Discussion

talking with other routers and advertising routing information to them. A number of
other software modules run in the router’s control plane, including the CLI and
accounting processes such as SNMP. Each of these modules runs as a separate process
in the JUNOS software, and, in some cases, multiple instances of a module
might be running (for instance, if two users are logged in to the router, two CLI
processes run).
The PFE is the router’s forwarding plane, housing the forwarding table and handling
most forwarding processing. Forwarding is the process of receiving a packet on an
inbound interface, de-encapsulating it, executing a number of packet-processing
activities (such as filtering, accounting, and next-hop determination), encapsulating
it, and queuing it on the outbound interface toward the packet’s destination. The
PFE consists of custom ASICs and the router’s input and output interfaces. The ASICs
use the forwarding table to perform route lookup, looking up the IP address prefix
and determining the output interface (next hop) for the packet. The link between the
Routing Engine and the PFE is a standard Fast Ethernet link (the fxp1 interface).
On the smaller J-series routers, the control and data planes are handled by the same
CPU, which runs the software for both the RE and PFE.
RPD installs all active routes from the routing tables into the forwarding table. The
JUNOS kernel maintains a master copy of the forwarding table and copies the table
to the PFE. The operation to update the forwarding table is done atomically, one
route at a time. This ensures that the forwarding table always has a single view of
how to forward traffic on the network.
IPv4 and IPv6 Addresses
IPv4 addresses are 32 bits long and are written in a dotted quad notation. Originally,
IPv4 addresses were divided into four classes, Classes A, B, C, and D. This type of
addressing is called classful. Classful addresses require an address and a network
mask. The address consists of a network portion and a host portion. The subnetwork
mask defines how to interpret the address bits in order to know which are being used
for the network portion and which for the host portion.
The IETF developed classless addresses in the late 1990s with the introduction of
Classless Interdomain Routing (CIDR). This was done as one way to increase the
number of network addresses available on the Internet. All IPv4 addresses on
JUNOS routers are expressed in CIDR format. Instead of network and host portions
and subnetwork masks, CIDR addresses have a prefix that represents the network
address, followed by a slash and the prefix length, which identifies the number of bits
being used for the network address. For example, one of the groups of routers used
as examples in this book has interface addresses in the network 10.0.1.0/24. Here,
the prefix is 10.0.1.0, and 24 bits are used for the network prefix. The remaining 8
bits are available for host addresses, so this network can have up to 256 hosts.
248 | Chapter 8: IP Routing
This is the Title of the Book, eMatter Edition
Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.