Users Guide

1338 BGP
routes. Delay and hold timers limit how often phase 2 of the decision process
runs. This phase 2 dampening limits route origination, as does IP event
dampening when interface flaps would otherwise cause rapid origination.
BGP originates a default route to all neighbors if the default-information
originate command is given and the default route is among the routes BGP
redistributes. Because this default origination depends on redistribution, BGP
normally only originates a default if a default is in the routing table. The
always option can be configured to relax this requirement. If the routing table
does not contain a default route, but the network administrator wants BGP to
originate a default route, the administrator can configure a static default
route. To prevent the static default route from affecting the local router's
forwarding, the default route can be given a preference of 255 (ip route 0.0.0.0
0.0.0.0 next-hop 255) or it can be configured as a reject route (ip route 0.0.0.0
0.0.0.0 Null0).
BGP can also originate a default to a specific neighbor using neighbor
default-originate. This form of default origination does not install a default
route in the BGP routing table (it will not appear in show ip bgp), nor does it
install a default route in the Adj-RIB-Out for the update group of peers so
configured (it will not appear in show ip bgp neighbor advertised-routes). A
neighbor specific default has no MED and the Origin is IGP. A neighbor-
specific default is only advertised if the Adj-RIB-Out does not include a
default learned by other means, either from default-information originate or
a default learned from a peer. This type of default origination is not
conditioned on the presence of a default route in the routing table.
Equal Cost Multipath (ECMP)
By default, Dell EMC Networking BGP selects a single next hop for each BGP
route. Dell EMC Networking BGP can be configured to install BGP routes
with up to 16 next hops in the common routing table (RTO). The network
administrator can independently configure the maximum number of next
hops for routes through internal and external peers.
Paths can be used to form an ECMP route when they are both internal or
both external, the resolved next hop is different, and the following attributes
are the same:
• local preference
• AS path length