S12508F-AF EBGP load balance problem

2020-06-23 19:13:48 Published
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Network Topology


12508 does EBGP with two uplink devices, and the neighbor relationship is normal.
Interconnect 158.1 and 159.1 are 4 100G ports respectively . 

12508>dis int brief | in NCS

HGE1/2/0/2           UP   UP       220.185.x3.198  

HGE1/2/0/3           UP   UP       220.185.x.182  

HGE1/2/0/4           UP   UP       220.185.x3.186 

HGE1/2/0/5           UP   UP       220.185.x3.194 

HGE2/2/0/1           UP   UP       220.185.x.190  

HGE2/2/0/3           UP   UP       220.185.63.210  

HGE2/2/0/4           UP   UP       220.185.63.202  

HGE2/2/0/5           DOWN DOWN     220.185.63.206 


Problem Description

Configure load balancing in the EBGP IPV4 address family as follows, used to advertise two default routes to 12508,

12508 configuration
peer xx.130.158.1 as-number 1xxx0
peer xx.130.158.1 group aaaa
peer xx.130.158.1
peer xx.130.159.1 as-number 1xxx0

address-family ipv4 unicast
dampening
balance ebgp 8  \\The relevant explanation is: https://www.h3c.com/cn/d_202001/1259040_30005_0.htm#aa_15
preference 20 200 200

But after configuring load balancing, check the routing table and see that there are only two interfaces for the next hop interface

S12508>dis ip routing-table 0.0.0.0
Summary Count: 2
Destination/Mask Proto Pre Cost NextHop Interface
0.0.0.0/0 BGP 20 0 xx.130.158.1 HGE1/2/0/4
0.0.0.0/0 BGP 20 0 xx.130.159.1 HGE2/2/0/4


View the BGP routing table,
<S12508>dis bgp routing-table ipv4 0.0.0.0
BGP local router ID: 220.xxx.58.223
Local AS number: 6xx40
Paths: 2 available, 2 best
BGP routing table information of 0.0.0.0/0:
From: xx.130.158.1 (xx.130.158.1)
Relay nexthop: 220.185.xx.185
Original nexthop: xx.130.158.1
OutLabel: NULL
AS-path: 136x90 4xx34
Origin: igp
Attribute value: pref-val 0
State: valid, external, best,


From: xx.130.159.1 (xx.130.159.1)
Relay nexthop: 220.185.xx.181
Original nexthop: xx.130.159.1
OutLabel: NULL
AS-path: 136190 4134
Origin: igp
Attribute value: pref-val 0
State: valid, external, best,

The current temporary replacement using eight default routes is not convenient for testing.
The following are manually configured static routes:
<S12508>display ip routing-table 0.0.0.0
Summary Count: 9
Destination/Mask Proto Pre Cost NextHop Interface
0.0.0.0/0 Static 10 0 220.185.xx.181 HGE1/2/0/3
220.185xx.185 HGE1/2/0/4
220.185.xx.189 HGE2/2/0/1
220.185.x.193 HGE1/2/0/5
220.185.x3.197 HGE1/2/0/2
220.185.x.201 HGE2/2/0/4
220.185.x.205 HGE2/2/0/5
220.185.x3.209 HGE2/2/0/3
  


Process Analysis

You can display fib 0.0.0.0 0 to see if there are 8 next hops in the default route.

The next hop in the ip routing table that iterates through EBGP routes on our device shows the next hop before the iteration. 

The next hop is the loopback address of the NCS. When it is actually forwarded, it will iterate to the remaining interfaces. fib table view.  

Solution

This phenomenon is normal. 

The display result of the current version of the platform is before the iteration. There are mentions of demand rectification. After E3603P01, it is the display result of B75 rectification.  

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