Controlling,Filtering,Routing Updates

Question 1

Routing Protocol Performance Solutions

Answer 1

Design changes, such as limiting the number of routing
protocols used.
Using passive interfaces to prevent routing protocol updates from
being advertised out an interface.
Route filtering techniques to block specific routes from being
advertised:
• Access control lists (ACLs, used to control “data plane traffic”)
• Route maps
• Distribute lists
• Prefix lists

Question 2

Route Filtering

Answer 2

Using route maps, distribute lists, or prefix lists instead of
access lists provides greater route filtering flexibility.
Filters can be configured to:
• Prevent updates through router interfaces.
• Control the advertising of routes in routing updates.
• Control the processing of routing updates.
If filters are not configured correctly or if filters are applied to
wrong interfaces, network performance issues may occur.

Question 3

Route Filtering Process

Answer 3

1. A router stores the incoming routing update in the buffer and triggers a decision.
2. Is there an incoming filter applied to this interface?
   • If no, then the routing update packet is processed normally.
3. Otherwise, is there an entry in the filter matching the routing update packet?
   • If no, then the routing update packet is dropped.
4. Otherwise, the router processes the routing update according to the filter.

Question 4

Controlling Routing Updates

Answer 4

• There is not one type of route filter that is appropriate for every
situation.
• A variety of techniques may be used to make the network run
smoothly.

Question 5

Controlling Routing Update Traffic

Answer 5

Passive interface:
This feature prevents all routing updates from
being sent through an interface. For EIGRP, OSPF, and IS-IS, this
method includes Hello protocol packets.

Default routes:
This feature instructs the router that if it does not
have a route for a given destination, it should send the packet to the
default route; therefore, no dynamic routing updates about the
remote destinations are necessary.

Static routes:
This feature allows routes to remote destinations to be
manually configured in the router; therefore, no dynamic routing
updates about the remote destinations are necessary.

Question 6

Static and Default Routes

Answer 6

Static routes are manually configured routes that are used to:
• Define specific routes to use when two autonomous systems must exchange routing information.
• Define routes to destinations over a WAN link to eliminate the need for a dynamic routing protocol.

Static route configuration considerations
• If you want a router to advertise a static route in a routing protocol, it might need to be redistributed.
• To reduce the number of static route entries, define a default static route.

Question 7

Passive Interfaces

Answer 7

Passive interfaces prevent routing updates from being sent
and/or received for a specified protocol.

• RIP interfaces listen but will not send updates.
• OSPF and EIGRP interfaces do not listen for or send updates and
therefore no neighbor adjacencies can be established.

Question 8

Route Map Applications

Answer 8

Route filtering during redistribution
• All IP routing protocols can use route maps for redistribution filtering.
• Applied using the redistribute protocol route-map router
configuration command.
Policy-based routing (PBR)
• PBR allows the operator to define routing policy other than basic
destination-based routing using the routing table.
• Applied using the ip policy route-map interface configuration
command.
NAT
• Route maps provide more control over which private addresses are
translated to public addresses.
BGP
• Route maps are the primary tools for implementing BGP policy.

Question 9

Route Maps

Answer 9

Route maps are similar to a scripting language for these reasons:
1. They work like a more sophisticated access list.
2.Lines are sequence-numbered for easier editing.
3.Route maps are named rather than numbered for easier documentation.

Match criteria and set criteria can be used, similar to the “if, then” logic in a scripting language.
They allow conditions to be tested using match commands and if the conditions match, actions specified by set commands can be taken to modify attributes of the packet or routes. Route maps are similar in function to ACLs, but provide far more control.