Brocade BigIron RX Series Configuration Guide - Manual (Page 908)

830 BigIron RX Series Configuration Guide
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Configuring a switch to allow routes with its own AS number
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The no-export keyword filters for routes with the well-known community “NO_EXPORT”. A route in
this community should not be advertised to any BGP4 neighbors outside the local AS. If the router
is a member of a confederation, the device advertises the route only within the confederation. For
information about confederations, refer to
“Configuring confederations” on page 837.
Configuring a switch to allow routes with its own AS number
BGP rejects routes that contain its own AS number within its AS_PATH attribute to prevent routing
loops. In an VPN hub and spoke topology this can stop legitimate routes from being accepted. In
this release, the allowas-in command eliminates this problem by allowing you to set a parameter
that disables the AS_PATH check function for routes learned from a specified location.
To configure a switch to disable the AS_PATH check function for routes sent to it by its BGP
neighbor for a maximum limit of 3 occurrences of the route, enter the following command at the
BGP configuration level.
BigIron RX(config-bgp-ipv4u)# neighbor 33.33.36.2 allowas-in 3
Syntax: neighbor <IPaddress> allowas-in <asn_limit>
The <IPaddress> variable is the IP address of the neighbor.
The asn_limit value prevents loops by limiting the number of occurrences that the AS number will
be accepted in routes that are received from the specified switch. The maximum limit is 10.
BGP Null0 routing
BGP can use the null0 route to resolve its next hop. Thus, null0 route in the routing table (for
example, static route) is considered as a valid route by BGP. If the next hop for BGP resolves into a
null0 route, the BGP route is also installed as a null0 route in the routing table.
The null0 routing feature allows network administrators to block certain network prefixes, by using
null0 routes and route-maps. The combined use of null0 routes and route maps blocks traffic from
a particular network prefix, telling a remote router to drop all traffic for this network prefix by
redistributing a null0 route into BGP.
Figure 123 shows a topology for a null0 routing application example.