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This chapter is from the book

This chapter is from the book

DNS and Routers

The DNS might be an unusual topic to put into a book covering ISP network essentials and Cisco IOS Software best practices. However, it is one of the most overlooked systems topics in the ISP industry—yet it is probably the most important part of the public visibility of the network to get right. If the DNS does not work, the public thinks that the network is broken—many newspaper headlines in the last few years have displayed such apocryphal headlines simply because of operational errors or problems with the DNS.

An ISP network engineer must pay attention to two aspects of the DNS. The first is the business of putting all the name-to-address-to-name mappings in the system so that routers can be recognized by their English-language names rather than by four boring decimal numbers separated by dots. Humans aren't good at remembering the latter. The second aspect is to actually enable support for the DNS in the routers themselves. This section covers only the router aspect—Chapter 5, "Operational Practices," describes configuration and placement of the DNS systems throughout the ISP backbone.

Mapping IP Addresses to Names

Mapping domain names to IP addresses and vice versa is one of those commonly overlooked areas in a new ISP's operations. Doing a trace from Australia across the backbones in the United States to a site in the United Kingdom gives you something like Example 2-3.

Example 2-2 Example Traceroute Across the Internet from Australia to the United Kingdom

traceroute to k.root-servers.net (, 30 hops max, 38 byte packets
 1 fe5-0.gw.apnic.net ( 0.707 ms 0.534 ms 0.497 ms
 2 Serial1-0-3.cha8.Brisbane.telstra.net ( 5.999 ms 5.131 ms 
  6.155 ms
 3 GigabitEthernet5-1.cha-core4.Brisbane.telstra.net ( 6.148 ms
  4.972 ms 4.537 ms
 4 Pos2-0.chw-core2.Sydney.telstra.net ( 19.355 ms 18.595 ms
  19.797 ms
 5 Pos4-0.exi-core1.Melbourne.telstra.net ( 32.120 ms 32.968 ms 
  32.544 ms
 6 Pos5-0.way-core4.Adelaide.telstra.net ( 50.088 ms 46.171 ms 
  44.896 ms
 7 Pos6-0.wel-core3.Perth.telstra.net ( 88.296 ms 75.545 ms 
  83.527 ms
 8 GigabitEthernet4-0.wel-gw1.Perth.telstra.net ( 78.172 ms
  76.116 ms 75.851 ms
 9 Pos1-0.paix1.PaloAlto.net.reach.com ( 305.915 ms 309.617 ms
   314.994 ms
10 fe0.pao0.verio.net ( 308.744 ms 304.431 ms 304.230 ms
11 p4-6-0-0.r02.mclnva02.us.bb.verio.net ( 380.061 ms 380.639 ms 
  380.292 ms
12 p16-0-0-0.r01.mclnva02.us.bb.verio.net ( 384.100 ms 384.124 ms 384.382 ms
13 p4-7-2-0.r00.nycmny06.us.bb.verio.net ( 390.487 ms 390.300 ms 
  396.328 ms
14 p4-0-2-0.r01.nycmny06.us.bb.verio.net ( 390.196 ms 384.921 ms 
  385.245 ms
15 gxn.d3-1-0-1.r01.nycmny06.us.bb.verio.net ( 321.844 ms
  319.204 ms 319.252 ms
16 se6-1-0-llb-x-ny2.NY1.core.rtr.xara.net ( 325.706 ms 320.925 ms
  320.557 ms
17 se5-1-llb-ny1.HU1.core.rtr.xara.net ( 325.264 ms 322.578 ms
  321.049 ms
18 po2-0-llb-hu1.TH30.core.rtr.xara.net ( 389.618 ms 390.177 ms
  388.401 ms
19 gb11-0-0-llb-x-many.TH1.core.rtr.uk.xo.net ( 398.421 ms 
  388.459 ms 390.471 ms
20 fa0-0.gxn-linx.transit1.linx.net ( 388.834 ms 391.937 ms
  389.687 ms
21 k.root-servers.net ( 387.544 ms 391.093 ms 387.059 ms

Notice that each router IP address has a corresponding DNS entry. These very descriptive DNS names help Internet users and operators understand what is happening with their connections and which route the outbound traffic is taking. The descriptive names are an invaluable aid to troubleshooting problems on the net.

Table 2-2 shows some examples of descriptive DNS formats used by various ISPs.

Table 2-2 DNS Formats


Example Use of the DNS



BBN Planet














Telstra BigPond








KDD Internet




DNS Resolver in IOS Software

You can specify a default domain name that the Cisco IOS Software will use to complete domain name requests for functions such as Telnet, TFTP, and other instances of name completion (for example, ip ospf domain-lookup). You can specify either a single domain name or a list of domain names. Any IP host name that does not contain a domain name will have the domain name that you specify appended to it before being added to the host table.

ip domain-name name
ip domain-list name

It is also advisable to include a name server for the router to resolve the DNS request:

ip name-server server-address1 [[server-address2]...server-address6]

Remember that the current practice on the Internet is to quote at least two DNS resolvers. The reason is the same as for any other situation: redundancy. If one DNS server disappears, the other one can take over. When both are there, the router will look up the servers in a round-robin fashion for each request.

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