Technical Note: Common issue when trying to advertise a subnet to BGP peers

alouvros · ‎05-28-2015

Description

This article describes the common issues when trying to advertise a subnet to BGP peers.

Scope

All firmware versions (BGP specific defined by the RFC 4271)

Solution

The BGP network command has a different use compared to the network command used by IGPs. This command is not used to enable BGP on interfaces (as a matter of fact, there is no such concept in BGP, as there is in IGPs), but it is used to inject routes from the routing table to the BGP table so they can be advertised to BGP peers. For this to happen, an exact route for the prefix that needs to be advertised should be installed in the routing table on the BGP-speaking router.

A very common mistake, while configuring such a scenario, is when the BGP network command does not reference the exact prefix for a route which is installed in the routing table, or such a route to this prefix is not installed in the routing table, at all.

The BGP network command must reference the exact prefix for which a route is currently installed in the routing table of the BGP-speaking router. When such a route for the exact prefix is not installed in the routing table, a workaround is to use a black hole route (outgoing interface null0, in other Vendors context) to this prefix. This way, the route in question will be installed in the routing table, and it will be injected into the BGP table and advertised to BGP peers.

CLI Configuration

# config router bgp

config network

edit 0

set prefix 10.0.0.0/8

end

# config router static

edit 0

set blackhole enable
set dst 10.0.0.0 255.0.0.0

Verification of Configuration and troubleshooting:

Routing Table	get router info routing-table all
RIB	get router info routing-table database
FIB (kernel routing table)	get router info kernel or diagnose ip route list
Routing Cache	diag ip rtcache list
BGP config	get router bgp
BGP info for networks	get router info bgp network