Conditional Advertisement Enabling Conditional Installation of Prefixes Use Cases


Networks are usually subdivided into smaller, more-manageable units called autonomous systems (ASs). When BGP is used by routers to form peer relationships in the same AS, it is referred to as internal BGP (IBGP). When BGP is used by routers to form peer relationships in different ASs, it is referred to as external BGP (EBGP).

After performing route sanity checks, a BGP router accepts the routes received from its peers and installs them into the routing table. By default, all routers in IBGP and EBGP sessions follow the standard BGP advertisement rules. While a router in an IBGP session advertises only the routes learned from its direct peers, a router in an EBGP session advertises all routes learned from its direct and indirect peers (peers of peers). Hence, in a typical network configured with EBGP, a router adds all routes received from an EBGP peer into its routing table and advertises nearly all routes to all EBGP peers.

A service provider exchanging BGP routes with both customers and peers on the Internet is at risk of malicious and unintended threats that can compromise the proper routing of traffic, as well as the operation of the routers.

This has several disadvantages:

  • Non-aggregated route advertisements—A customer could erroneously advertise all its prefixes to the ISP rather than an aggregate of its address space. Given the size of the Internet routing table, this must be carefully controlled. An edge router might also need only a default route out toward the Internet and instead be receiving the entire BGP routing table from its upstream peer.

  • BGP route manipulation—If a malicious administrator alters the contents of the BGP routing table, it could prevent traffic from reaching its intended destination.

  • BGP route hijacking—A rogue administrator of a BGP peer could maliciously announce a network’s prefixes in an attempt to reroute the traffic intended for the victim network to the administrator’s network to either gain access to the contents of traffic or to block the victim’s online services.

  • BGP denial of service (DoS)—If a malicious administrator sends unexpected or undesirable BGP traffic to a router in an attempt to use all of the router’s available BGP resources, it might result in impairing the router’s ability to process valid BGP route information.

Conditional installation of prefixes can be used to address all the problems previously mentioned. If a customer requires access to remote networks, it is possible to install a specific route in the routing table of the router that is connected with the remote network. This does not happen in a typical EBGP network and hence, conditional installation of prefixes becomes essential.

ASs are not only bound by physical relationships but by business or other organizational relationships. An AS can provide services to another organization, or act as a transit AS between two other ASs. These transit ASs are bound by contractual agreements between the parties that include parameters on how to connect to each other and most importantly, the type and quantity of traffic they carry for each other. Therefore, for both legal and financial reasons, service providers must implement policies that control how BGP routes are exchanged with neighbors, which routes are accepted from those neighbors, and how those routes affect the traffic between the ASs.

There are many different options available to filter routes received from a BGP peer to both enforce inter-AS policies and mitigate the risks of receiving potentially harmful routes. Conventional route filtering examines the attributes of a route and accepts or rejects the route based on such attributes. A policy or filter can examine the contents of the AS-Path, the next-hop value, a community value, a list of prefixes, the address family of the route, and so on.

In some cases, the standard “acceptance condition” of matching a particular attribute value is not enough. The service provider might need to use another condition outside of the route itself, for example, another route in the routing table. As an example, it might be desirable to install a default route received from an upstream peer, only if it can be verified that this peer has reachability to other networks further upstream. This conditional route installation avoids installing a default route that is used to send traffic toward this peer, when the peer might have lost its routes upstream, leading to black-holed traffic. To achieve this, the router can be configured to search for the presence of a particular route in the routing table, and based on this knowledge accept or reject another prefix.

Example: Configuring a Routing Policy for Conditional Advertisement Enabling Conditional Installation of Prefixes in a Routing Table explains how the conditional installation of prefixes can be configured and verified.