Information Technology Reference
In-Depth Information
BGP Convergence Tuning
This section examines performance aspects of BGP convergence from an uninitialized
state. This state is typical for a router that has just been reloaded or is newly deployed. BGP
convergence from an uninitialized state is of special interest because of the significant
impact on the network.
The term
BGP convergence
requires clarification to ensure that a common definition is
being used. A BGP router is said to have converged when the following criteria have
been met:
•
NOTE
All routes are accepted.
All routes are installed in the routing table.
•
The table version counter for all peers must equal the table version of the BGP table.
•
The BGP InQ and OutQ for all peers must be 0.
•
This chapter defines convergence as the amount of time it takes from the establishment of
the first peer until the router has a fully populated RIB and has updated all its BGP peers.
The placement of the initializing router in the network is significant in determining the
scope of impact on convergence. The focus is on convergence internal to the network. The
following examples describe three different BGP convergence scenarios:
•
Scenario 1: The edge router initializes
—An edge router is an ISP router that is used
for customer aggregation. A more-detailed explanation of the BGP topology for an
edge router is provided in Chapter 9, “Service Provider Architecture.” An edge router
initializes its BGP sessions with the customers and with its two upstream route reflec-
tors. The route reflectors send down the full internal table of 125,000 prefixes. The
edge router sends upstream 500 prefixes, received from the customers, to the route
reflectors. The edge router receives approximately 250,000 paths and advertises 500
prefixes to both the route reflectors.
•
Scenario 2: The peering router initializes
—A peering router is an ISP router that is
used for connecting to another ISP, a special case of the edge router. A more-detailed
explanation of peering is provided in Chapter 9. A peering router initializes its BGP
sessions with the external peers and its two upstream route reflectors. If the peering
router receives 80,000 unique prefixes from its various peers, they are sent upstream
to the route reflectors. The route reflectors advertise the internal table, which consists
of 125,000 prefixes, to the peering router.
