Information Technology Reference
In-Depth Information
We i g h t i s u s e d t o i n f l u e n c e t h e p a t h o f o u t b o u n d t r a f f i c f r o m a s i n g l e r o u t e r, c o n f i g -
ured locally.
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This section covers policy-based routing (PBR), route summarization, route filtering, and
route redistribution. You can use PBR to modify the next hop of packets from what is se-
lected by the routing protocol. PBR is useful when the traffic engineering of paths is re-
quired. Routes are summarized to reduce the size of routing tables and at network
boundaries. Redistribution between routing protocols is required to inject route informa-
tion from one routing protocol to another. Route filtering is used to control network ad-
dresses that get redistributed or control access to certain parts of the network. The CCDA
must understand the issues with the redistribution of routes.
PBR
Yo u c a n u s e P B R t o m o d i f y t h e n e x t - h o p a d d r e s s o f p a c k e t s o r t o m a r k p a c k e t s t o r e c e i v e
differential service. Routing is based on destination addresses; routers look at the routing
table to determine the next-hop IP address based on a destination lookup. PBR is com-
monly used to modify the next-hop IP address based on the source address. You can also
use PBR to mark the IP precedence bits in outbound IP packets so that you can apply QoS
policies. In Figure 11-16, Router A exchanges routing updates with routers in the WAN.
The routing protocol might select Serial 0 as the preferred path for all traffic because of
the higher bandwidth. The company might have business-critical systems that use the T1
but does not want systems on Ethernet 1 to affect WAN performance. You can configure
PBR on Router A to force traffic from Ethernet 1 out on Serial 1.
Ethernet 0: 192.168.1.0/24
Serial 0: T-1
Router A
WAN
Serial 1: 512 k
Ethernet 1: 192.168.2.0/24
Figure 11-16
Policy-Based Routing
Route Summarization
Large networks can grow quickly from 500 routes to 1000, to 2000, and higher. Network
IP addresses should be allocated to allow for route summarization. Route summarization
reduces the amount of route traffic on the network and unnecessary route computation.
Route summarization also allows the network to scale as a company grows.
Key
To p i c
The recommended location for route summarization is to summarize at the distribution
layer of the network topology. Figure 11-17 shows a hierarchical network. It has a network
core, regional distribution routers, and access routes for sites.
