BGP route reflector
A BGP device selects a preferred BGP4 route for a specific prefix 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 configurations 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 traffic 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 traffic miss-forwarding or packet drop can occur.
When a BGP device is configured as only a route reflector 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 reflector 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.
Configuring BGP route reflector
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 fill 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 configurable in any order under the BGP4 address
family configuration level.
Perform the following steps to advertise a preferred BGP4 route not installed in the RTM.
1. Configure 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 configure a BGP4 unicast route for a specified VRF instance, use the
vrf
vrf-name
parameter. The
vrf
vrf-name
parameter allows you to create a VPN routing or forwarding instance specified by the
vrf-name
variable. The
vrf-name
variable specifies 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 reflector
FastIron Ethernet Switch Layer 3 Routing
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