Install guide

48 BGP Enhancements Release Note
Software Version 2.7.5
C613-10454-00 REV A
Automatic Summarising: Advertising as Few Routes
as Possible
Problem When BGP learns routes, it imports and advertises every route, even if some
are routes to subnets of the same network. For example, if you used the subnets
192.168.1.64/26 and 192.168.1.128/26, BGP would advertise routes to both of
these. Depending on the router or switch’s role in your network, this may be
undesirable because it:
■ exposes network topology
■ creates more update messages than necessary
■ increases the size of the routing table
Solution With Software Version 2.7.5, there are two available solutions:
■ The new automatic summarising feature, which enables BGP to
automatically summarise all locally-originated prefixes into their class A, B
or C networks. This option allows BGP to summarise prefixes when it
imports OSPF, RIP, interface and statically-configured routes.
Automatic Summarising describes this new feature.
■ The existing route aggregation feature, which is useful when you want to
summarise subnet routes that are within particular class A, B or C
networks. This option allows BGP to summarise subnets from any source,
including from BGP peers.
Route aggregation is an existing feature, but we have improved the
Software Reference’s description of it. Aggregating Routes contains the
new description.
Automatic Summarising
About automatic
summarising
When BGP imports routes from another routing source, such as OSPF, by
default it stores and advertises every route, no matter how specific. If your
LAN is divided into subnets, this means BGP advertises a route to each subnet.
You can avoid this by enabling automatic summarising. This feature
summarises prefixes into networks and only advertises a route to that network.
It is particularly useful on the external speaker for an AS—the router or switch
that links an internal network to a public network.
When you enable automatic summarising, the router or switch summarises
subnets into their Class A, B or C network. Instead of writing the route to the
subnet into the BGP routing table and advertising that subnet route, it writes a
single route to the summary network. For example, instead of storing and
advertising routes to 192.168.1.64/26 and 192.168.1.128/26, BGP would have
one route to 192.168.1.0/24.
Caution Only turn on automatic summarising if you own the whole classful
network for your locally-generated routes. Otherwise, you advertise yourself as
the next hop for subnets that you do not own.
For example, if you owned 202.202.202.0/24, you could use automatic
summarising. However, if you only owned 202.202.202.64/26, you must not use
automatic summarising.