1. Route Optimisation
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2. Preventing Propagation of routing information
When using an on-demand WAN link
To minimize, or stop entirely, the exchange of routing update information across this type of link; otherwise, the link will remain up constantly
To prevent routing loops
When a path is learned to the same destination by two different routing protocols, you may want to filter the propagation of one of the paths
To preserve bandwidth
To ensure maximum bandwidth availability for data traffic by reducing unnecessary routing update traffic
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3. Controlling/Preventing Propagation
Passive interface
Prevents all routing updates from being sent through an interface
Default routes
Instructs the router that if it does not have a route for a given destination, to send the packet to the default route
Static routes
A route to a destination that is configured in the router
Route update filtering
Use access lists to filter route update traffic about specific networks
Route Maps – Policy Routing
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4. Passive Interface
With most protocols passive interface stops the router from sending updates to a particular neighbor, but continues to listen and use routing updates from that neighbor
In OSPF the interface address you specify as passive appears as a stub network in the OSPF domain.
OSPF routing information is neither sent nor received through the specified router interface
In EIGRP, passive interface causes the router to stop sending hello packets.
When this happens, the router can't form neighbor adjacencies on the interface or send or receive routing updates
To achieve the common effect of passive interface with EIGRP, use the distribute-list command
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5. Using the passive-interface Command
We have all used the passive-interface command. However, the new objectives for the BSCI exam adds the passive-interface default command in which all interfaces are set to passive. Then you can configure which interfaces are not passive with the no passive-interface interface command. This would be very helpful in situations where you have many interfaces and a majority of them are passive.
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6. Gateway of Last Resort
The gateway of last resort is a routing entry that the router forwards packets to when it lacks a more specific route to a network
When IP routing is Enabled
1. The ip default-network to tag a route as a gateway of last resort
Router(config)#ip default-network
Propagated differently depending on which protocol is propagating the default route
For IGRP/EIGRP to, the network specified must be a IGRP or EIGRP derived network in the routing table or advertised into these protocols using ‘network’ command
2. Create a Static Route to quad zero ( )
Router (config)# ip route
IGRP can not understand a route to
When IP routing is disabled,
Router(config) # ip default-gateway
Can a router have multiple IP default networks entered ?
2513#show ip route
Gateway of last resort is not set
/24 is subnetted, 1 subnets
C is directly connected, Ethernet0
/24 is subnetted, 1 subnets
C is directly connected, Serial0
S /24 [1/0] via

7. Example: Default network2
2513(config)#ip default−network ,
2513#show ip route
Gateway of last resort is to network
/24 is subnetted, 1 subnets
C is directly connected, Ethernet0
/24 is subnetted, 1 subnets
C is directly connected, Serial0
S* /24 [1/0] via
Add another default gateway
2513(config)#ip route
2513(config)#ip default−network 1
The ip default−network command is classful. This means that if the router has a route to the subnet
indicated by this command, it installs the route to the major net. At this point neither network has been
flagged as the default network. The ip default−network command must be issued again, using the major net,
in order to flag the candidate default route. 3
2513#show ip route
2513#show ip route
Gateway of last resort is not set
/24 is subnetted, 1 subnets
C is directly connected, Ethernet0
/24 is subnetted, 1 subnets
C is directly connected, Serial0
S /24 [1/0] via
Gateway of last resort is to network
/16 is variably subnetted, 2 subnets, 2 masks
S /16 [1/0] via
S /24 [1/0] via
/24 is subnetted, 1 subnets
C is directly connected, Ethernet0
/24 is subnetted, 1 subnets
C is directly connected, Serial0
S* /24 [1/0] via
Why is the network not flagged as default network
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8. Issues with flagging Default Routes
The ip default−network command is classful. This means that if the router has a route to the subnet indicated by this command, it installs the route to the major net.
At this point neither network has been flagged as the default network. The ip default−network command must be issued again, using the major net, in order to flag the candidate default route.
2513(config)#ip default−network
2513(config)#^Z
2513#show ip route
Gateway of last resort is to network
* /16 is variably subnetted, 2 subnets, 2 masks
S* /16 [1/0] via
S /24 [1/0] via
/24 is subnetted, 1 subnets
C is directly connected, Ethernet0
/24 is subnetted, 1 subnets
C is directly connected, Serial0
S* /24 [1/0] via
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9. Null0
Significant overhead can be created by packets for routes that do not exist.
the router may want to send an Internet Control Message Protocol (ICMP) host or network- unreachable message for each un-routable packet
One solution is to configure a route to the null0 interface
a legitimate interface that accepts the packets and then throws them away
No ICMP host or network-unreachable messages are sent for packets forwarded to null0
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10. Route Map Route maps are similar to a scripting language:
A list of statements composes a route map.
The list is processed top-down like an access list.
The first match found for a route is applied.
The sequence number is used for inserting or deleting specific route map statements.
route-map my_bgp permit 10
{ match statements }
{ set statements }
route-map my_bgp deny 20
:: :: ::
route-map my_bgp permit 30
Route maps operate in a manner similar to ACLs. When determining which routes will be redistributed from one protocol to the next, the router checks each route against the route map, beginning with the top line.
Each line is sequence-numbered, both for top-down processing purposes and for editing purposes. Lines can be added or removed from a route map as changes are required.
Each line has a permit or deny statement. If a route is matched in the matching statements and the line statement is “permit,” then the router sets the metrics or other defined conditions and permits the redistribution of that route. The route map stops processing at the first match.
If the packet is matched and the route map line is “deny,” then the router stops at the matched line in the map and does not redistribute that route. Routes are filtered by this method.
Routes are checked from line to line looking for a match. If there is not a match and the bottom of the route map is reached, then the router denies the route from being redistributed. There is always an implicit deny at the end of a route map.
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11. route-map Commands Defines the route map conditions
router(config)#
route-map map-tag [permit | deny] [sequence-number]
Defines the route map conditions
router(config-route-map)#
match {conditions}
Defines the conditions to match
router(config-route-map)#
set {actions}
This slide shows the parameters for the route-map commands.
map-tag Specifies the name of the route map
permit | deny Specifies the action to be taken if the route map match conditions are met
permit = permit the matched route to be redistributed
deny = deny the matched route from being redistributed
sequence-number Specifies the sequence number that indicates the position that a new route map statement will have in the list of route map statements already configured with the same route map name
We will look at the match and set commands next.
When used for redistribution filtering, a route map is applied to the route redistribution process by adding the route-map command and map-tag to the end of the redistribute protocol command.
Defines the action to be taken on a match
router(config-router)#
redistribute protocol [process id] route-map map-tag
Allows for detailed control of routes being redistributed into a routing protocol
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12. Route Map Operation (Cont.)
The match statement may contain multiple references.
Multiple match criteria in the same line use a logical OR.
Matching statements in a route map can be complex. Multiple match criteria in the same line are processed with OR logic. Separate match criteria can also be applied vertically under a route map line. In this case, each match uses AND logic.
A route map may consist of multiple route map statements. The statements are processed top-down, like an ACL. The first match found for a route is applied. The sequence number is used for inserting or deleting specific route map statements in a specific place in the route map.
The match route map configuration commands define the conditions to be checked. The set route map configuration commands define the actions that you should follow if there is a match.
The single-match statement may contain multiple conditions. At least one condition in the match statement must be true to consider the statement a match (logical OR). A route map statement may contain multiple match statements. All match statements in the route map statement must be true to consider the route map statement a match (logical AND).
The sequence number specifies the order in which conditions are checked. For example, if there are two statements in a route map named MYMAP, one with sequence 10 and the other with sequence 20, sequence 10 is checked first. If the match conditions in sequence 10 are not met, then sequence 20 is checked.
Like an ACL, there is an implicit deny any at the end of a route map. The consequences of this deny depend on how the route map is used.
Each vertical match uses a logical AND.
All match statements must permit the route for it to remain a candidate for redistribution.
Route map permit or deny determines if the candidate will be redistributed.
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13. Example - Policy Routing
With normal routing, all the packets from /8 network to the Internet will take the path through interface ethernet 0/0 of Cisco WAN Router (via /24 subnet) as it is the best path with least metric.
Policy-based routing can be used such that these packets take the path through the Firewall to the Internet, normal routing behavior has to be overridden by configuring policy routing.
The firewall translates all the packets from /8 network going to the Internet, which is however not necessary for policy routing to work.
Policy Routing
Another purpose of route maps is to enable policy routing. Use the ip policy route-map or ipv6 policy route-map command, in addition to the route-map command, and the match and set commands to define the conditions for policy routing packets. The match commands specify the conditions under which policy routing occurs. The set commands specify the routing actions to perform if the criteria enforced by the match commands are met. You might want to policy route packets some way other than the obvious shortest path.
The sequence-number argument works as follows:
1. If no entry is defined with the supplied tag, an entry is created with the sequence-number argument set to 10.
2. If only one entry is defined with the supplied tag, that entry becomes the default entry for the following route-map command. The sequence-number argument of this entry is unchanged.
3. If more than one entry is defined with the supplied tag, an error message is printed to indicate that the sequence-number argument is required.
interface Ethernet3/0
ip address
ip policy route-map net-10
access-list 111 permit ip any
route-map net-10 permit 10
match ip address 111
set interface Ethernet0/1
route-map net-10 permit 20

14. The match commands Command Description match community
Matches a BGP community
match interface
Matches any routes that have the next hop out of one of the interfaces specified
match ip address
Matches any routes that have a destination network number address that is permitted by a standard or extended ACL
match ip next-hop
Matches any routes that have a next-hop router address that is passed by one of the ACLs specified
match ip route-source
Matches routes that have been advertised by routers and access servers at the address that is specified by the ACLs
match length
Matches based on the layer 3 length of a packet
match metric
Matches routes with the metric specified
match route-type
Matches routes of the specified type
match tag
Matches tag of a route
The table presents a general list of match criteria. Some criteria are used for BGP policy, some criteria are used for policy-based routing, and some criteria are used for redistribution filtering.

15. Filtering Routing Updates
A distribute-list is used to control routing updates either coming TO router or leaving your router.
Example: we want to filter out route /32.
define an ACL that identifies that route, denies it, and allows all other traffic
Create a Distribute-List that references the ACL and defines the direction
Go into the routing process
we want to filter OSPF routes so we go into the OSPF routing process configuration
Verify that the route has been removed
Router# show ip route
Gateway of last resort is not set
/8 is variably subnetted, 3 subnets, 2 masks
O /32 [110/11] via , 00:00:10, Ethernet0
O /32 [110/11] via , 00:00:10, Ethernet0
C /24 is directly connected, Loopback0
/24 is subnetted, 1 subnets
C is directly connected, Ethernet0
Define Access list
Router(config)# access-list 50 deny  
Router(config)# access-list 50 permit any
OSPF ROUTING PROCESS CONFIGURATION
Router(config)# router ospf 10
Router(config-router)# distribute-list 50 in
Verify route
Router# sh ip route …
/8 is variably subnetted, 2 subnets, 2 masks
O /32 [110/11] via , 00:11:39, Ethernet0
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16. Modifying Administrative Distance
Router(config-router)#
distance administrative distance [address wildcard-mask [access-list-number | name]]
Used for all protocols except EIGRP and BGP redistribution
Router(config-router)#
distance eigrp internal-distance external-distance
In some cases, a router selects a suboptimal path if it believes a routing protocol with a better administrative distance, even though it is actually a routing protocol with a worse route.
Assigning an undesired routing protocol a larger administrative distance ensures that routers select routes from the desired routing protocol. The figure illustrates the commands for changing the default administrative distance.
The distance command can be used to change the default administrative distance for all protocols except EIGRP and BGP.
For EIGRP, use the distance eigrp command with the following parameters:
internal-distance Specifies the administrative distance for EIGRP internal routes. Internal routes are routes that are learned from another entity within the AS.
external-distance Sets the administrative distance for EIGRP external routes. External routes are routes for which the best path is learned from a neighbor external to the AS.
Used for EIGRP
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