Why multiple Loopback Inerafces

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Dec 19th, 2007
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Just curious why a router/L3 switch would be configured to have more than one Loopback interface IP address? Would they configure multiple Loopback intrefaces to match with multiple GRE tunnels?


Just curious, thanks.

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Richard Burts Wed, 12/19/2007 - 14:01
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Glenn


There might be many reasons why a router/L3 switch would be configured with multiple loopback interfaces. At one point I was configuring a router that was going to have multiple OSPF processes and I configured a loopback interface for each process so that each process would have a router ID based on a loopback interface.


We frequently use loopback interfaces as the source address for various types of traffic sourced from the router (FTP, telnet, syslog, SNMP, etc). There might be a situation where to be able to control traffic (access list filtering or firewall somewhere in the network) you might want management traffic like syslog or SNMP sourced from one address but telnet sourced from a different address. So multiple loopback interfaces would make sense here.


There are other scenarios where multiple loopback interfaces are useful. But these are a beginning of the discussion.


HTH


Rick

gwhansen59 Thu, 12/20/2007 - 05:45
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Thanks Rick. One other question, if you multiple Loopback interfaces configured on a router/switch, how can you determine which Loopback interface is the one used as management purposes? You can't tell by the IP address can you (i.e. highest priority like when using OSPF)?


In other words, you can't just execute a 'sh ip int b' command and know which Loopback is the management interface, right? I would think in most situations, Loopback0 is used as the management interface.


Thanks,

Glenn

Richard Burts Thu, 12/20/2007 - 05:51
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Glenn


You are correct that you can not tell by looking at show ip interface brief which interface is used as the management interface. And in fact it is quite possible that different interfaces will be used for different parts of management of the router. You might be using one interface as the source for authentication with your AAA ACS server, and another interface as the source for your syslog, and perhaps another interface as the polling address for network management purposes.


So to know which interface(s) are being used for management purposes you really need to look through the config and identify the various management functions and what interface is used for each.


HTH


Rick

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