BGP route reector
A BGP device selects a preferred BGP4 route for a specic prex learned from multiple peers by using the BGP best path selection
algorithm, and installs the BGP4 route in the Routing Table Manager (RTM). The BGP device marks the preferred BGP4 route as the
best route, and advertises the route to other BGP4 neighbors. Generally, the RTM route table size is larger than the number of unique
BGP4 routes in the BGP4 route table. All preferred BGP4 routes are installed in RTM and are marked as the best BGP4 routes.
However, in certain congurations it is possible that the total number of preferred BGP4 routes may exceed the RTM route table size
limit. Therefore, some preferred BGP4 routes may not be installed in the RTM, and the BGP device is not able to forward trac correctly
for those BGP4 routes. Those BGP4 routes are not considered as the best BGP4 routes, and are not advertised to other BGP4
neighbors because trac miss-forwarding or packet drop can occur.
When a BGP device is congured as only a route reector server, and is not placed directly in the forwarding path, it is possible to mark
all preferred BGP4 routes as the best routes to be advertised to other BGP4 neighbors even if the routes are not installed in the RTM. To
support the behavior of a BGP device as a route reector server in such a scenario, use the always-propagate command and the rib-
route-limit command.
NOTE
The always-propagate command and the rib-route-limit command are supported.
Conguring BGP route reector
The always-propagate command enables a device to mark a preferred BGP4 route not installed in the RTM as the best route, and
advertise the route to other BGP4 neighbors. The same process for outbound route policy continues to apply to all best BGP4 routes.
The rib-route-limit command limits the number of BGP4 Routing Information Base (RIB) routes that can be installed in the RTM. The
RTM must be able to reserve enough entries for Interior Gateway Protocol (IGP) routes because the IGP routes are required by BGP4 to
resolve BGP4 next-hop entries. If the RTM is not able to reserve enough entries for IGP routes, BGP4 RIB routes can ll the entire RTM
with only BGP4 route entries. The rib-route-limit command enables IGP and BGP4 route entries to be installed in the RTM.
NOTE
The always-propagate command and the rib-route-limit command are congurable in any order under the BGP4 address
family conguration level.
Perform the following steps to advertise a preferred BGP4 route not installed in the RTM.
1. Congure a BGP4 unicast route. Enter a command such as the following.
device(config-bgp)# address-family ipv4 unicast
Syntax: address-family ipv4 unicast [ vrf vrf-name ] | ipv6 unicast
NOTE
To congure a BGP4 unicast route for a specied VRF instance, use the vrf vrf-name parameter. The vrfvrf-name
parameter allows you to create a VPN routing or forwarding instance specied by the vrf-name variable. The vrf-name
variable species the name of the VRF instance you want to create.
2. Enter the always-propagate command to enable a preferred BGP4 route (not installed in the RTM) to be advertised to other
BGP4 neighbors.
device(config-bgp)# always-propagate
Syntax: always-propagate
BGP route reector
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