Users Guide

BGP 1403
Networking BGP retains the NEXT_HOP address if it is an address on the
subnet used to connect the peers but is not the peer's IP address. Otherwise,
Dell EMC Networking BGP sets the NEXT_HOP path attribute to the local
IP address on the interface to the peer. Dell EMC Networking BGP does not
support “first party” next hop. Dell EMC Networking does not allow the
network operator to disable third party next hop. Dell EMC Networking does
not support multihop EBGP. (RFC 4271 section 5.1.3)
The Multi Exit Discriminator (MED) attribute is sent to external peers when
advertising routes that originate within the local AS. The MED value may be
configured for redistributed routes, either using the metric option on the
redistribution command or by configuring a default-metric. If the MED is
not configured for a redistributed route, the route is advertised without a
MED attribute. Routes originated through the network command set the
MED to the metric of the IGP route to the same network. The MED may also
be set on locally-originated routes using a route map. The MED for non-
locally-originated routes is propagated to internal peers. By default, MEDs are
only compared when two routes are received from external peers in the same
AS. There is a configuration option to force BGP to compare MEDs for paths
received from different autonomous systems.
When BGP receives an UPDATE message from an external peer, it assigns a
local preference value during phase 1 of the decision process. Local preference
is set to a fixed, configured value which is the same for all paths received from
all neighbors. This value is attached to the path in the LOCAL_PREF path
attribute when the path is advertised to internal peers. The configured
default local preference is assigned to all locally-originated routes and to the
paths for all active aggregate addresses. LOCAL_PREF can be configured to
different values on different routers to influence the exit point from the AS
that other routers select for each destination. An inbound route map can
override the default local preference. LOCAL_PREF is never included in
paths sent to external peers. If the user changes the default local preference
while BGP is running, BGP automatically initiates an immediate soft
inbound reset for all external peers, updates the local preference for all locally-
originated routes, and re-computes routes.
For each aggregate address configured, the network administrator may specify
whether to advertise an AS_SET of the AS numbers in the paths from which
the aggregate was formed. When the aggregate is advertised with an empty
AS Path, the ATOMIC_AGGREGATE path attribute is attached to the path.
In either case, the AGGREGATOR path attribute is attached.