BGP Router

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Description

The BGP router process runs about once per second to check for work. BGP convergence defines the duration between the time when the first BGP peer is established and the point at which BGP is converged. In order to ensure the shortest possible convergence times, BGP router consumes all free CPU cycles. However, after it starts, it relinquishes (or suspends) the CPU intermittently. Convergence time is a direct measurement of how long BGP router spends on the CPU, not the total time. This procedure shows a high CPU condition during BGP convergence and exchanges BGP prefixes with two external BGP (eBGP) peers.

1. Capture a baseline for normal CPU utilization before starting the test.

   router# show process cpu
    CPU utilization for five seconds: 0%/0%; one minute: 4%; five minutes: 5%
   

2. Once the test starts, the CPU reaches 100 percent utilization. The show process cpu command shows that the high CPU condition is caused by BGP router, denoted by 139 (the IOS process ID for BGP router) in the following output.

   router# show process cpu
    CPU utilization for five seconds: 100%/0%; one minute: 99%; five minutes: 81%
   
   !--- Output omitted.
   
   139     6795740   1020252       6660 88.34% 91.63% 74.01%   0 BGP Router
   

3. Monitor the router by capturing multiple outputs of the show ip bgp summary and show process cpu commands during the event. The show ip bgp summary command captures the state of the BGP neighbors.

   router# show ip bgp summary
    Neighbor    V    AS  MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down State/PfxRcd
    10.1.1.1    4  64512  309453  157389    19981    0  253 22:06:44 111633
    172.16.1.1  4  65101  188934    1047    40081   41    0 00:07:51 58430
   

4. When the router completes prefix exchange with its BGP peers, the CPU utilization rates should return to normal levels. The computed one minute and five minute averages will settle back down as well and may show higher than normal levels for a longer period than the five seconds rate.

   router# show process cpu
    CPU utilization for five seconds: 3%/0%; one minute: 82%; five minutes: 91%
   

5. Use the captured output of the above show commands to calculate the BGP convergence time. In particular, use the "Up/Down" column of the show ip bgp summary command and compare the start and stop times of the high CPU condition. Typically, BGP convergence can take several minutes when exchanging a large Internet routing table.

Note: The high CPU on the device could also be due to the instability of the BGP table. If the router is receiving two copies of the routing table, one from the EBGP peering with the ISP and the other from the IBGP peering in the network. The root cause for this is the amount of memory on the device. Cisco recommends a minimum of 1 Gig of RAM for a single copy of the internet routing table. To circumvent this instability, increase the RAM on the device or filter the prefixes so that the BGP table and the memory occupied by it is relieved.