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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


The importance of the loopback interface should never be overlooked, especially for general operations and management of the router. Indeed, it is surprising how few ISPs make use of this time-saving resource. The chapter continues with a discussion on how to configure router interfaces and check their status.

Following the discussion of basic management configuration is an introduction to the CEF and NetFlow capabilities that ISPs should be using on their routers. The chapter finishes with a brief look at Nagle before discussing the importance of the DNS in an ISP's operation.

Many of the features discussed here are described in the context of the ISP software covered in Chapter 1, "Software and Router Management."

IOS Software and Loopback Interfaces

The use of the loopback interface is mentioned in many instances throughout this book. Although this is not a feature unique to IOS Software, there are many and considerable advantages in making full use of the capability that the loopback interface allows. This section brings together all the occasions where the loopback interface is mentioned throughout the book and describes how they can be useful to the ISP network engineer.

Motivation for Using the Loopback Interface

ISPs endeavor to minimize the unnecessary overhead present in their networks. This unnecessary overhead can be the number of networks carried in the IGP, the number of skilled engineering staff to operate the network, or even network security. The utilization of one feature, the loopback interface on the router, goes a long way to help with each of the three scenarios mentioned here.

Control of the size of the IGP is attended to by summarization of point-to-point addresses at PoP or regional boundaries, the use of IP unnumbered on static WAN interfaces, and a carefully designed network addressing plan. ISP network security is of paramount importance, and any techniques that make the management simpler are usually welcomed. For example, when routers access core servers, ISPs apply filters or access lists to these servers so that the risk of compromise from the outside is reduced. The loopback interface is helpful here as well.

It is very common to assign all the IP addresses used for loopback interfaces from one address block. For example, an ISP with around 200 routers in a network might assign a /24 network (253 usable addresses) for addressing the loopback interface on each router. If this is done, all dependent systems can be configured to permit this address range to access the particular function concerned, whether it is security, unnumbered WAN links, or the iBGP mesh. Some examples of the use of the loopback interface in the ISP environment follow in the rest of this section.

BGP Update Source

In the following example, the iBGP mesh is built using the loopback interface on each router. The loopback doesn't ever disappear, which results in a more stable iBGP, even if the underlying physical connectivity is less than reliable.

hostname gateway1
interface loopback 0

ip address
router bgp 200
 neighbor remote-as 200
 neighbor  update-source loopback 0

neighbor remote-as 200
 neighbor   update-source loopback 0


Router ID

If a loopback interface is configured on the router, its IP address is used as the router ID. This is important for ensuring stability and predictability in the operation of the ISP's network.

OSPF chooses the designated router (DR) on a LAN as the device that has the highest IP address. If routers are added or removed from the LAN, or if a router gains an interface with a higher address than that of the existing DR, the DR likely will change if the DR or backup designated router (BDR) fails. This generally is undesirable in an ISP network because ISPs prefer to have the DR and BDR routers established deterministically. This change in DR and BDR can be avoided by ensuring that the loopback interface is configured and in use on all routers on the LAN.

The loopback interface is used for the BGP router ID. If the loopback isn't configured, BGP uses the highest IP address on the router. Again, because of the ever-changing nature of an ISP network, this value can change, possibly resulting in operational confusion. Configuring and using a loopback interface ensures stability.


If the router has two or more loopback interfaces configured, the router ID is the highest IP address of the configured loopback interfaces at the time of booting the router.

Exception Dumps by FTP

Cisco routers can be configured to dump core memory to an FTP server as part of the diagnostic and debugging process. However, this core dump should be to a system not running a public FTP server, but one heavily protected by filters (TCP Wrapper even) that allow only the routers access. If the loopback interface address is used as source address from the router and is part of one address block, the filter is very easy to configure. A 200-router network with 200 disparate IP addresses makes for a very large filter list on the FTP server. Examine the following example IOS Software configuration:

ip ftp  source-interface Loopback0

ip ftp username cisco
ip ftp password 7 045802150C2E
exception protocol ftp
exception dump

TFTP Server Access

TFTP is the most common tool for uploading and downloading configurations. The TFTP server's security is critical, which means that you should always use security tools with IP source addresses. IOS Software allows TFTP to be configured to use specific IP interfaces address. This allows a fixed ACL on the TFTP server based on a fixed address on the router (for example, the loopback interface).

ip tftp source-interface Loopback0

SNMP Server Access

If SNMP is used in the network, the loopback interface again can be brought into use for security access issues. If SNMP traffic from the router is sourced from its loopback interface, it is easy to protect the SNMP management station in the NOC. A sample IOS Software configuration follows:

access-list 98 permit
access-list 98 permit
access-list 98 deny  any
snmp-server community 5nmc02m RO 98
snmp-server  trap-source Loopback0

snmp-server trap-authentication
snmp-server host 5nmc02m 
snmp-server host 5nmc02m

TACACS/RADIUS Server Source Interface

Most ISPs use TACACS+ or RADIUS for user authentication. Very few define accounts on the router itself because this offers more opportunity for the system to be compromised. A well-protected TACACS+ server accessed only from the router's loopback interface address block offers more security of user and enable accounts. A sample configuration for standard and enable passwords follows:

aaa new-model
aaa authentication login default tacacs+ enable
aaa authentication enable default tacacs+ enable
aaa accounting exec start-stop tacacs+
ip tacacs  source-interface Loopback0

tacacs-server host
tacacs-server host
tacacs-server key CKr3t#

When using RADIUS, either for user administrative access to the router or for dial user authentication and accounting, the router configuration to support loopback interfaces as the source address for RADIUS packets originating from the router looks like this:

radius-server host auth-port 1645 acct-port 1646
radius-server host auth-port 1645 acct-port 1646
ip radius source-interface Loopback0


NetFlow Flow Export

Exporting traffic that flows from the router to a NetFlow Collector for traffic analysis or billing purposes is quite common. Using the loopback interface as the source address for all exported traffic flows from the router allows for more precise and less costly filtering at or near the server. A configuration example follows:

ip flow-export destination 9996
ip flow-export  source Loopback0

ip flow-export version 5 origin-as
interface Fddi0/0/0
 description FDDI link to IXP
 ip address
 ip route-cache flow
 ip route-cache distributed
 no keepalive

Interface FDDI0/0/0 has been configured to capture flow records. The router has been configured to export Version 5–style flow records to the host at IP address on UDP port 9996, with the source address being the router's loopback interface.

NTP Source Interface

NTP is the means of keeping the clocks on all the routers on the network synchronized to within a few milliseconds. If the loopback interface is used as the source interface between NTP speakers, it makes filtering and authentication somewhat easier to maintain. Most ISPs want to permit their customers to synchronize only with their time servers, not everyone else in the world. Look at the following configuration example:

clock timezone SST 8
access-list 5 permit
access-list 5 permit
ntp authentication-key 1234 md5 104D000A0618 7
ntp authenticate
ntp trusted-key 1234
ntp  source Loopback0

ntp access-group peer 5
ntp update-calendar
ntp peer
ntp peer

Syslog Source Interface

Syslog servers also require careful protection on ISP backbones. Most ISPs prefer to see only their own systems' syslog messages, not anything from the outside world. Denial-of-service attacks on syslog devices are not unknown, either. Protecting the syslog server is again made easier if the known source of syslog messages comes from a well-defined set of address space—for example, that used by the loopback interfaces on the routers. See the following configuration example:

logging buffered 16384
logging trap debugging
logging  source-interface Loopback0

logging facility local7

Telnet to the Router

This might seem to be an odd example in a document dedicated to IOS Software essentials. However, remember that a loopback interface on a router never changes its state and rarely has any need to change its IP address. Physical interfaces can be physically swapped out or renumbered, and address ranges can change, but the loopback interface will always be there. So, if the DNS is set up so that the router name maps to the loopback interface address, there is one less change to worry about during operational and configuration changes elsewhere in the ISP backbone. ISP backbones are continuously developing entities. Here's an example from the DNS forward and reverse zone files:

; net.galaxy zone file
net.galaxy.   IN   SOA   ns.net.galaxy. hostmaster.net.galaxy. (
                1998072901 ; version == date(YYYYMMDD)+serial
                10800   ; Refresh (3 hours)
                900    ; Retry (15 minutes)
                172800   ; Expire (48 hours)
                43200 )  ; Minimum (12 hours)
        IN   NS   ns0.net.galaxy.
        IN   NS   ns1.net.galaxy.
        IN   MX   10 mail0.net.galaxy.
        IN   MX   20 mail1.net.galaxy.
localhost    IN   A
gateway1    IN   A
gateway2    IN   A
gateway3    IN   A
;etc etc
; 1.17.215.in-addr.arpa zone file
1.17.215.in-addr.arpa. IN   SOA   ns.net.galaxy. hostmaster.net.galaxy. (
                1998072901 ; version == date(YYYYMMDD)+serial
                10800   ; Refresh (3 hours)
                900    ; Retry (15 minutes)
                172800   ; Expire (48 hours)
                43200 )  ; Minimum (12 hours)
IN   NS   ns0.net.galaxy.
        IN   NS   ns1.net.galaxy.
1        IN   PTR   gateway1.net.galaxy.
2        IN   PTR   gateway2.net.galaxy.
3        IN   PTR   gateway3.net.galaxy.
;etc etc

On the router, set the Telnet source to the loopback interface:

ip telnet  source-interface Loopback0

RCMD to the Router

RCMD requires the operator to have the UNIX rlogin/rsh clients to enable access to the router. Some ISPs use RCMD for grabbing interface statistics, uploading or downloading router configurations, or taking a snapshot of the routing table. The router can be configured so that RCMD connections use the loopback interface as the source address of all packets leaving the router:

ip rcmd  source-interface Loopback0
2. Interface Configuration | Next Section

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