Information Technology Reference
In-Depth Information
BGP takes advantage of the reliable transport service provided by Transmission Control
Protocol (TCP). This eliminates the need in BGP to implement update fragmentation,
retransmission, acknowledgment, and sequencing, because TCP takes care of these
functions. Additionally, any authentication scheme used by TCP may be used for BGP.
After the session is established, BGP uses regular keepalives to maintain session integrity.
Update messages also reset the hold timer, which is typically three times the keepalive
timer. A BGP session is closed if three consecutive keepalives are missed and no Update
messages are received.
Accurate routing information is important for reliable forwarding. BGP uses several
measures to increase accuracy. When updates are received,
AS_PATH
(a BGP attribute that
lists the autonomous systems the route has traversed) is checked to detect loops. Updates
sourced from the current AS or that have passed through the AS are denied. Inbound filters
can be applied to all updates that ensure conformance to local policies. Reachability of the
next hop is regularly verified before a BGP route is considered valid.
To maintain the accuracy of the routing information, it is also important to remove
unreachable routes in a timely manner. BGP withdraws them promptly from the peers as
the routes become unreachable.
Stability
A routing protocol's stability is critical for large networks. Given the size of the current
Internet, flapping of large numbers of routes can be catastrophic.
By implementing various timers, BGP suppresses the impact of interface or route up/down
events on the network. For example, a BGP speaker can generate updates up to only the
Minimal Advertisement Interval. In Cisco IOS software, the interval is 30 seconds for
external BGP (eBGP) sessions and 5 seconds for internal BGP (iBGP) sessions, plus some
jitters to avoid synchronization of updates. The subject of eBGP versus iBGP is discussed
in Chapter 2, “Understanding BGP Building Blocks.”
Route dampening is another BGP feature that suppresses instability. The router tracks a
route's flapping history. Unstable routes are penalized and are subject to suppression. Route
dampening is discussed in several chapters of this topic.
Stability can be increased if sessions do not have to be reset when a policy changes. Fea-
tures such as soft reconfiguration and route refresh, both of which are covered in Chapter 3,
“Tuning BGP Performance,” are useful for changing BGP policy without resetting the BGP
session. Both of these features allow new updates to be requested or sent dynamically.
If a session must be reset, all BGP routing and forwarding information for that session is
cleared. This might lead to packet loss until a new forwarding database is built. Nonstop
Forwarding (NSF) or Graceful Restart allows a router to continue forwarding with the
existing information (retained from the previous session) while the session is being reset.
NSF is discussed in detail in Chapter 3.
