Information Technology Reference
In-Depth Information
ORIGINATOR_ID
ORIGINATOR_ID is used as a loop-prevention mechanism inside an AS when route
reflectors (RRs) are deployed. It is created by the first RR and is not modified by subsequent
RRs. ORIGINATOR_ID is the router ID of either of the following:
•
The BGP speaker that originates the route in the local AS, such as routes injected
using the
network
command.
•
The BGP border router of the same AS if the route is learned via eBGP.
The ORIGINATOR_ID is 32 bits long and should be received only from iBGP peers. On
an RR, the ORIGINATOR_ID is used in place of the router ID in the path selection. When
an iBGP speaker receives updates containing its own ORIGINATOR_ID, it discards the
routes, breaking the routing information loop. A BGP speaker should not create an
ORIGINATOR_ID attribute if one already exists.
CLUSTER_LIST
CLUSTER_LIST is another loop-prevention mechanism inside an AS when RRs are
deployed. This attribute records the list of CLUSTER_IDs that a prefix has traversed in
an RR environment. When an RR reflects a route from its clients to nonclients outside the
cluster, from nonclients to clients, or from one client to another client, it prepends the local
CLUSTER_ID to the CLUSTER_LIST. If the update has an empty CLUSTER_LIST, the
RR creates one. Using this attribute, an RR can identify if the routing information is looped
back to the same cluster. If the local CLUSTER_ID is found in the CLUSTER_LIST, the
update is discarded, breaking the routing information loop. A detailed discussion of the con-
figuration and design of CLUSTER_LIST and CLUSTER_ID is presented in Chapter 7,
“Scalable iBGP Design and Implementation Guidelines.”
Understanding Internal BGP
BGP was designed to provide a loop-free path among a series of autonomous systems on
the Internet. The mechanism to ensure a loop-free topology is the AS_PATH attribute.
Consider Figure 2-2, in which three autonomous systems are interconnected. If router R1
in AS 65000 advertises a prefix to R3 in AS 65001, R1 prepends 65000 to the AS_PATH
list for the prefix when it sends the prefix to R3. If that same prefix is received by AS 65000
again, a border BGP speaker rejects the prefix, because it detects a loop in the AS_PATH
attribute.
