Install guide

50 BGP Enhancements Release Note
Software Version 2.7.5
C613-10454-00 REV A
Aggregating Routes
About route
aggregation
When BGP receives routes from its peers or imports them from the RIB, by
default it advertises every route, no matter how specific. You can reduce the
number of routes BGP advertises, by configuring aggregate prefix entries. If the
router or switch receives a route to a subset of the entry’s prefix, BGP adds the
aggregate prefix to its database, as well as the route for the more specific prefix.
You can set the router or switch to advertise only the aggregate.
Consider a configuration in which you create an aggregate entry of
192.168.1.0/24 and set the aggregate entry to advertise only the aggregate. If
the router or switch receives routes to the prefixes 192.168.1.64/26 and
192.168.1.128/26, it stores all three prefixes but only advertises 192.168.1.0/24.
Note that the router or switch does not use the aggregate route for IP routing.
The router or switch only uses the aggregate to determine which routes to
advertise.
The router or switch advertises the aggregate route as coming from the router
or switch’s autonomous system, and sets the aggregate’s atomic_aggregate
attribute.
Caution Make sure that you own all the IP addresses in the aggregate entry.
Otherwise, you advertise yourself as the next hop to addresses that you do not
own.
For example, if you own 202.202.202.0/24, you can configure that as an
aggregate entry. However, if you only own 202.202.202.64/26, you must not
configure an aggregate of 202.202.202.0/24.
Configuring route
aggregation
To aggregate subnets and only advertise the aggregate prefix, use the
command:
add bgp aggregate=prefix[/0..32] [mask=mask] summary=yes
[routemap=routemap]
The aggregate parameter specifies the network that BGP aggregates subnets
into.
The summary parameter controls advertisement. If this parameter is yes, the
router or switch only advertises the route to the aggregate. Note that
unadvertised routes are still displayed in output of the show bgp route
command, but are marked with an “s”.
Creating an aggregate entry does not immediately add the aggregate prefix to
the BGP routing table. BGP adds the aggregate prefix when it receives an
advertisement of a more specific subnet.
Command Changes
The following table summarises the new and modified commands for
automatic summarising (see Command Reference Updates).
Command Change
enable bgp autosummary New command
disable bgp autosummary New command
show bgp New Auto summary field