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[ACX/M/MX] BGP route advertisement happens with a delay in a scaled network

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Article ID: KB36325 KB Last Updated: 24 Nov 2020Version: 1.0
Summary:

This article explains that the delay in BGP convergence that takes place in a scaled network is expected behavior and goes to provide more details about such behavior.

 

Symptoms:

The following logs show that the interface was shut down at 2:31:49, which resulted in BGP neighborship going down as well followed by route withdrawal:

Sep 24 02:31:49.742  jtac-mx480-r2054-re0 mgd[59057]: UI_COMMIT_COMPLETED:  : commit complete

The first update message with MP unreach is sent at 2:31:50:

Sep 24 02:31:50.712977 brt_active_set:3168: send proc: active table set for peer 62.241.26.3 (Internal AS 43976), rib 5, priority 0
Sep 24 02:31:50.714765 bgp_peer_send_msg_start:2582: Cloned new peer send msg into build area 0xf7fcae0 from group, parts=4, bytes=4039
Sep 24 02:31:50.714773 BGP_43976.62.241.26.3: send proc: send via threaded I/O
Sep 24 02:31:50.714777 sending 4039 bytes
Sep 24 02:31:50.714782
Sep 24 02:31:50.714782 BGP SEND 62.241.26.1+58561 -> 62.241.26.3+179
Sep 24 02:31:50.714787 BGP SEND message type 2 (Update) length 4039
Sep 24 02:31:50.714792 BGP SEND Update PDU length 4039
Sep 24 02:31:50.714801 BGP SEND flags 0x90 code MP_unreach(15): AFI/SAFI 1/128
Sep 24 02:31:50.714807 BGP SEND     Add-path enabled peer 62.241.26.3 nlri inet-vpn-unicast

The last message is sent at 2:32:49

Sep 24 02:32:49.403286 BGP SEND 62.241.26.1+58561 -> 62.241.26.3+179
Sep 24 02:32:49.403290 BGP SEND message type 2 (Update) length 2654
Sep 24 02:32:49.403294 BGP SEND Update PDU length 2654
Sep 24 02:32:49.403299 BGP SEND flags 0x90 code MP_unreach(15): AFI/SAFI 1/128
Sep 24 02:32:49.403303 BGP SEND     Add-path enabled peer 62.241.26.3 nlri inet-vpn-unicast

Route withdrawal takes place as follows:

Sep 24 02:32:30.354909 BGP SEND  43976:100:20.20.20.20/32 (label 0)  [ADDPATH Path-ID: 1] , 43976:100:81.17.219.132/32 (label 0)  [ADDPATH Path-ID: 1] , 43976:100:59.167.0.0/16 (label 0)  [ADDPATH Path-ID: 1] , 43976:100:59.166.0.0/16 (label 0)  [ADDPATH Path-ID: 1]

Note that it takes almost a minute to achieve full convergence here.

As shown, convergence also depends on when the prefix that we are trying to reach gets advertised. If it gets advertised sooner, traffic to that prefix will resume faster. That is why there is variation of time when the ping traffic to 20.20.20.20 starts to work between multiple attempts. 

 

Solution:

No action is required because this behavior is expected.

Typically BGP deals with millions of routers. If you look at a specific route (for example, 8.8.8.8), the change in route details (MP unreach in the case of a vrf route) may happen based on the order of route processing.

BGP cannot send MP reach for all 800,000 routes simultaneously. Doing so would trigger an MP unreach based on the route entries in the list of 800,000. If the route that we are interested in falls within the range of the first few routes in the 800,000 prefixes, withdrawal would be faster. If the route that we are interested in is at the end of the 800,000 list, prefix withdrawal may happen with a delay.

 

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