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bfd session between NE20E and AR2200 can't go up

Publication Date:  2017-06-30 Views:  392 Downloads:  0
Issue Description

The NE20E established an EBGP peer relationship with the AR2200. After a BFD session was bound to BGP, the BFD session between the NE20E (10.X.X.9) and AR (10.X.X.240) failed to go Up.

The AR used the physical address 10.X.X.240 to establish BFD sessions with the NE20E and NE40E. The NE20E used the loopback0 address 10.X.X.9, and the NE40E used the loopback0 address 10.X.X.1. The BFD session between the AR and NE20E was Down, but the BFD session between the AR and NE40E was Up.

Handling Process

                          Step 1          Checked BFD link establishment information.

NE40E:

<DC01-WAN-R1>dis bfd sess peer-ip 10.X.X.240 verbose

--------------------------------------------------------------------------------

Session MIndex : 37945     (Multi Hop) State : Up        Name : dyn_13369

--------------------------------------------------------------------------------

  Local Discriminator    : 13369            Remote Discriminator   : 8233

  Session Detect Mode    : Asynchronous Mode Without Echo Function

  BFD Bind Type          : Peer IP Address

  Bind Session Type      : Dynamic

  Bind Peer IP Address   : 10.X.X.240

  Bind Interface         : -

  FSM Board Id           : 2                TOS-EXP                : 7

  Min Tx Interval (ms)   : 250              Min Rx Interval (ms)   : 250

  Actual Tx Interval (ms): 1000             Actual Rx Interval (ms): 1000

  Local Detect Multi     : 3                Detect Interval (ms)   : 3000

  Echo Passive           : Disable          Acl Number             : -

  Destination Port       : 3784             TTL                    : 253

  Proc Interface Status  : Disable          Process PST            : Disable

  WTR Interval (ms)      : -

  Active Multi           : 3                DSCP                   : -

  Last Local Diagnostic  : No Diagnostic

  Bind Application       : BGP

  Session TX TmrID       : -                Session Detect TmrID   : -

  Session Init TmrID     : -                Session WTR TmrID      : -

  Session Echo Tx TmrID  : -

  PDT Index              : FSM-400008A | RCV-98 | IF-41C0000 | TOKEN-0

  Session Description    : -

--------------------------------------------------------------------------------

 

     Total UP/DOWN Session Number : 1/0

NE20E:

<PM9-WAN-R1>dis bfd sess peer-ip 10.X.X.240 ver

--------------------------------------------------------------------------------

  (One Hop)   State : Down                  Name : dyn_16406

--------------------------------------------------------------------------------

  Local Discriminator    : 16406            Remote Discriminator   : 0

  Session Detect Mode    : Asynchronous Mode Without Echo Function

  BFD Bind Type          : Interface(GigabitEthernet0/3/8)

  Bind Session Type      : Dynamic

  Bind Peer IP Address   : 10.X.X.240

  Bind Interface         : GigabitEthernet0/3/8

  Track Interface        : -

  Bind Source IP Address : 10.X.X.9

  FSM Board Id           : 3                TOS-EXP                : 7

  Min Tx Interval (ms)   : 500              Min Rx Interval (ms)   : 500

  Actual Tx Interval (ms): -                Actual Rx Interval (ms): -

  Local Detect Multi     : 3                Detect Interval (ms)   : -

  Echo Passive           : Disable          Acl Number             : -

  Destination Port       : 3784             TTL                    : 255

  Proc Interface Status  : Disable          Process PST            : Disable

  WTR Interval (ms)      : -                Config PST             : Disable

  Active Multi           : -

  Last Local Diagnostic  : No Diagnostic

  Bind Application       : BGP

  Session TX TmrID       : -                Session Detect TmrID   : -

  Session Init TmrID     : -                Session WTR TmrID      : -

  Session Echo Tx TmrID  : -

  Session Not Up Reason  : In negotiation

  Session Description    : -

--------------------------------------------------------------------------------

 

Total UP/DOWN Session Number : 0/1

AR2240:

<msk02-pan-r1>dis bfd sess peer 10.160.0.1 ver

--------------------------------------------------------------------------------

Session MIndex : 297       (Multi Hop) State : Up        Name : dyn_8233

--------------------------------------------------------------------------------

  Local Discriminator    : 8233             Remote Discriminator   : 13369

  Session Detect Mode    : Asynchronous Mode Without Echo Function

  BFD Bind Type          : Peer IP Address

  Bind Session Type      : Dynamic

  Bind Peer IP Address   : 10X.X.1 (to NE40)

  Bind Interface         : -

  FSM Board Id           : 0                TOS-EXP                : 7

  Min Tx Interval (ms)   : 1000             Min Rx Interval (ms)   : 1000

  Actual Tx Interval (ms): 1000             Actual Rx Interval (ms): 1000

  Local Detect Multi     : 3                Detect Interval (ms)   : 3000

  Echo Passive           : Disable          Acl Number             : -

  Destination Port       : 3784             TTL                    : 253

  Proc Interface Status  : Disable          Process PST            : Disable

  WTR Interval (ms)      : -

  Active Multi           : 3                DSCP                   : -

  Last Local Diagnostic  : No Diagnostic

  Bind Application       : BGP

  Session TX TmrID       : -                Session Detect TmrID   : -

  Session Init TmrID     : -                Session WTR TmrID      : -

  Session Echo Tx TmrID  : -

  PDT Index              : FSM-3 | RCV-0 | IF-0 | TOKEN-0

  Session Description    : -

--------------------------------------------------------------------------------

 

     Total UP/DOWN Session Number : 1/0

AR:

 

<msk02-pan-r1>dis bfd sess peer-ip 10.X.X.9 verbose

--------------------------------------------------------------------------------

Session MIndex : 306       (Multi Hop) State : Down      Name : dyn_8242

--------------------------------------------------------------------------------

  Local Discriminator    : 8242             Remote Discriminator   : 0

  Session Detect Mode    : Asynchronous Mode Without Echo Function

  BFD Bind Type          : Peer IP Address

  Bind Session Type      : Dynamic

  Bind Peer IP Address   : 10.X.X.9 (to NE20)

  Bind Interface         : -

  FSM Board Id           : 0                TOS-EXP                : 7

  Min Tx Interval (ms)   : 500              Min Rx Interval (ms)   : 500

  Actual Tx Interval (ms): 11500            Actual Rx Interval (ms): 11500

  Local Detect Multi     : 3                Detect Interval (ms)   : -

  Echo Passive           : Disable          Acl Number             : -

  Destination Port       : 3784             TTL                    : 253

  Proc Interface Status  : Disable          Process PST            : Disable

  WTR Interval (ms)      : -

  Active Multi           : -                DSCP                   : -

  Last Local Diagnostic  : No Diagnostic

  Bind Application       : BGP

  Session TX TmrID       : 31907            Session Detect TmrID   : -

  Session Init TmrID     : -                Session WTR TmrID      : -

  Session Echo Tx TmrID  : -

  PDT Index              : FSM-3 | RCV-0 | IF-0 | TOKEN-0

  Session Description    : -

--------------------------------------------------------------------------------

 

     Total UP/DOWN Session Number : 0/1

                          Step 2          Analyzed the symptom.

The AR established an EBGP BFD session with the peer loopback address, and therefore the route from the AR to the peer is a non-direct route. In this case, a multi-hop BFD session was established. The theoretical case complies with the actual case.

The NE20E and NE40E established an EBGP BFD session with the peer's directly connected interface address, and therefore the route to the peer is a direct route. In this case, a single-hop BFD session should be established theoretically.

For the NE20E, the theoretical case complies with the actual case. For the NE40E, however, the actual case does not comply with the theoretical case.

As a result, the NE40E established an EBGP BFD session with the AR's directly connected interface address, but the NE20E did not.

Root Cause

When the NE40E used V600R007C00, the confederation EBGP peer relationship affected the type of a BFD session established for the directly connected EBGP peer relationship. As a result, a multi-hop BFD session was established. However, the NE20E used V800R007. In this version, the confederation EBGP peer relationship did not affect the type of a BFD session established for the directly connected EBGP peer relationship, and a single-hop BFD session was established. The AR's non-directly-connected EBGP peer relationship used multi-hop BFD to establish sessions. Therefore, the AR could establish a multi-hop BFD session with the NE40E, but cannot establish a BFD session with the NE20E.

Solution

l   The confederation EBGP actually simulated the IBGP function. It is more reasonable to use loopback addresses to establish links. It is recommended that a loopback address should be used to establish a confederation EBGP peer relationship on the AR side, considering the following advantages:

       Using a loopback address to establish a peer relationship is more stable than using a physical interface address to establish a peer relationship. If the physical interface goes Down, the peer relationship is not affected only if the route to the peer link address is reachable. Additionally, route flapping is prevented.

       When a loopback address is used to establish a link, using BFD can meet the actual application. When a physical interface address is used to establish a link, BFD does not need to be configured and the device itself can quickly detect faults.

When loopback addresses are used to establish a link on both ends, the live network has low risks. The EBGP peers' routing mechanisms are the same. Generally, the difference between the upstream and downstream route convergence speeds is small, which facilitates live-network maintenance.

END