Contents
Route Selection – PBR ............................................................................................................................................ 1
1.
Introduction ...................................................................................................................................................... 1
2.
Cisco Express Forwarding (CEF) ................................................................................................................. 1
2.1 Operation of packet switching ................................................................................................................ 2
2.2 Operation of Fast Switching ..................................................................................................................... 2
2.3 Operation of Cisco Express Forwarding (CEF) ................................................................................... 2
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11. Route Selection – learning about PBR
1. Introduction
Path control is a thing that contains everything. In order to deliver a packet to source to
destination, it need routing and protocols. Thus practically every topic is covered under this
path control title. This section is covered three path control topics with more details.
 Cisco Express Forwarding (CEF) – this feature enables to find the route quickly and
easier. We will discuss later about CEF such as processes of switching.
 Policy Based Routing (PBR) – we discuss it here. This is also called as policy routing.
Influence in IP data plane. Change the forwarding decisions which router makes. But it
won’t change the routing table.
 IP Service-Level Agreement (IP SLA) – this will monitor network health and reachability.
Then router can decide when to use or ignore routes based on the status determine
by the IP SLA.
Physical router can be divided into multiple virtual routers which can perform it won routing
selection. That thing is the ultimate goal of this topic which is about VRF-Lite configuration. In
this Virtual Routing and Forwarding (VRF) concept you can run multiple instances even in a
single router.
2. Cisco Express Forwarding (CEF)
Literally, router functions can be divide into three basic operational planes.
 Management plane: concern the management of the device. Ex: admin access the
router via SSH through router’s VTY lines.
 Control plane: concern with packet forwarding decisions. Ex: operation of routing
protocols is a control plane operation.
 Data plane: concern with forwarding data through a router. Ex: end user traffic going
from user’s PC to web server on a different network would go via data plane.
From the above two planes, the control and data planes are important when considering about
how fast packets can flow through a router.
Let’s consider this operation in those two planes and examine three different approaches that
Cisco routers can take to forward packets arriving on an ingress interface and being sent out
from egress interface, process called packet switching. Thus, Cisco router supports three
primary modes of packet switching.
 Process switching
 Fast switching
 Cisco Express Forwarding (CEF)
2.1 Operation of packet switching
When router routes a packet, the router removes the packet’s L2 header, examines the L3
addressing and decide how to forward the packet.
Then L2 header re-written (which might changing the SRC/DEST MAC addresses and
computing a new cyclic redundancy check [CRC]). Then packet is forwarded to appropriate
interface. Here, router CPU directly involved with packet switching. Thus the performance of a
router may get slow.
An interface can be configured for process switching by disabling the fast switching on that
interface. In interface configuration command used to disable the fast switching as # no ip
route-cache.
2.2 Operation of Fast Switching
This used fast cache maintained in a router’s data plane. Fast cache contains details about how
traffic from different data flows should be forwarded. 1st packet in data flow process switched
by router’s CPU. After determining how to forward the 1st frame of a data flow, the forwarding
details are stored in the fast cache.
Subsequently, packets in that same data flow are forwarded based on the details in the fast
cache. Thus fast switching reduce the CPU utilization of the router when compared to process
switching. Fast switching can enable in interface configuration via # ip route-cache.
2.3 Operation of Cisco Express Forwarding (CEF)
Maintain two table in data plane. Forwarding Information Based (FIB) maintains L3 forwarding
information, whereas the adjacency table maintains L2 information for next hop listed in the
FIB. Using those two table, with router’s IP routing table and ARP cache, CEF can efficiently
make forwarding decisions.
As in the fast switching, CEF doesn’t want to require the 1st packet of data flow to be process
switched. Entire data flow can be forwarded at the data plane.
Many routers, CEF enable by default. If not, you can globally enable it by # ip cef command. If
CEF enable in the router, but not in the interface, you can enable it on the interface
configuration via # ip route-cache cef command.
Other commands:
# ip cef
# ip route-cache cef
# show ip cef (display the contents of a router’s FIB)
# show adjacency details
# show ip interface interface-id
To illustration the configuration and operation of CEF….