Specifications

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in the routing table. Additional CLI commands allow the user to display
prefixes which share some set of community attributes. Finally, the CLI
allows the user to display the prefixes being suppressed because of route
flap as well as the accumulated dampening state on a per-prefix basis.
§ Maximum Prefixes Limits - Upper bounds may be set on the number of
prefixes we will allow to be received per neighbor per RIB
§ Multiple Local ASes - BGP can be configured with a different local AS
number for each EBGP session
§ Capability Negotiation for BGP – as per RFC 2842. It provides a
mechanism for a router to send a message to its peers requesting them to
send their previously advertised routes again. This features needs BGP
mechanisms to add new “capabilities” to BGP :
o BGP Sender: BGP OPEN message may include optional
capabilities supported
o BGP Receiver: Learns of sender capabilities via OPEN message.
When using capability negotiation, three things can occur :
o The BGP receiver accepts capabilities and uses them
o The BGP receiver sends NOTIFICATION to sender with Error
subcode “Unsupported Option Parameter” . In this case, the BGP
speaker would attempt to re-establish a BGP session with the peer
without using capabilities
o The BGP receiver sends NOTIFICATION to sender with error
subcode “Unsupported Capability” with the list of capabilities
contained in the OPEN which are not supported
3.2.5 Routing Policies
JUNOS routing policy allows you to control (filter) which routes a routing protocol
imports into the routing table and which routes a routing protocol can export from
the routing table. Routing policy also allows you to set the information associated
with a route as it is being imported into or exported from the routing table.
Applying routing policy to routes being imported to the routing table allows you to
control the routes that the routing protocol process uses to determine active
routes. Applying routing policy to routes being exported from the routing table
allows you to control the routes that a protocol advertises to its neighbors.
The routing policy definition language determines what routes are accepted into
the routing table, what routes are advertised to peers, and the specific
modifications that are made to attributes on both import and export. The control
provided by the policy definition language is strategic to backbone networks
because it is the fundamental tool that controls how the networks are used. Policy
language determines the paths selected across the Internet and can play a role in
the path selected across the service provider's network.
The JUNOS policy definition language is similar to a programming language.
JUNOS routing policy lets you selectively filter the routing information that is
transferred between different routing databases. The routes can be filtered on
many variables but can include prefixes and protocol identifiers.
A JUNOS routing policy consists of policy terms. Each term consists of two
components: the conditions that a route must match and one or more actions to
take if a match occurs.
The conditions are circumstances that a route must match. If a route matches the
conditions, the action specified in the policy is applied to that route. You can
define various match conditions, including a route's source and destination; the
interface on which the route was received; the OSPF path; the IS-IS level; the AS
path; and various BGP path attributes, including community, local preference,
Neighbors
Protocol
Routing
Table
Forwarding
Table
Neighbors
Protocol
Import Policy #1
Routes Routes
PFE
Route Selection
Policy
Import Policy #2
Export Policy #1
Export Policy #2