BGP conditional advertisements

Normally, routes are propagated regardless of the existence of a different path. The BGP conditional advertisement feature allows a route not to be advertised based on existence or non-existence of other routes. With this new feature, a child table under bgp.neighbor is introduced. Any route matched by one of the route-map specified in the table will be advertised to the peer based on the corresponding condition route-map.

You can enable and disable conditional advertisements using the CLI.

 

To configure BGP conditional advertisements – CLI:

config router bgp set as 3

config neighbor

edit “10.10.10.10” set remote-as 3

config conditional-advertise

edit “route-map-to-match-sending”

set condition-routemap “route-map-to-match-condition” set condition-type [exist | non-exist]

next end

next end

 

BGP Neighbor Groups

The BGP Neighbor Groups feature allows a large number of neighbors to be configured automatically based on a range of neighbors’ source addresses.

 

To configure BGP Neighbor Groups – CLI:

Start by adding a BGP neighbor group:

config router bgp

config neighbor-group

edit <neighbor-group-name>

set remote-as 100

…

(All options for BGP neighbor are supported except password.)

 

end

Then add a BGP neighbor range:

 

config router bgp

config neighbor-range

edit 1

set prefix 192.168.1.0/24 set max-neighbor-num 100

set neighbor-group <neighbor-group-name>

next end

 

Network Layer Reachability Information (NLRI)

Network Layer Reachability Information (NLRI) is unique to BGP-4. It is sent as part of the update messages sent between BGP routers, and contains information necessary to supernet, or aggregate route, information. The

NLRI includes the length and prefix that when combined are the address of the aggregated routes referred to. There is only one NLRI entry per BGP update message.

BGP attributes

Each route in a BGP network has a set of attributes associated with it. These attributes define the route, and are modified as required along the route.

BGP can work well with mostly default settings, but if you are going to change settings you need to understand the roles of each attribute and how they affect those settings.

The BGP attributes include:

AS_PATH    A list of ASes a route has passed through. See AS_PATH on page 346.

MULTI_EXIT_DESC (MED)        Which router to use to exit an AS with more than one external connection.

See MULTI_EXIT_DESC (MED) on page 346.

COMMUNITY     Used to apply attributes to a group of routes. See COMMUNITY on page 347.

NEXT_HOP                                 Where the IP packets should be forwarded to, like a gateway in static rout- ing. See NEXT_HOP on page 347.

ATOMIC_AGGREGATE            Used when routes have been summarized to tell downstream routers not to de-aggregate the route. See ATOMIC_AGGREGATE on page 347.

ORIGIN             Used to determine if the route is from the local AS or not. See ORIGIN on page 348.

LOCAL_PREF   Used only within an AS to select the best route to a location (like MED)

Inbound policies on FortiGate units can change the NEXT-HOP,LOCAL-PREF, MED and AS-PATH attributes of an internal BGP (iBGP) route for its local route selection purposes. However, outbound policies on the unit cannot affect these attributes.