BGP Policy Control Overview
Table of Contents
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.