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BGP Prefix-Based Filtering Lab Sim

Question

After the network administrator modified the configuration on the WGR1 router, the WGR1 router is not able to receive any prefixes from AS22. The network administrator original intent was to only filter out a particular prefix from AS22.

Fix the configuration on the WGR1 router to solve the problem. You have correctly solved the problem once the WGR1 router is able to receive 23 prefixes from AS22.

Enable secret password is cisco

Prefix_Based_BGP.jpg

 

Answer and Explanation

First check the configuration of this router with the “show running-config”

WGR1# show running-config

We should also check the ip prefix-list configuration with the “show ip prefix-list” as well:

Currently we are not sure what you will see when typing these two “show” commands but we guess it can be something like this:

WGR1# show ip prefix-list
ip prefix-list 1: 1 entries
seq 5 deny 10.0.0.0/8

(Note: 10.0.0.0/8 is just an example)

The problem here is that there is always an implicit “deny all” entry at the end of each prefix-list. If the packet doesn’t match any entry of the prefix-list, it will match the implicit “deny all” and the packet will be filtered out.

Therefore we have to permit all packets with the command:

WGR1#configure terminal
WGR1(config)#ip prefix-list 1 seq 10 permit 0.0.0.0/0 le 32
WGR1(config)#exit

The “/0” part says “check 0 bit” of “0.0.0.0”; meaning that I don’t care what any of the bits are.  The “le 32” part says that the subnet mask can be 32 bits or less (down to the number of bits we are checking) -> down to 0. So we aren’t going to check any bits and the network can have a subnet mask of anything between 0 and 32 bits. This would be ‘any’.

Force BGP to clear its table and reset all BGP sessions:

WGR1#clear ip bgp *

Display the status of all BGP connections

WGR1#show ip bgp summary

WGR1#copy running-config startup-config

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