BGP Policy Control Overview

Introduction

BGP is know for its very flexible control of routing information. This pages list some of the common policy control features.

BGP Policy Control

  • Prefix-list
  • AS-Path List
  • Distribute-list
  • Route-maps
  • Policy Lists
  • Community List
  • Local-AS - Used for migration of AS numbers.
  • Conditional Advertisement - mechanism to withhold/send routes when a condition is met for prefixes that are already advertised in BGP.
  • Aggregation - reduces the number of BGP prefixes through summarization.
  • De-aggregation - reverse of summarization, where specific routers are injected back based on a summary route.
  • QoS Policy Propagation via BGP (QPPB) - Based on BGP attributes, prefixes can be mapped to a QoS policy on a router. Uses table-map under the BGP process to match attributes to QoS markings via route-maps. QoS marking can be IP Precedence, QoS-group ID or Traffic-Index. The BGP-destination command is used to tie it to an interfaces.

Filtering Processing Order

  • There is an order of which policy filtering is processed. This order is different depending on the direction of Updates either inbound or outbound.

Inbound Order

1. Route-Map
2. Filter-list
3. Distribute-list xor Prefix-list

  • Whatever passes Route-map is then passed to filter-list and then prefix-list (if avail).
  • If any of these deny a prefix (list route-map) that prefix is dropped.
  • If a route-map matches a prefix and allows it, the evaluation process continues.

Outbound Order

1. Distribute-list xor Prefix-list
2. Prefix-list via ORF (Outgoing Route Filtering)
3. Filter-list
4. Route-map

  • The outbound filtering is reversed to inbound.

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