Cisco Cisco Aironet 350 Wireless Bridge 문제 해결 가이드
Process switching is the most basic way of handling a packet. The packet is placed in the queue corresponding
to the Layer 3 protocol while the scheduler schedules the corresponding process. The waiting time depends on
the number of processes waiting to run and the number of packets waiting to be processed. The routing
decision is then made based on the routing table and the Address Resolution Protocol (ARP) cache. After the
routing decision has been made, the packet is forwarded to the corresponding outgoing interface.
to the Layer 3 protocol while the scheduler schedules the corresponding process. The waiting time depends on
the number of processes waiting to run and the number of packets waiting to be processed. The routing
decision is then made based on the routing table and the Address Resolution Protocol (ARP) cache. After the
routing decision has been made, the packet is forwarded to the corresponding outgoing interface.
Fast switching is an improvement over process switching. In fast switching, the arrival of a packet triggers an
interrupt, which causes the CPU to postpone other tasks and handle the packet. The CPU immediately does a
lookup in the fast cache table for the destination Layer 3 address. If it finds a hit, it rewrites the header and
forwards the packet to the corresponding interface (or its queue). If not, the packet is queued in the
corresponding Layer 3 queue for process switching.
interrupt, which causes the CPU to postpone other tasks and handle the packet. The CPU immediately does a
lookup in the fast cache table for the destination Layer 3 address. If it finds a hit, it rewrites the header and
forwards the packet to the corresponding interface (or its queue). If not, the packet is queued in the
corresponding Layer 3 queue for process switching.
The fast cache is a binary tree containing destination Layer 3 addresses with the corresponding Layer 2
address and outgoing interface. Because this is a destination−based cache, load sharing is done per destination
only. If the routing table has two equal cost paths for a destination network, there is one entry in the fast cache
for each host.
address and outgoing interface. Because this is a destination−based cache, load sharing is done per destination
only. If the routing table has two equal cost paths for a destination network, there is one entry in the fast cache
for each host.
Fast Switching vs. CEF Switching
Both fast switching and Cisco Express Forwarding (CEF) switching were tested with the Cisco Aironet bridge
design. It was determined that Enhanced IGRP dropped neighbor adjacencies under heavy loads less often
using CEF as the switching path. The main drawbacks of fast switching include:
design. It was determined that Enhanced IGRP dropped neighbor adjacencies under heavy loads less often
using CEF as the switching path. The main drawbacks of fast switching include:
The first packet for a particular destination is always process switched to initialize the fast cache.
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The fast cache can become very big. For example, if there are multiple equal−cost paths to the same
destination network, the fast cache is populated by host entries instead of the network.
destination network, the fast cache is populated by host entries instead of the network.
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There's no direct relation between the fast cache and the ARP table. If an entry becomes invalid in the
ARP cache, there is no way to invalidate it in the fast cache. To avoid this problem, 1/20th of the
cache is randomly invalidated every minute. This invalidation/repopulation of the cache can become
CPU intensive with very large networks.
ARP cache, there is no way to invalidate it in the fast cache. To avoid this problem, 1/20th of the
cache is randomly invalidated every minute. This invalidation/repopulation of the cache can become
CPU intensive with very large networks.
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CEF addresses these issues by using two tables: the forwarding information base table and the adjacency
table. The adjacency table is indexed by the Layer 3 addresses and contains the corresponding Layer 2 data
needed to forward a packet. It is populated when the router discovers adjacent nodes. The forwarding table is
an mtree indexed by Layer 3 addresses. It is built based on the routing table and points to the adjacency table.
table. The adjacency table is indexed by the Layer 3 addresses and contains the corresponding Layer 2 data
needed to forward a packet. It is populated when the router discovers adjacent nodes. The forwarding table is
an mtree indexed by Layer 3 addresses. It is built based on the routing table and points to the adjacency table.
While another advantage of CEF is the ability to allow load balancing per destination or per packet, the use of
per−packet load balancing is not recommended and was not tested in this design. Bridge pairs may have
different amounts of latency, which could cause problems with per−packet load balancing.
per−packet load balancing is not recommended and was not tested in this design. Bridge pairs may have
different amounts of latency, which could cause problems with per−packet load balancing.
Other Design Considerations
Quality of Service
Quality of Service (QoS) features can be used to increase the reliability of routing protocols. In situations with
heavy traffic loads, congestion management or avoidance techniques can prioritize routing protocol traffic to
ensure timely communication.
heavy traffic loads, congestion management or avoidance techniques can prioritize routing protocol traffic to
ensure timely communication.
Full Duplex
Setting the Fast Ethernet bridge ports and associated Layer 2 switch ports to 10−Mbps full duplex will
increase reliability by causing congestion to be queued at the switch instead of the bridge, which has limited
increase reliability by causing congestion to be queued at the switch instead of the bridge, which has limited