Users Guide
1328 BGP
5
Prefer the route with the lower MED. By default, MEDs are compared for
routes from any AS, but a configuration option limits comparison of
MEDs to the same AS. A route with no MED is considered to have a MED
of 0.
6
Prefer an eBGP route to an iBGP route
7
Prefer the route with the lower IGP cost to the BGP NEXT HOP
8
Prefer the route learned from the peer with the lower router ID
9
Prefer the route learned from the peer with the lower peer IP address
Limiting Phase 2 CPU Usage
In a network with a large number of prefixes, phase 2 of the decision process
can consume a significant amount of time. If the BGP hold timers are
configured to be shorter than the duration of the decision process, the timers
can expire causing a loss of adjacency. If the decision process runs frequently,
it may consume significant CPU resources, starving other processes. Two
mechanisms mitigate these potential issues. First, a hold timer prevents phase
2 from running too often. The hold time is explained in more detail in the
next paragraph. Second, phase 2 is limited to running no more than
approximately one second without yielding to other tasks.
When an event triggers phase 2, a short delay (usually 100 ms) is imposed
before the decision process runs. This delay allows other RIB changes to
complete before computing new routes. When the trigger occurs, if the
decision process was previously run within the hold time, the next decision
process is scheduled to run after the hold time expires. The initial hold time is
one second. Each time one or more new triggers occur while the hold timer is
running, the hold timer is doubled, up to a maximum of 4 seconds. When the
hold timer expires without BGP receiving a new phase 2 trigger, the hold time
is reset to the minimum hold time.
Path Attributes
Dell EMC Networking supports all path attributes described in RFC 4271.
Dell EMC Networking BGP sets the ORIGIN path attribute to IGP for routes
originated through the network command and to INCOMPLETE for routes
originated through route redistribution. Dell EMC Networking BGP never
sets the ORIGIN path attribute to EGP.