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Route Filtering with Different Routing Protocol

 Route Filtering with Any Routing Protocol

Route filtering is used to selectively control which routes are advertised or received from neighboring routers, helping manage traffic flows, reduce memory utilization, or enhance security. In vector-based routing protocols, filtering occurs during the advertisement of routes. In link-state protocols like OSPF, filtering typically happens at the Area Border Routers (ABRs) when routes enter or leave an area.

Example of Route Filtering in OSPF

In this example, ip prefix-list is used to create filtering rules that deny or permit specific routes. These rules are applied using the area command to control which routes enter or exit an area.

  • Technical Tip: When configuring OSPF route filtering with prefix lists, always keep in mind the hierarchical structure of areas and the flow of LSAs. Filtering usually applies at ABRs to prevent specific LSAs from being propagated across areas.

R2 Configuration:

ip prefix-list PREFIX-FILTER seq 5 deny 172.16.1.0/24

ip prefix-list PREFIX-FILTER seq 10 permit 0.0.0.0/0 le 32

router ospf 1

 area 0 filter-list prefix PREFIX-FILTER in

EIGRP Filtering

EIGRP Route Filtering can be done using distribute lists, route-maps, or access-lists. Distribute lists can be applied in inbound or outbound directions to filter updates before they are added to the routing table or before being sent to neighbors.

  • Technical Tip: Always use the correct direction (in or out) when applying distribute lists to avoid unintended filtering of important routes.

Offset List: Used in EIGRP to influence route selection by modifying the metric for a given prefix. Example: offset-list 1 in 100 Ethernet 0/0 adds 100 to the metric of the matched prefix.

OSPF Filtering

In OSPF, routes are not advertised directly; instead, LSAs are propagated. Filtering LSAs is more complex because OSPF requires all routers in an area to have identical Link-State Databases (LSDBs). Filtering typically happens through:

  • Type 3 LSA Filtering: Prevents ABRs from generating specific Type 3 LSAs (summary routes) for other areas. Example: area 34 filter-list prefix PREFIX-FILTER in.
  • Distribute Lists: Filters routes before they are installed in the local routing table but does not affect the LSDB. Example: distribute-list prefix FILTER-LIST in.
  • Technical Tip: Be prepared to explain the limitations of OSPF filtering and how it differs from other protocols like EIGRP. In particular, know that filtering in OSPF focuses on LSAs, not routes, and how distribute lists work in OSPF.

ABR LSA Suppression: You can also use the area range no-advertise option to prevent the generation of specific summary LSAs.

Database Filter: Prevents LSAs from being flooded out of a specific interface. Example:

interface Serial 1/5

ip ospf database-filter all out

Local OSPF Filtering

In scenarios where routes need to be filtered locally, distribute lists can prevent specific routes from being installed in the Routing Information Base (RIB) without modifying the LSDB. However, remember that this affects only local routing decisions and does not impact LSA propagation.

  • Technical Tip: Filtering in OSPF is often tricky since LSAs still propagate. Be ready to explain how local filtering only impacts the RIB and doesn't affect how routes are distributed across OSPF neighbors.

BGP Filtering

BGP Filtering is used to control route advertisements between Autonomous Systems. Methods include:

  • Outbound Route Filtering (ORF): Allows a router to send a prefix list to its neighbor, asking it not to send certain routes. This reduces the routing table size on both sides.
  • Prefix Lists and Route-Maps: BGP uses prefix lists and route-maps extensively for filtering inbound or outbound advertisements.
  • Technical Tip: In interviews, highlight that BGP filtering is critical in large networks for controlling routing policies and preventing the advertisement of unnecessary routes.

Summary of Technical Tips:

  • OSPF Route Filtering: Focuses on LSAs, not routes. Use distribute lists carefully, as they only affect local routing tables.
  • EIGRP Filtering: Use distribute lists and offset lists to manipulate EIGRP routing decisions effectively. Be clear about the direction (in or out) of the filtering.
  • BGP Filtering: Emphasize the importance of prefix-lists and route-maps in controlling BGP advertisements and how ORF reduces overhead.

By understanding how route filtering works in different protocols, you'll be prepared to answer both conceptual and technical questions in interviews, focusing on real-world use cases where route filtering optimizes traffic flow and network performance.

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