13.4.5 Route refresh
A fundamental aspect of BGP4 is that only route updates are advertised. In contrast, the route refresh capability forcibly re-advertises previously propagated routes, which otherwise would not be advertised in BGP4.
Route refresh covers the re-advertisement of routes from the local device and from the remote BGP4 peer. You can select which types of routes to re-advertise. This can be executed by using the clear ip bgp command.
The following table describes the route refresh capability.
|
Capability |
Route type |
Direction of re-advertisement |
|---|---|---|
|
Resend IPv4 unicast routes |
IPv4 unicast route |
Re-advertised from local peer to remote peer. |
|
Re-receive IPv4 unicast routes |
Re-advertised from remote peer to local peer. |
The following figure illustrates how the route refresh capability operates.
|
|
![[Figure Data]](./GRAPHICS/ZU311135.GIF)
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(1) Notes on using route refresh
For a route to be resent from the remote device, both routers in a peering relationship must support the route refresh capability. To use the route refresh capability, its use must be negotiated between the routers at the time of BGP4 peer establishment.
When the inbound parameter is specified in the neighbor soft-reconfiguration configuration command, because the routes suppressed by the learned route filter are retained as invalid routes, the remote peer does not send any route refresh requests for route re-advertisement to the local peer.
The route refresh capability of the Switch complies with RFC 2918. The capability codes used in negotiation are the RFC-2918-compliant code (value:2) and a private code (value: 128). Note that other vendors may use Capability code (valuesfrom 128 to 255), which are private codes defined in RFC2434.
Take care when using the route refresh capability between the Switch and devices from other vendors.