how long for Cisco ASR 1000 to load the BGP table

[MonkeyBear007]

how long for Cisco ASR 1000 to load the BGP table
If the ISP is doing default does the router need to wait until the BGP is load or just use default route?

[M02@rt37]

Hello @MonkeyBear007 

That's a good question. I don't think there an exact time of processing.

The time it takes for a asr 1000 series router to load the BGP table can vary widely depending on several factors, including the size of the BGP table (i.e., the number of routes), the performance of the router, the speed and stability of the BGP peer connections, and the configuration of BGP policies (like route filters and prefix lists). For a full Internet BGP table, which typically contains over 900,000 routes, the process can take anywhere from a few minutes to potentially few minutes. During this time, the router processes incoming BGP updates, installs routes into the BGP table, and applies any configured policies. ASR 1000 series, with its advanced processing capabilities, generally handles this task more efficiently than lower-end routers, but the exact duration will still depend on the specific context and conditions mentioned...

If the ISP is providing a default route, the router does not necessarily need to wait for the full BGP table to be loaded to begin routing traffic. A default route, often provided by the ISP as a simpler and lighter alternative to a full BGP table, directs all outbound traffic to a single gateway when no specific route is found in the routing table. This means that the router can start forwarding packets using this default route as soon as the BGP session is established and the default route is received. This can significantly reduce the time to begin routing traffic, as the router does not need to process and install the full BGP table to start operating effectively. In practice, having a default route allows the network to remain functional and responsive while the full BGP table continues to load and be processed in the background. This approach is particularly advantageous in scenarios where rapid convergence and minimal downtime are critical.

[Joseph W. Doherty]

Not sure its current impact, but years ago discovered insuring router had PMTUD enabled and TCP RWIN larger than default dramatically sped up the initial BGP load.

[M02@rt37]

Hello @Joseph W. Doherty 

Yes ! Enabling PMTUD and increasing the TCP RWIN size can indeed optimize the initial BGP table load times by ensuring efficient use of available network bandwidth and reducing latency due to packet fragmentation and acknowledgment wait times. 

[Joseph W. Doherty]

". . .  packet fragmentation . . ."

BTW, normally not an issue even if PMTUD is disabled.

What can happen, though, if PMTUD is disabled BGP peers might use the min standard packet size, 576, rather than MTU.  Also note, I think that's only possible if BGP peer isn't a direct/same subnet connection.

Interestingly, years ago, if 576 was being used rather than whatever MTU was, it loaded up on router's CPU.  Why?  Well, at least for software based routers, their throughput often dramatically increases with packet size, as it appears much of their processing was dealing with packet/frame overhead rather than the (BGP) data within the packet.  The ASR, and "better" routers, often have dedicated hardware for dealing with the "usual" L2/L3 processing needs.  I.e. wouldn't expect on such a router quite the bump in performance I saw years ago.

[MonkeyBear007]

I work with ASR 9000 and I was waiting for the BGP table to load before I can do anything.
This was like 9 years ago i'm sure tech has change since then
this is not the same for ASR 1000?

[M02@rt37]

Hello @MonkeyBear007 

Over the past decade, significant technological advancements have transformed how routers, such as the Cisco asr 9000 and asr1000 series, handle BGP table loads. Historically, with routers like the asr 9000, network engineers often had to wait for the entire BGP table to load before the router could effectively route traffic. This was due to the sheer volume of routes, which required extensive processing time.

However, improvements in hardware and software have dramatically reduced these wait times. Modern ASR 9000 routers benefit from faster CPUs, increased memory, and optimized data plane processors, enabling more efficient handling of BGP updates. Additionally, Cisco IOS Xr and IOS Xe have undergone numerous optimizations, improving memory management and route processing algorithms. Protocol-level enhancements, such as BGP Route Refresh and Graceful Restart, further contribute to quicker convergence times and minimal downtime during BGP updates.

In contrast, the ASR 1000 series, designed for smaller-scale operations compared to the ASR 9000, has also seen substantial improvements in BGP processing efficiency. These routers now leverage advanced forwarding engines and robust processing capabilities, significantly reducing the time required to load large BGP tables. Modern ASR 1000 routers can use default routes provided by ISPs almost immediately after establishing BGP sessions, allowing them to route traffic efficiently without waiting for the complete BGP table to load. This capability ensures that network operations remain functional and responsive even as the BGP table is being populated in the background. 

[MHM Cisco World]

Sorry I see ypu ask me in other post and I see ypur this post 

The different I think is in scan of bgp' in asr9k scan done after full receive bgp table in asr1k scan for each prefix.

Sorry I dont have exact answer 

MHM