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Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring Routing Information (Rev. 03)
Controlling and Modifying Routing Information
© 2008 Foundry Networks, Inc.
Page 22 of 48
Metric/Cost
Description
Bandwidth
Bandwidth is the metric used by protocols that consider the transfer-rate
capacity of a link, measured in bits per second. Links that support high-
transfer rates are preferred to low-capacity links (for example, Gigabit links
have preference over 100 Mbit links). These protocols take into account the
bandwidth capacity of each link along the path up to the destination, and
choose the path with the overall highest bandwidth as the best path.
Delay
Delay represents the time required for a router to process, queue and transmit
a packet. Delay values are measured in microseconds (indicated by the
symbol
μ
). Protocols that use this metric determine the delay values for all
links along the path up to the destination. The path with the lowest delay is
considered to be the best path.
Reliability
Generally, this metric can be configured as a fixed value measured
dynamically over a specific time. If it is user-defined it allows routers to
examine attached links and to report problems.
Load
Load is a variable value, generally measured over a determinate period
indicating the traffic load over a specific link. If defined, by definition the
load measures the amount of traffic that occupies the link over this period as
a percentage of the link total capacity.
Cost
Users can change the way routers make path decisions by setting arbitrary
metric values on links along the path end-to-end. These arbitrary values are
typically single integers, with lower values indicating better paths.
Routing Filters
When routing filters are configured, the device evaluates the routing information in the following
sequence:
1:
The device checks whether the evaluated packet matches the criteria defined in the filter
clauses.
2:
The device decides what action to undertake with the result, depending on the routing
protocol.
3:
Based on the result of the filter match and the filter assignment set in the routing process
context, the device decides what action to take. When no match condition is met, the
default policy is to drop the control packets. Foundry Networks switches express these
filtering rules in the form of access lists, prefix lists, and route maps.
Understanding Routing Access Lists
Routing access lists control the transmission of packets on an IP interface, manage virtual terminal
line access, and restrict contents of routing updates.
The access list is a group of statements. Each statement defines a rule that should be matched in
incoming packets. The access lists control the access to a network by manipulating routing
information based on the IP addresses of the source and destination. Access lists are set up on the
device to match specific criteria and to reject all packets that do not match these criteria.
As a packet arrives at the device, the access list is scanned in the order that it was created, for a
statement that matches the attributes of the incoming packet. A permit or deny rule determines the
action to be applied, depending on the routing protocol.