Table of Contents
Advertisement
------------------ ------- --------------- -------------- ------------
10.100.3.3/32
10.63.42.23/32
Access Lists
An access list is a sequential collection of permit and deny conditions that you can
use to filter inbound or outbound routes. You can use different kinds of access lists
to filter routes based on either the prefix or the AS path.
Filtering Prefixes
To filter routes based on the prefix, you can do any of the following:
The router compares each route's prefix against the conditions in the list or tree one
by one. If the first match is for a permit condition, the route is accepted or passed.
If the first match is for a deny condition, the route is rejected or blocked. The order
of conditions is critical because testing stops with the first match. If no conditions
match, the router rejects or blocks the address; that is, the last action of any list is
an implicit deny condition for all routes. The implicit rule is displayed by show
access-list and show configuration commands.
You cannot selectively place conditions in or remove conditions from an access list,
prefix, list, or prefix tree. You can insert a new condition only at the end of a list or
tree.
Consider the network structure in Figure 21 on page 84.
I1- ISIS level 1, I2- ISIS level2,
I- route type intra, IA- route type inter, E- route type external,
i- metric type internal, e- metric type external,
O- OSPF, E1- external type 1, E2- external type2,
N1- NSSA external type1, N2- NSSA external type2
Prefix/Length
Type
Bgp
Bgp
Define an access list with the access list command and apply the list to routes
received from or passed to a neighbor with the neighbor distribute-list
command.
Define a prefix list with the ip prefix-list command and apply the list to routes
received from or passed to a neighbor with the neighbor prefix-list command.
Define a prefix tree with the ip prefix-tree command and apply the list to routes
received from or passed to a neighbor with the neighbor prefix-tree command.
Next Hop
Dist/Met
10.12.12.1
33/44
10.45.2.31
33/44
Configuring BGP Routing Policy
Chapter 1: Configuring BGP Routing
Intf
ATM5/1.12
ATM5/1.14
83
Advertisement
Table of Contents
Troubleshooting
