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Different BGP Routes Are Selected When an NE40E-X3 Replaces an NE20E

Publication Date:  2018-10-09 Views:  67 Downloads:  0
Issue Description
1. Problem description: The summary route is configured on both NE20-1 and NE20-2 which interconnect with each other. The configuration is as follows:

ipv4-family unicast
  undo synchronization
  preference 20 180 255 
aggregate //The same configuration is performed on the peer end.

2. The topology is as follows:
An IBGP peer relationship is established between NE20-1 and NE20-2.

3. Symptom

3.1 Before NE20-1 is replaced by an NE40E: The IP routing table on NE20-1 shows that the route is learned from NE20-2, with the next hop of NE20-2.

<ne20-1>         dis ip routing-table
Route Flags: R - relay, D - download to fib
Routing Tables: Public
        Destinations : 261      Routes : 264
Destination/Mask    Proto  Pre  Cost     Flags NextHop         Interface BGP 180 0 RD 10.1.Y.2 GigabitEthernet1/0/1 //Next hop of NE20-2 before the replacement.

3.2 After NE20-1 is replaced by an NE40E running a V600R008 version:

[ne40-1]dis ip routing-table protocol bgp
Route Flags: R - relay, D - download to fib
Public routing table : BGP
         Destinations : 155      Routes : 155     
BGP routing table status : <Active>
         Destinations : 152      Routes : 152
Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface IBGP 255 0 D NULL0 //After the replacement, the next hop learned is NULL0, and the route changes.

4. Question

Why is NE20-1 selects the next hop of NE20-2 from the IP routing table? Why are next hops inconsistent before and after the replacement?
Handling Process
1. Compare key configurations:

On the NE40E:
ipv4-family unicast
  undo synchronization
  preference 20 180 255

On NE20-1:
ipv4-family unicast
  undo synchronization
  preference 20 180 255
The configuration remains unchanged after the replacement.

2. Check the product documentation of the corresponding version, find the differences between BGP route selection, and compare with the previous configuration.
Root Cause
The BGP route selection principles of NE20-1 (V200R005) differ from those of an NE40E (V600R008).

1.1 On an NE20 (V200R005), BGP selects routes based on the following principle:

BGP selects a locally originated route with a lower preference value. Preference values of various protocol routes (including direct routes and static routes) in the IP routing table can be queried using the display ip routing-table command. A route with a smaller preference value has a higher priority.

1.2 On an NE40E (V600R008), BGP selects routes based on the following principles:
When multiple routes to the same destination are available, BGP preferentially selects the route with the largest PrefVal value.
A. PrefVal is a Huawei-specific attribute, valid only on the device where it is configured.
B. BGP preferentially selects the route with the highest Local_Pref value.
C. If a route does not carry Local_Pref, the default value 100 takes effect. To modify the default Local_Pref value of BGP routes, run the default local-preference command. A locally originated route is preferentially selected. (A locally originated route takes precedence over a route learned from a peer.)

2. In conclusion, the NE20E compares the learned routes according to their preference values. After the preference 20 180 255 command is run, the protocol preferences for local BGP routes and IBGP routes are 255 and 180, respectively, indicating IBGP routes are preferentially selected. Before NE20E-1 is replaced by an NE40E, the next hop address learned by NE20E-1 is the peer's IP address. After NE20E-1 is replaced by an NE40E, the NE40E preferentially selects a locally originated route. As a result, the routes selected before and after the replacement are different.
The problem is caused by different BGP route selection rules. You are advised to optimize the rules.