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Two Equal-Cost BGP Routes Exist in VPN Instance A on an NE40E-X8 Are Imported to VPN Instance B Through RTs, But Only One Static Route Is Learned in the Routing Table of VPN Instance B

Publication Date:  2019-04-22 Views:  99 Downloads:  0

Issue Description

Two VPN instances are configured. In VPN instance A, BGP is used to learn two equal-cost routes (with load balancing enabled), with the next-hop IP addresses of x.x.x.1 and x.x.x.2. The routes are imported from VPN instance A to VPN instance B through RTs. However, only one static route with the next-hop IP address of x.x.x.1 is learned in the VPN routing table of VPN instance B.

Handling Process

1. View the routing table in VPN instance A. Check that both of the two routes are added to the routing table. This indicates that the next-hop IP addresses of the two routes are both reachable.

[NE40E-X8]display ip routing-table vpn-instance A

--------------------------------------------------
0.0.0.0/0 IBGP 255 100 RD x.x.x.1 Eth-Trunk0
IBGP 255 100 RD x.x.x.2 Eth-Trunk1

2. View the routing table in VPN instance B.

[NE40E-X8]display ip routing-table vpn-instance B

------------------------------------------------------------------------------
0.0.0.0/0 IBGP 255 100 RD x.x.x.1 Eth-Trunk0

3. Check the two routes learned in BGP VPN instance A.

 

[NE40E-X8]display bgp vpnv4 vpn-instance A routing-table

      Network        NextHop      MED      LocPrf    PrefVal Path/Ogn

 *>i  0.0.0.0         x.x.x.1     100        100        0      ?
 * i                     x.x.x.2     100        100        0      ?

Although the two equal-cost static default routes exist in VPN instance A, one of them is not preferentially selected. This is the IBGP equal-csot load balancing feature of NE routers. Even if the maximum-path configuration is performed, BGP load balancing exists in the routing table. However, only one BGP route is preferentially selected, and this route is forwarded to the BGP peer.

Root Cause

This is the IBGP equal-csot load balancing feature of NE routers. Even if the maximum-path configuration is performed, BGP load balancing exists in the routing table. However, only one BGP route is preferentially selected, and this route is forwarded to the BGP peer.

Solution

After BGP equal-cost load balancing is enabled, load-balancing routes exist in the routing table. However, only the optimal route in the BGP routing table can be transmitted the BGP peer.

END