Topology: Business Case: Advertise only the networks that are locally-originated to avoid transit networkLimit the size of the routing table by accepting only prefixes greater than or equal to 8 bits and less than or equal to 24 bits.Prefer the primary ISP.

BGP Peer Groups provide a template to be applied to multiple BGP neighbors, which assigns one policy/update to each of them versus defining multiple policies/updates for multiple neighbors. This effectively saves CPU resources by building a single update for all peer group members. There are a number of parameters that can be consolidated within a … Continue reading 12.30 BGP Peer Groups →

After BGP is established, changes to policy on routers take effect immediately but are not applied until an update is triggered. So ultimately, this must be done manually. There are two methods for triggering an immediate update: hard and soft resets. To perform a hard reset, which is disruptive and will drop BGP adjacency: clear … Continue reading 12.29 Clearing the BGP Session →

Multiple filters can be applied to a neighbor at once, so there is an order of operations which must be understood. It is as follows. For inbound advertisements received from a BGP neighbor, the router processes the following filter mechanisms in this order before the route reaches the BGP forwarding table. Route-Map inFilter-List inPrefix-List in … Continue reading 12.28 Filtering Order →

Consider the following scenario: The customer wants to filter and allow only a default route from both ISPs, and prefer the circuit through AS 65100 as primary. The filtering is accomplished using the ip prefix-list command, which is applied to both route map statements to limit the advertisements from the ISP to default routes only. … Continue reading 12.27 Route Map Use Case Example →

Note all the various match and set options available to manipulate BGP updates:

AS Path access-lists have several use cases. First, note the following regular expressions: _ matches any delimiter, including beginning, end, space, tab, and comma^ matches the beginning of a string$ matches the end of a string Scenarios and examples: To announce only locally-originated routes (to prevent transit AS), this filter provides an empty AS Path … Continue reading 12.25 Implementing AS Path Access Lists →

Consider the following configuration example, which pertains to inbound BGP route filtering for prefixes greater than or equal to 8 bits and less than or equal to 24 bits. Any updates with masks that fall outside the range of the prefix-list will be dropped.

Filtering of BGP prefixes is important when peering with EBGP neighbors, lest you become a transit AS. Consider the following example: If filtering is not applied on GW1 in the outbound direction, then ISP1 may receive routing information about the network update originally sent by ISP2, effectively making AS 65000 a transit AS. Filtering can … Continue reading 12.23 Filtering of BGP Routing Updates →

Consider the following example. There are four routers - 2 CE routers in AS 65000 and 2 PE routers in AS 65100 (ISP). We use a route-map on router GW2 to set the BGP metric (MED) - the 6th value in the path selection process - to influence ISP router path selection to use the … Continue reading 12.22 Setting MED with a Route Map →