Troubleshooting BGP

There are some features in BGP that are used to deal with problems that may arise. Typically the problems with a BGP network that has been configured, involve routes going offline frequently. This is called route flap and causes problems for the routers using that route.

 

Clearing routing table entries

To see if a new route is being properly added to the routing table, you can clear all or some BGP neighbor connections (sessions) using the execute router clear bgp command.

For example, if you have 10 routes in the BGP routing table and you want to clear the specific route to IP address 10.10.10.1, enter the command:

execute router clear bgp ip 10.10.10.1

To remove all routes for AS number 650001, enter the command:

execute router clear bgp as 650001

 

Route flap

When routers or hardware along a route go offline and back online that is called a route flap. Flapping is the term if these outages continue, especially if they occur frequently.

Route flap is a problem in BGP because each time a peer or a route goes down, all the peer routers that are connected to that out-of-service router advertise the change in their routing tables which creates a lot of administration traffic on the network. And the same traffic happens again when that router comes back online. If the problem is something like a faulty network cable that wobbles on and offline every 10 seconds, there could easily be overwhelming amounts of routing updates sent out unnecessarily.

Another possible reason for route flap occurs with multiple FortiGate units in HA mode. When an HA cluster fails over to the secondary unit, other routers on the network may see the HA cluster as being offline resulting in route flap. While this doesn’t occur often, or more than once at a time, it can still result in an interruption in traffic which is unpleasant for network users. The easy solution for this problem is to increase the timers on the HA cluster, such as TTL timers, so they do not expire during the failover process. Also configuring graceful restart on the HA cluster will help with a smooth failover.

The first method of dealing with route flap should be to check your hardware. If a cable is loose or bad, it can easily be replaced and eliminate the problem. If an interface on the router is bad, either avoid using that interface or swap in a functioning router. If the power source is bad on a router, either replace the power supply or use a power conditioning backup power supply. These quick and easy fixes can save you from configuring more complex BGP options. However if the route flap is from another source, configuring BGP to deal with the outages will ensure your network users uninterrupted service.

Some methods of dealing with route flap in BGP include:

• Holddown timer
• Dampening
• Graceful restart
• Bi-directional forwarding detection (BFD)

Holddown timer

The first line of defence to a flapping route is the hold down timer. This timer reduces how frequently a route going down will cause a routing update to be broadcast.

Once activated, the holddown timer won’t allow the FortiGate unit to accept any changes to that route for the duration of the timer. If the route flaps five times during the timer period, only the first outage will be recognized by the FortiGate unit — for the duration of the other outages there will be no changes because the Fortigate unit is essentially treating this router as down. After the timer expires, if the route is still flapping it will happen all over again.

Even if the route isn’t flapping — if it goes down, comes up, and stays back up — the timer still counts down and the route is ignored for the duration of the timer. In this situation the route will be seen as down longer than it really is, but there will be only the one set of route updates. This is not a problem in normal operation because updates are not frequent.

Also the potential for a route to be treated as down when it is really up can be viewed as a robustness feature. Typically you do not want most of your traffic being routed over an unreliable route. So if there is route flap going on, it is best to avoid that route if you can. This is enforced by the holddown timer.

 

How to configure the holddown timer

There are three different route flapping situations that can occur: the route goes up and down frequently, the route goes down and back up once over a long period of time, or the route goes down and stays down for a long period of time. These can all be handled using the holddown timer.

For example, your network has two routes that you want to set the holddown timer for. One is your main route ( to 10.12.101.4) that all your Internet traffic goes through, and it can’t be down for long if its down. The second is a low speed connection to a custom network that is used infrequently ( to 10.13.101.4). The holddown timer for the main route should be fairly short, lets say 60 seconds instead of the default 180 seconds. The second route timer can be left at the default or even longer since it is rarely used. In your BGP configuration this looks like:

config router bgp config neighbor edit 10.12.101.4

set holddown-timer 60 next

edit 10.13.101.4

set holddown-timer 180 next

end end

 

Dampening

Dampening is a method used to limit the amount of network problems due to flapping routes. With dampening the flapping still occurs, but the peer routers pay less and less attention to that route as it flaps more often. One flap doesn’t start dampening, but the second starts a timer where the router will not use that route — it is considered unstable. If the route flaps again before the timer expires, the timer continues to increase. There is a period of time called the reachability half-life after which a route flap will only be suppressed for half the time. This half-life comes into effect when a route has been stable for a while but not long enough to clear all the dampening completely. For the flapping route to be included in the routing table again, the suppression time must expire.

If the route flapping was temporary, you can clear the flapping or dampening from the FortiGate units cache by using one of the execute router clear bgp commands:

execute router clear bgp dampening {<ip_address> | <ip/netmask>}

or

execute router clear bgp flap-statistics {<ip> | <ip/netmask>}

For example, to remove route flap dampening information for the 10.10.0.0/16 subnet, enter the command:

execute router clear bgp dampening 10.10.0.0/16

The BGP commands related to route dampening are:

config router bgp

set dampening {enable | disable}

set dampening-max-suppress-time <minutes_integer>

set dampening-reachability-half-life <minutes_integer>

set dampening-reuse <reuse_integer>

set dampening-route-map <routemap-name_str>

set dampening-suppress <limit_integer>

set dampening-unreachability-half-life <minutes_integer>

end