIVE message BGP4 routers do not regularly exchange UPDATE messages to maintain the BGP4 sessions. For example, if a device configured to perform BGP4 routing has already sent the latest route information to its peers in UPDATE messages, the router does not send more UPDATE messages. Instead, BGP4 routers send KEEPALIVE messages to maintain the BGP4 sessions. KEEPALIVE messages are 19 bytes long and consist only of a message header; they contain no routing data. BGP4 routers send KEEPALIVE messages at a regular interval, the Keep Alive Time. The default Keep Alive Time on device is 60 seconds. A parameter related to the Keep Alive Time is the Hold Time. A BGP4 router's Hold Time determines how many seconds the router will wait for a KEEPALIVE or UPDATE message from a BGP4 neighbor before deciding that the neighbor is dead. The Hold Time is negotiated when BGP4 routers exchange OPEN messages; the lower Hold Time is then used by both neighbors. For example, if BigIron RX Series Configuration Guide 743 53-1002253-01