Route Aggregation and Generation
Route Aggregation and Generation Overview
Route aggregation is a method of generating a more general summary route, given the presence of a more specific route. It is used, for example, at an autonomous system border to generate a route to a network to be advertised via BGP, given the presence of one or more subnets of that network learned via RIP. No aggregation is performed unless explicitly requested in an aggregate command.
Route aggregation is also used by regional and national networks to reduce the amount of routing information passed. With careful allocation of network addresses to clients, regional networks can announce one route to regional networks instead of hundreds.
Aggregate routes are not actually used for packet forwarding by the originator of the aggregate route, only by the receiver (if it wishes). A router is supposed to respond with an ICMP network unreachable message if the router receives a packet that does not match one of the component routes that led to the generation of an aggregate route. This message is to prevent packets for unknown component routes from following a default route into another network, where they would be forwarded back to the border router, and around and around again and again, until their TTL expired. Sending an unreachable message for a missing piece of an aggregate is only possible on systems with support for reject routes.
A difference between route aggregation and generation is the generation of a route based on the existence of certain conditions. This is sometimes known as the "route of last resort." This route inherits the next hops and AS path from the contributor specified with the lowest (most favorable) preference. The most common usage for this is to generate a default based on the presence of a route from a peer on a neighboring backbone.
aggregate-address
Named
aggregate-address - allows the aggregation of specific routes into one
Syntax
aggregate-address [(address (mask | masklen)?) |
prefix ] protocol [aggregate | all | bgp | direct | kernel | ospf | rip | ripng | static]
[match-map map_name]? (rib [unicast | multicast |
unicast-multicast])? [preference pref]? generate?
noinstall? blackhole? bgp? brief?
no aggregate-address [(address (mask | masklen)?) |
prefix ] protocol [aggregate | all | bgp | direct |
isis | kernel | ospf | rip | ripng | static]
[match-map map_name]? (rib [unicast | multicast |
unicast-multicast])? [preference pref]? generate?
noinstall? blackhole? bgp? brief?
Mode
Aggregate Router Configuration Mode
Parameters
address (mask | masklen) - the IPv4 address to be aggregated or generated. Specify either an address or a prefix. The address can be accompanied by an address mask or masklen. The address mask is specified in dotted-quad format for IPv4 addresses. The masklen specifies the number of contiguous bits at the beginning of a mask.
prefix - specifies a valid IPv4 prefix
protocol - specify the protocol whose rules this aggregate will use to determine whether to include each route. For IPv4 routes, valid values include the following: aggregate, all, bgp, direct, kernel, ospf, rip, and static.
match-map map_name - optionally specify a route map that will be used to generate this route
rib [ unicast | multicast | unicast-multicast ] - specifies whether the aggregate is restricted to the unicast RIB, the multicast RIB, or both. The default is to restrict the aggregate to both the unicast and multicast RIB.
brief - specifies that the AS path should be truncated to the longest common leading AS path. This is option is only useful if the configured protocol is BGP.
bgp - specifies that this aggregate will use BGP rules to determine whether to include each route. This is a valid option only if the configured protocol is BGP.
preference pref - specifies the aggregation preference value to be assigned to the contributor route. This can be an integer between 1 and 255, inclusive. Note: If two different aggregate commands are specified for the same prefix, then the preference specified most recently will be used by Advanced Routing Suite, and that preference value will display in the show running-config query for each configured aggregate route.
noinstall - specifies whether to install the generate address if any contributor route is active. The default behavior is to install the route.
generate - generates a default route. Route generation is a method of generating a more general route, given the presence of a specific route. generate differs from aggregate only in terms of the route installed in the kernel.
blackhole - specifies whether to blackhole the route; thus enabling the router to refuse to route various prefixes
Description
Route aggregation is a method of generating a more general summary route, given the presence of a more specific route. generate differs from aggregate only in terms of the route installed in the kernel.
Note: Using the generate keyword in the no form restores the default for the generate route but has no effect on the aggregate route. To restore the default for the aggregate route, use the no form of the command without the generate keyword. This will have a cascading effect on restoring the default generated route.
Default
No aggregate/generate routes are configured by default.
Command History
NGC 2.2 - This command was introduced.
Examples
Example 1
In the following example, an aggregate route to 10/8 is generated based on routes formed via BGP that match route map "rm1".
(config)# router aggregate
(config-aggregate)# aggregate-address 10.0.0.0 255.0.0.0
protocol bgp match-map rm1
(config-aggregate)# exit
(config)#
Example 2
The following example removes the configuration in Example 1 and restores the defaults.
(config)# router aggregate
(config-aggregate)# no aggregate-address 10.0.0.0
255.0.0.0 protocol bgp
(config-aggregate)# exit
(config)#
Example 3
In the following example, a generate route to 10/8 is created based on routes formed via BGP that match route map "rm1".
(config)# router aggregate
(config-aggregate)# aggregate-address 10.0.0.0 255.0.0.0
protocol bgp match-map rm1 generate
(config-aggregate)# exit
(config)#
Example 4
The following example removes the configuration in Example 3 and restores the defaults.
(config)# router aggregate
(config-aggregate)# no aggregate-address 10.0.0.0
255.0.0.0 protocol bgp generate
(config-aggregate)# exit
(config)#
Example 5
The following example creates an aggregate route that includes all OSPF routes matching route map "rm1" and that are more specific than 10/8. It also restricts it to the unicast RIB.
(config)# router aggregate
(config-aggregate)# aggregate-address 10.0.0.0 255.0.0.0
protocol ospf match-map rm1 rib unicast
(config-aggregate)# exit
(config)#
Example 6
The following example creates a generate route from all OSPF routes that match route map "rm1" and that are more specific than 10/0. It restricts the generate route to the unicast RIB. The routes, however, will not be installed if the contributor route is not active.
(config)# router aggregate
(config-aggregate)# aggregate-address 10.0.0.0 255.0.0.0 protocol ospf match-map rm1 rib unicast generate noinstall
(config-aggregate)# exit
(config)#
Example 7
The following example restores the defaults in Example 6.
(config)# router aggregate
(config-aggregate)# no aggregate-address 10.0.0.0
255.0.0.0 protocol ospf rib unicast generate
(config-aggregate)# exit
(config)#
Example 8
The following example creates an aggregate route that includes all ISIS routes matching route map "rm1" and that are more specific than 10/8. In addition, it sets the preference for these routes to 10.
(config)# router aggregate
(config-aggregate)# aggregate-address 10.0.0.0 255.0.0.0
protocol isis match-map rm1 preference 10
(config-aggregate)# exit
(config)#
Example 9
The following example creates an aggregate route that includes all RIP routes matching route map "rm1" and that are more specific than 10/8. It also sets the preference for the aggregate route to 10.
(config)# router aggregate
(config-aggregate)# aggregate-address 10.0.0.0 255.0.0.0
protocol rip match-map rm1 preference 10
(config-aggregate)# exit
(config)#
Example 10
The following example creates a generate route from all RIP routes matching route map "rm1" and that are more specific than 10/8. It also sets the preference for these routes to 10. The routes, however, will not be installed if the contributor route is not active.
(config)# router aggregate
(config-aggregate)# aggregate-address 10.0.0.0 255.0.0.0
protocol rip match-map rm1 preference 10 generate noinstall
(config-aggregate)# exit
(config)#
Example 11
The following example creates an aggregate route that includes all static routes matching route map "rm1" and that are more specific than 10/8. IT also sets the preference for the aggregate route to 10.
(config)# router-aggregate
(config-aggregate)# aggregate-address 10.0.0.0 255.0.0.0
protocol static match-map rm1 preference 10
(config-aggregate)# exit
(config)#
Example 12
The following example creates a generate route from all static routes that match route map "rm1" and that are more specific than 10/8. It also sets the preference for these routes to 10.
(config)# router aggregate
(config-aggregate)# aggregate-address 10.0.0.0 255.0.0.0
protocol static match-map rm1 preference 10 generate
(config-aggregate)# exit
(config)#
router aggregate
Name
router aggregate - enters the user into Aggregate Router Configuration mode
Syntax
router aggregate
Mode
Global Configuration
Parameters
none
Description
Use the router aggregate command to enter Aggregate Router Configuration mode. This mode allows the aggregation of specific routes into a single route.
Default
This command is not explicitly configured by default.
Command History
NGC 2.2 - This command was introduced.
Examples
The following example shows how to enter Aggregate Router Configuration mode.
(config)# router aggregate
(config-aggregate)#
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