Layer 3 loop brought by route Summary - solution experiment

A summary of the problem

We all know , Do route summary to reduce route entries , However, improper configuration may cause a three-layer loop , And will cause equipment CPU Increase the load .

 

The topology

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AR1 Analog Internet ,AR2 Is an exit routing device ,LSW1 As a three-layer switch, it is divided into 5 individual vlan, And is PC1-PC5 Gateway for

The experiment starts with the establishment of normal network interworking , Then test and trigger the loop , Finally, how to solve this problem

 

Equipment configuration and network connectivity test

LSW1：

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       [LSW1]vlan batch 10 20 30 40 50 100
       
       
[LSW1]int vlanif 10
       
       
[LSW1-Vlanif10]ip add 10.1.1.1 24
       
       
[LSW1-Vlanif10]int vlanif 20
       
       
[LSW1-Vlanif20]ip add 10.1.2.1 24
       
       
[LSW1-Vlanif20]int vlanif 30
       
       
[LSW1-Vlanif30]ip add 10.1.3.1 24
       
       
[LSW1-Vlanif30]int vlanif 40
       
       
[LSW1-Vlanif40]ip add 10.1.4.1 24
       
       
[LSW1-Vlanif40]int vlanif 50
       
       
[LSW1-Vlanif50]ip add 10.1.5.1 24
       
       
// because ensp in S5700 This device cannot be configured after it is converted to a three-layer interface ip, So here we use vlanif100 Instead of 
       
       
[LSW1-Vlanif50]interface Vlanif100
       
       
[LSW1-Vlanif100]ip address 172.16.1.2 255.255.255.252
       
       
[LSW1-Vlanif100]interface GigabitEthernet0/0/1
       
       
[LSW1-GigabitEthernet0/0/1]port hybrid pvid vlan 100
       
       
[LSW1-GigabitEthernet0/0/1]port hybrid untagged vlan 100
       
       
[LSW1-GigabitEthernet0/0/1]un sh
       
       
#
       
       
[LSW1-GigabitEthernet0/0/1]interface GigabitEthernet0/0/2
       
       
[LSW1-GigabitEthernet0/0/2]port link-type access
       
       
[LSW1-GigabitEthernet0/0/2]port default vlan 10
       
       
#
       
       
// The interface configuration method of the remaining connecting hosts is similar , It is omitted here ...
       
       
#
       
       
// Configure the default route 
       
       
[LSW1]ip route-static 0.0.0.0 0.0.0.0 172.16.1.1
      
      
      
      

       
       
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AR2：

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       [AR2]interface GigabitEthernet0/0/0
       
       
[AR2-GigabitEthernet0/0/0]ip address 100.100.100.2 255.255.255.252 
       
       
[AR2-GigabitEthernet0/0/0]un sh
       
       
[AR2-GigabitEthernet0/0/0]interface GigabitEthernet0/0/1
       
       
[AR2-GigabitEthernet0/0/1]ip address 172.16.1.1 255.255.255.252
       
       
[AR2-GigabitEthernet0/0/1]un sh
       
       
[AR2-GigabitEthernet0/0/1]quit
       
       
// Here, multiple network segments are summarized , And configure static routing 
       
       
[AR2]ip route-static 10.1.0.0 255.255.248.0 172.16.1.2
      
      
      
      

       
       
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AR1：

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       [AR1]interface GigabitEthernet0/0/0
       
       
[AR1-GigabitEthernet0/0/0]ip address 100.100.100.1 255.255.255.252 
       
       
[AR1-GigabitEthernet0/0/0]quit
       
       
[AR1]ip route-static 0.0.0.0 0.0.0.0 100.100.100.2
      
      
      
      

       
       
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PC1 – PC5 Each is well prepared ip Address
use PC1 ping measuring 100.100.100.1, Can communicate , For normal , Besides, we tracert Follow up , It's normal
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Trigger the loop and analyze the problem

So let's use PC1 Go to tracert Track the existing and non-existent network segments .

Discovery tracking PC2(2 paragraph ) normal , That's because LSW1 Is directly connected to all PC Of , There must be a destination in the routing table PC2 Direct route entry for , Direct routing is the highest priority , So we arrived smoothly .

track 6 Paragraph and 7 Duan found jumping back and forth , It means that the loop has been lifted ,LSW1 There is no direct connection route of these two network segments in the routing table of , Only the default route can be matched AR2 了 ,AR2 After receiving, find the route table to match the summarized route entries 10.1.0.0/21, So she asked LSW1 Sent back ,LSW1 After receiving, it will be sent back .

And tracking 8 The segment shows *, It means that the forwarding cannot reach the destination address , The bag was discarded , That's because packets arrive AR2 when , No route entries were found to match ,10.1.0.0/21 The last of the paragraph ip The address is 10.1.7.255, So arrive AR2 Discard the packet .
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terms of settlement

How to solve ？ It's simple , Just in LSW1 Add one that is also 10.1.0.0/21 Aggregate black hole routing for , So when the destination is received ip Is this paragraph and ip When there is no , It won't send out any more .

Log in and copy

      
      

       
       [LSW1]ip route-static 10.1.0.0 21 NULL0
      
      
      
      

       
       
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Reuse PC1 track , Just as expected

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One last reminder , Experiments and cases can play the role of understanding and reference , In practice, we still have to be flexible ！！ Sharing is here , If you have other ways that work , Welcome to the comments section