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Unbalanced Egress Traffics on the Core Egress Device Due to Different BGP Route Selection Policies in Different Versions

Publication Date:  2013-09-30 Views:  58 Downloads:  0
Issue Description

The egress traffics of the two core egress devices are unbanlanced in multiple cities. In some cities, one of the two core egress devices is Huawei NE5000E and the other one is CRS1XXXX from company C. In some sities, the two core egress devices are NE5000Es. However, the network structures in these cities are the same. This case takes city A as an example.
Traffics of ports on an NE5000E in city A are listed as follows:
Pos3/0/0                    up    up         27%    34% 1352435561        0
Pos4/0/0                    up    up         28%    34%  278424347          0

The egress traffic of the two ports is about 6.8 Gbit/s.
Traffics of CRS related ports in city A:
A-CR12-MB01#show int pos 4/0/0  
     5 minute output rate 1266791000 bits/sec, 267328 packets/sec
A-CR12-MB01#show int pos 14/0
     5 minute output rate 1279011000 bits/sec, 265380 packets/sec

The egress traffic of the two ports is about 2.4 Gbit/s.
 

Handling Process

The symptom is caused by the difference of BGP route selection policies in different versions:
The BGP route selection policy of V300R005 (city A)
1. The locally originated route with a low preference value is preferred
2. The next hop unattainable route is firstly discarded.
3. The IPv4 route with labels is preferred unconditionally.

4. The route with the largest PrefVal is preferred.
5. The local route with higher Local_Pref is preferred.
6. The aggregation route (Aggregation route has a higher priority than the non aggregation route) is preferred.
7. The route with the shortest AS path is preferred.
8. The preference sequency for routes with Origin attribute is: IGP, EGP, Incomplete.
9. The route with the lowest MED value is preferred.
10. The route learned form the EBGP is preferred. (The EBGP route has a higher priority than the IBGP route)
11. The AS internal IGP route with the smallest Metric is preferred.
12. The route with the shortest cluster list is preferred.
13. The route with the smallest originator ID is preferred.
14. The route with the smallest router ID is preferred.
15. The route learned from the peer with the smaller IP address is preferred.
The configuration on the NE5000E in city A is as follows:
#
 ipv4-family unicast
  undo synchronization
  preference 20 200 200

The first policy for BGP route selection is "the locally originated route with a low preference value is preferred". Therefore, the route learned from EBGP with a preference value of 20 is selected. Theoratically, the route from the EBGP is preferred compared with the router from the IBGP with the same prefix.

The BGP route selection policy of V300R007 (city G)
1. The route with the largest PrefVal is preferred.
2. The route with the largest local pref is preferred.
3. The locally originated route is prferred. (The locally originated route has a higher priority than the route learned from the neighbour.)
4. The route with the shortest AS path is preferred.
6. The route with the lowest multi exit discriminator (MED) is preferred.
7. The router learned from the EBGP is preferred. (The EBGP route has a higher priority than the IBGP route)
8. The router with the smallest IGP metric to the next hop is preferred.
9. The router with the shortest cluster list is preferred.
10. The router released by the router with the smallest router ID is preferred.
11. The route learned from the peer with the smaller IP address is preferred.
In V300R007, according to the route selection policy "the route with the lowest multi exit discriminator (MED) is preferred", the traverse route of MED 2000 is selected. The route from the IBG neighbour with th esmaller MED is preferred.
Root Cause

BGP route queried on the NE5000E in city A and NE5000E in city G are as follows:

NE5000E in city A:
<A-HR5KE-MB02>dis bgp rout 1.56.0.0 18
BGP local router ID : 218.21.155.250
Local AS number : 64659
Paths:   2 available, 1 best
BGP routing table entry information of 1.56.0.0/18:
From: 219.158.1.93 (219.158.1.93)
Relay Nexthop: 110.19.20.149

Original nexthop: 219.158.1.93   ---Uplink EBGP neighbour
Convergence Priority: 0
 AS-path 4837 64930, origin igp, MED 2050, pref-val 0, valid, external, best, pre 20
Advertised to such 2 peers:
    61.138.69.169
    218.21.154.250
BGP routing table entry information of 1.56.0.0/18:
From: 218.21.154.250 (218.21.154.250)
Relay Nexthop: 61.138.69.21

Original nexthop: 218.21.154.250  ---Traverse IBGP neighbour
Convergence Priority: 0
 AS-path 4837 64930, origin igp, MED 2000, localpref 100, pref-val 0, valid, internal, pre 200
Not advertised to any peer yet

NE5000E in city G:
<G-HR5KE-MB02>dis bgp rout 1.56.0.0 18
 
BGP local router ID : 218.21.155.246
Local AS number : 64653
Paths:   2 available, 1 best, 1 select
BGP routing table entry information of 1.56.0.0/18:
From: 218.21.154.245 (218.21.154.245) 
---Traverse IBGP neighbour

Route Duration: 07h57m46s
 Relay IP Nexthop: 110.19.21.53
Relay IP Out-Interface: GigabitEthernet12/0/0
Original nexthop: 218.21.154.245
Qos information : 0x0
AS-path 4837 64930, origin igp, MED 2000, localpref 100, pref-val 0, valid, internal, best, select, active, pre 200
Advertised to such 1 peers:
    202.99.226.125
BGP routing table entry information of 1.56.0.0/18:
From: 219.158.1.93 (219.158.1.93)
---Uplink EBGP neighbour
Route Duration: 17h56m58s
 Relay IP Nexthop: 110.19.20.109
Relay IP Out-Interface: Pos1/0/0
Original nexthop: 219.158.1.93  
Qos information : 0x0
AS-path 4837 64930, origin igp, MED 2050, pref-val 0, valid, external, pre 20, not preferred for MED
Not advertised to any peer yet
As shown in the preceding BGP rout selections, the NE5000E in city A selects the uplink route and the NE5000E in city G selects the traverse route. In addition, no route sent by the IBGP is selected on the NE5000E in city A. In the IP route list, none of the next hop of the BGP route is 218.21.154.250.
Solution
Upgrade the NE5000E to V300R007C00SPC500.
Suggestions
None

END