Dell BigIron RX Series, Руководство по настройке

Страница: 819 / 1458

BigIron RX Series Configuration Guide 743
53-1002253-01
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.
<IP address> / <CIDR prefix> .
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