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Cisco ISP Software and Router Management

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jun 7, 2002.

Chapter Description

This chapter covers general features that ISPs should consider for their routers and network implementations. Most are good design practices and don't leverage particular unique Cisco IOS Software features, but each demonstrates how IOS Software can aid the smooth operation of an ISP's business.

From the Book

Cisco ISP Essentials

Cisco ISP Essentials


Interface Status Checking

Some useful hidden IOS Software commands enable the operator to check the status of the interfaces in IOS Software. Three useful commands are show interface switching, show interface stats, and show idb.

show interface switching

The IOS Software command show interface switching provides useful information about the switching status of the router's interfaces, either on an individual interface basis or over the whole router. The full command format is show interface [int n/n] switching, where an optional argument is the specific interface in question. Command completion cannot be used for switching—it needs to be typed in up to and including the second i. Sample output might look like the following:

gw>show interface FastEthernet 1/0 switching

FastEthernet1/0 Production LAN

Throttle count 0

Drops RP 0 SP 0

SPD Flushes Fast 0 SSE 0

SPD Aggress Fast 0

SPD Priority Inputs 2421 Drops 0


Protocol Path Pkts In Chars In Pkts Out Chars Out

Other Process 0 0 74633 4477980

Cache misses 0

Fast 0 0 31653 2957994

Auton/SSE 0 0 0 0

IP Process 5339594 516613071 5622371 851165330

Cache misses 5391487

Fast 256289350 1125491757 257803747 2058541849

Auton/SSE 0 0 0 0

ARP Process 16919 1015300 34270 2056200

Cache misses 0

Fast 0 0 0 0

Auton/SSE 0 0 0 0

CDP Process 12449 4083272 12440 4142520

Cache misses 0

Fast 0 0 0 0

Auton/SSE 0 0 0 0


This sample output shows SPD1 activity, as well as other activity on that particular interface on the router. Note the references to autonomous/SSE switching—this is applicable only to the Cisco 7000 series with Silicon Switch Engine only (a product that is now discontinued but was a significant part of the Internet core in the mid-1990s). Fast switching refers to all packets that have not been process-switched, which would include Optimum switching, NetFlow, and CEF.

show interface stats

The IOS Software command show interface stats is the second useful command to show interface status. It shows the number of packets and characters inbound and outbound on an individual router interface or all of them. The full command format is show interface [int n/n] stats, where an optional argument is the specific interface in question. Command completion cannot be used for stats—at least st needs to be typed in at the command prompt. Sample output might look like this:

gw>show interface stats

Interface FastEthernet0/0 is disabled



Switching path Pkts In Chars In Pkts Out Chars Out

Processor 5371378 521946816 5746126 862068168

Route cache 256413200 1149405512 257960291 2072462774

Total 261784578 1671352328 263706417 2934530942


As for interface switching, the output differentiates between packets that go via the processor and those that have been processed via the route cache. This is useful to determine the level of process switching taking place on the router.

On a router that supports distributed switching (for example, 7500 with VIP interfaces), the output will look like the following:

gw>show interface stats


Switching path Pkts In Chars In Pkts Out Chars Out

Processor 207745 14075132 270885 21915788

Route cache 0 0 0 0

Distributed cache 93 9729 0 0

Total 207838 14084861 270885 21915788

Notice that packets that have been processed via the distributed cache are counted separately from those handled via the central route cache and the processor.

show IDB

Each interface on the router has an associated interface descriptor block allocated to it. In the early days, each physical interface mapped to one IDB, and routers generally could support up to 300 IDBs (for example, the Cisco AGS+).

However, with the increasing numbers of new connection services, and with ATM and Frame Relay providing large numbers of subinterfaces, routers have had to scale to supporting several thousand IDBs. show IDB recently has become a visible command in IOS Software (CSCds89322); it allows ISPs to find out how many IDBs are configured on the router:

gw#show idb

24 SW IDBs allocated (2368 bytes each)


21 HW IDBs allocated (4040 bytes each)

HWIDB#1 1 FastEthernet0/0 (HW IFINDEX, Ether)

HWIDB#2 2 FastEthernet1/0 (HW IFINDEX, Ether)

HWIDB#3 3 Serial2/0 (HW IFINDEX, Serial)

HWIDB#4 4 Serial2/1 (HW IFINDEX, Serial)

HWIDB#5 5 Serial2/2 (HW IFINDEX, Serial)

HWIDB#6 6 Serial2/3 (HW IFINDEX, Serial)

HWIDB#7 7 FastEthernet3/0 (HW IFINDEX, Ether)

HWIDB#8 8 FastEthernet5/0 (HW IFINDEX, Ether)

HWIDB#9 20 Dialer0 (HW IFINDEX, Serial)

HWIDB#10 21 Loopback0 (HW IFINDEX)



To find out how many IDBs are supported on different router platforms, consult Cisco.com documentation—for example, http://www.cisco.com/warp/public/63/idb_limit.html. Although most smaller router platforms still support only 300 IDBs at maximum, some of the larger platforms can go as high as 10,000 (7200/12.2T release). These values might change as future enhancements are made to Cisco IOS Software.

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