Information Technology Reference
In-Depth Information
Redistribution of an IGP into BGP (Continued)
Example 9-4
access-list 40 permit 15.124.0.0
access-list 40 permit 15.24.0.0 0.7.0.0
!
access-list 50 permit 15.202.208.0
access-list 50 permit 15.238.200.0
BGP Administrative Distance
The IOS assigns an administrative distance to eBGP and iBGP routes, as it does with other
routing protocols. For the same prefix, the route with the lowest administrative distance is
selected for inclusion in the IP forwarding table. Because iBGP learned routes do not have
metrics associated with the route as IGPs (OSPF and EIGRP) do, iBGP learned routes are less
trusted. The following are the administrative distances for BGP:
eBGP routes—20
iBGP routes—200
BGP Filters
Several methods filter BGP updates, including community lists, prefix filters, distribute lists,
and AS path filters. Route maps also create route filters that are based on detailed parameters.
(Route maps are covered in Chapter 10, “Administrative Distance, Access Lists, Route
Manipulation, and IP Multicast.”
BGP Communities
A group of policies can be applied to a community of networks or destinations with community
filters. Each destination can belong to multiple communities. Communities provide a method
to control distribution and filter routing information.
The communities attribute is an optional, transitive, global attribute that is in the numerical
range from 1 to 4,294,967,200. BGP communities are optional because BGP has the option to
support the attribute. Transitive attributes are passed along to other BGP routers. A few
predefined, well-known communities follow:
internet —Advertises this route to the Internet community. All routers belong to it.
no-export —Does not advertise this route to other eBGP peers.
no-advertise —Does not advertise this route to any peer (internal or external).
local-as —Sends this route to peers in other sub-autonomous systems within the local
confederation; does not advertise this route to a peer outside the confederation.
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