ferrix/docs/content/docs/guides/passing-routes-on.md

+++ title = "Route Advertisement" description = "Describes how Ferrix advertises originated and learned routes, how BGP roles and communities restrict advertisement, and how to configure export policy." weight = 6 +++

Ferrix advertises two kinds of routes to its neighbors:

• Originated routes, configured with the originate statement. Ferrix advertises them to every neighbor.
• Learned routes, received from other neighbors. Ferrix advertises the best path for each prefix to the neighbors whose export policy enables learned_routes.

Configuring Originated Routes

The following example originates one IPv4 and one IPv6 prefix:

originate { prefix: "198.51.100.0/24" }
originate {
  prefix: "2001:db8:100::/48"
  med: 50
  as_path_prepend: 1
  large_communities: "64500:2:0"
}

Ferrix advertises an originated route with an AS path that contains the local AS number and, on each session, the local address of the session as the next hop. For the optional attributes, see OriginatedRoute.

Advertising Learned Routes

To advertise learned routes to a neighbor, enable learned_routes in its export policy:

export { learned_routes: true }

The top-level export statement applies to every neighbor that does not have its own export statement. A neighbor's export statement replaces the top-level statement entirely.

Ferrix does not advertise a learned route in the following cases. These rules cannot be disabled:

• To the neighbor from which the route was received.
• To a neighbor whose AS number is in the AS path of the route.
• From one IBGP neighbor to another IBGP neighbor. Route reflection is not supported.
• To an EBGP neighbor, if the route has the NO_EXPORT community; to any neighbor, if the route has the NO_ADVERTISE community.

Attributes of Advertised Routes

Table 1 lists how Ferrix modifies the attributes of a learned route for each type of neighbor.

Neighbor type	AS path	Next hop	MED	Local preference
EBGP	Local AS prepended	Local address of the session	Removed	Not sent
IBGP	Unchanged	Local address of the session	Unchanged	Sent; 100 if the route has none
ROUTE_SERVER_CLIENT (RFC 7947)	Unchanged	Unchanged, if the client can use it	Unchanged	Not sent

Ferrix advertises unknown optional transitive attributes with the Partial bit set, and does not advertise unknown non-transitive attributes.

Roles and Route Advertisement

Ferrix adds the Only to Customer (OTC) attribute to routes received from providers, peers, and route servers (RFC 9234), and advertises routes with the OTC attribute only to customers. Routes received from customers are advertised to all neighbors. Therefore, when every neighbor has a role, the local router provides transit only to its customers.

export { learned_routes: true }
peers { name: "customer-a" role: CUSTOMER ... }
peers { name: "transit-a"  role: PROVIDER ... }
peers { name: "ixp-a-rs"   role: ROUTE_SERVER ... }

Caution

A neighbor without a role is not subject to this restriction. If learned_routes is enabled for such a neighbor, Ferrix advertises all learned routes to it, including routes from providers and peers. Ferrix logs a warning at startup for each such neighbor.

Configuring Export Policy

The export policy can add and remove communities on advertised routes, and prepend the local AS number to the AS path:

export {
  remove_communities: "64500:2:*"
  add_large_communities: "64500:3:1"
  as_path_prepend: 2
}

Statement	Description
remove_communities	Removes matching communities before communities are added. A pattern has two fields for a standard community and three for a large community; each field is a number or *.
add_communities, add_large_communities	Adds communities to every advertised route.
as_path_prepend	Prepends the local AS number the specified number of additional times. Range: 0 through 16. Default: 0.

Control Communities

Control communities allow a route's communities to restrict the neighbors to which Ferrix advertises it. The format is the one used by Internet exchange route servers. To enable control communities, configure the following top-level statement:

control_communities: true

Table 2 lists the control communities, where L is the local AS number and P is a neighbor's AS number.

Standard community	Large community	Action
0:P	L:0:P	Do not advertise to the neighbor with AS P.
L:P	L:1:P	Advertise to the neighbor with AS P. If present, advertise only to neighbors named in such communities.
0:L	L:0:0	Do not advertise to any neighbor.

Control communities are evaluated as follows:

• "Do not advertise" takes precedence over "advertise".
• A route without control communities is advertised as the other rules permit.
• Standard communities can express AS numbers up to 65535 only. Use the large community forms for four-octet AS numbers.
• Ferrix removes control communities from a route when it advertises the route.
• Control communities apply to originated routes and to communities added by the import policy.

The following example prevents the advertisement of an originated route to AS 64496:

control_communities: true
originate {
  prefix: "198.51.100.0/24"
  large_communities: "64500:0:64496"
}

Important

When control communities are enabled, Ferrix interprets the standard community L:n as "advertise to AS n", and the large communities L:0:n and L:1:n as listed in Table 2. Use large communities L:2:n and higher for informational communities. Do not remove customers' control communities with remove_communities.

Update Generation

Ferrix groups routes with identical attributes into a single UPDATE message. It collects changes for 50 milliseconds before it sends them.

When a neighbor requires the full routing table, at session establishment or after a ROUTE-REFRESH message, Ferrix collects the entire table before it sends it, so that prefixes with identical attributes share UPDATE messages. At most four sessions collect a full table concurrently; other sessions wait.

Each session has its own export task, which records the routes advertised on the session. If the neighbor reads slowly, only that session's task waits; other sessions are not affected.

In the transit benchmark, in which one neighbor advertises 1.23 million routes and Ferrix advertises them to a second neighbor, the second neighbor receives all routes after 2.1 seconds, with an average of four routes in each UPDATE message.

Verifying Route Advertisement

The metric bgp_routes_exported_total{change} counts learned routes that Ferrix advertised (announced) and withdrew (withdrawn). Ferrix does not provide a view of the routes advertised to a neighbor (Adj-RIB-Out); to display them, use the neighbor's own tools.

Limitations

• Per-client best path selection for route servers is not supported. Every client receives the same best path.
• ADD-PATH is not supported for advertisement; only the best path is advertised.
• Route reflection is not supported.
• UPDATE messages are generated separately for each session, even when several sessions receive identical routes.