Information Technology Reference
In-Depth Information
In a BGP architecture, the edge routers are route reflector clients of the aggregation routers.
They also terminate eBGP peering sessions with customers. Several BGP functions are
performed on the edge routers:
•
Route dampening
—The route dampening function works on only external prefixes.
The edge routers contain externally learned prefixes from customers, transit, and
peering. The suppression of prefixes at the network edge causes that prefix to be
removed from the core and aggregation routers.
•
Route aggregation
—Prefix aggregation is also performed at the edge of the network.
An ISP may have an /8 that is distributed to customers across the entire network. The
specific prefixes are needed internally to provide reachability. However, the ISP will
want to reduce the number of prefixes advertised externally by aggregating these
longer customer prefixes into summaries.
•
Resetting the next hop
—Prefixes learned from an external peer have the next-hop
attribute for that prefix set to the remote peer's address. The next hop for a prefix must
be reachable for the path to be included in the BGP decision process. This situation is
shown in Figure 9-4.
The next-hop reachability requirements mean that the edge routers need to include the
prefixes for the customer link in the IGP or reset the next hop for the BGP prefixes
received. The most common method is to set
next-hop-self
on the iBGP session
upstream to the aggregation routers. The inclusion of link addressing for all customer
connections does not scale and dramatically increases the amount of information
contained in the IGP.
•
Zeroing BGP Multi-Exit Discriminators (MEDs)
—If BGP MEDs are used, it is
common practice to zero them on reception. This is because MEDs received from dif-
ferent autonomous systems have no relation to each other. Resetting the BGP MED
of incoming prefixes prevents routing oscillation. If BGP MEDs are accepted, pre-
venting routing oscillation requires you to configure
always-compare-med
, as dis-
cussed in Chapter 7, “Scalable iBGP Design and Implementation Guidelines.”
•
Route information filtering
—All filtering of routing information is performed on the
eBGP session. This includes filtering based on prefix lists, distribute lists, filter lists,
and community lists.
•
Policy application
—The application of BGP policy is performed at the network
edge. This includes attribute manipulation based on communities received from the
customer, such as local preference or MEDs. This also includes setting communities
for prefixes received to identify them for future policy application, such as filtering
prefix advertisements to external peers.
