Route summarization reduces memory use on
routers, CPU for recalculations and routing-protocol network
traffic. Requirements for summarization to work correctly are as follows:
- Multiple IP addresses must share the
same high-order bits.
- Routing tables and protocols must base
their routing decisions on a 32-bit IP address and prefix length that
can be up to 32 bits.
- Routing protocols must carry the prefix
length (subnet mask) with the 32-bit IP address.
For example, consider the binary equivalent
of 172.21.136.0/16 and 172.21.140.0/16, as follows:
10101100.00010101.10001000.00000000
10101100.00010101.10001100.00000000
You can count the common high order bits at
21 and then summarize these routes as 172.21.136.0/21. The first 21 bits
are common between the two addresses. The number "21" is used as
the prefix.
Route Summarization Operation in Cisco
Routers
This section discusses the generalities of
how Cisco routers handle route summarization. Details about how route
summarization operates with a specific IP routing protocol are discussed
in the specific protocol chapter.
Cisco routers manage route summarization in
two ways:
- Sending route summaries ---
Routing
information advertised out an interface is automatically summarized at
major (classful) network address boundaries only for some protocols,
such as IGRP and RIP. A classful network address is a traditional
address that has defined standard boundaries. For example, the address
12.0.0.0 is a classful address using the first byte for the network
portion (12.0.0.0/8). Specifically, this automatic summarization occurs
for those routes whose classful network address differs from the major
network address of the interface to which the advertisement is being
sent. For protocols such as OSPF, you must configure summarization.
Route summarization is not always a good
solution. You would not want to use route summarization if you needed to
advertise all networks across a boundary, such as when you have
discontiguous networks (discussed next in this section). Protocols such as
EIGRP and RIP2 allow you to disable automatic summarization.
- Selecting routes from route summaries
---
If
more than one entry in the routing table matches a particular
destination, the longest prefix match in the routing table is used.
Several routes might match one destination, but the longest matching
prefix is used.
For example, if a routing table has
different paths to 172.168.0.0/16 and to 172.168.5.0/24, packets addressed
to 172.168.5.99 would be routed through 172.168.5.0/24 because that
address has the longest prefix match.
Administrative Distances
The term metric has already been explained
as describing the overall desirability of a route to a remote (not locally
attached) network.
With Cisco routers, there is another
concept called administrative distance, which is a subjective analysis of
the believability of a routing protocol. In other words, if a router
running two different routing protocols learns about the same network from
both protocols, the protocol that has the lower distance associated with
it will be the one whose entry is installed in the router's routing table.
The types of distances used in the next scenario, which is on running
multiple routing protocols concurrently, are as follows:
- Connected interface Distance 0
- A subnet that is assigned to an
interface in the router takes precedence over an advertised route for
the same subnet. If RouterB advertised a route to 168.71.5.0, RouterA
would ignore it because it has a connected route to this subnet
already.
- Static route Distance 1
- A route that has been manually
configured into a router. If RouterA had a static route indicating
that 168.71.8.0 was reachable via the next hop of 168.71.6.2 (RouterB),
this route would override any dynamic route RouterA received from
RouterC, even though the RouterA-to- RouterC path is actually shorter
in terms of hops.
- IGRP Distance 100
- Cisco's proprietary Interior Gateway
Routing Protocol.
- RIP Distance 120
- An IETF standard routing protocol.
There are several other types of routing
protocols and associated distances. When using a combination of routing
protocols, keep in mind that the default distance values may cause a
router to use a path that you did not intend it to use.
Summarizing Routes in a Discontiguous
Network
Classful routing protocols summarize
automatically at network boundaries. This behavior, which cannot be
changed with RIP and IGRP, has the following important results:
- Subnets are not advertised to a
different major network.
- Discontiguous subnets are not visible to
each other.
In the main figure,
the 172.16.5.0/24 and 172.16.6.0/24 subnetworks are not advertised by RIP
because RIP cannot advertise subnets. Therefore, each router advertises
172.16.0.0, which leads to confusion when routing across network
172.168.14.0 because this network receives routes about 172.16.0.0 from
two different directions, so it cannot make a correct routing decision.
This situation is resolved by RIP2, OSPF,
and Enhanced IGRP when summarization is not used because the routes could
be advertised with their actual subnet masks.
Advertisements are configurable with OSPF
and Enhanced IGRP. Cisco IOS software also provides an IP unnumbered
feature that permits noncontiguous (or nonsequential) subnets separated by
an unnumbered link. An unnumbered link does not have an address assigned
to it.
Note RIP1
and IGRP do not advertise subnets or support noncontiguous subnets.
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