Smallest Multi-Exit Discriminator

The purpose of the MED (or MULTI_EXIT_DISC) is to allow routers in one AS to tell routers in a neighboring AS how good a particular route is. In fact, because of how MED works, it is often called the BGP metric, even though it is not close to the top of the BGP decision process. Figure 13-11 shows a classic case for the use of MED, where a customer has two links connecting it to a single ISP. The ISP, aware of its best routes for 11.0.0.0/8 and 12.0.0.0/8, can set MED so that..

The OSPF network Command

Once OSPF is turned on, you must define the interfaces that are to participate in OSPF and the area that they reside in by using the following command:  Router(config-router)#network network-number wildcard-mask area area-number  Be careful with this command. It is the cause of many errors in configuration, normally due to a misapplication of the wildcard-mask parameter, either including too many or too few interfaces in a particular OSPF area.  Like RIP, th..

Maintaining Syslog Files on the Server

The Bourne shell script in Example 18-1 automatically rotates router logfiles to ensure that these files don't become too big and cumbersome to navigate. The script is intended to be invoked via a cron job on a daily basis, but you can also run it manually. By default, the script retains seven days worth of archived logfiles and compresses files older than two days. No arguments are required or expected.   Example 18-1. rotatelog.sh #!/bin/sh # #  rotatelog.sh -- a ..

Restricting Peers

You can use the ntp access-group command to restrict which devices you want your router to allow NTP associations with: Router#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router(config)#access-list 88 permit host 172.25.1.1 Router(config)#access-list 88 permit host 10.1.1.1 Router(config)#access-list 99 permit 172.25.0.0 0.0.255.255 Router(config)#access-list 99 permit 10.2.0.0 0.0.255.255 Router(config)#clock timezone EST -5 Router(..

Disabling OSPF on Certain Interfaces

The passive-interface configuration command effectively disables OSPF on an interface by preventing it from forming OSPF adjacencies: Router3#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router3(config)#router ospf 44 Router3(config-router)#network 0.0.0.0 255.255.255.255 area 100 Router3(config-router)#passive-interface Ethernet0 Router3(config-router)#exit Router3(config)#end Router3# A useful variant of this command allows you to..