Information Technology Reference
In-Depth Information
router to request that a remote peer resend its BGP Adj-RIB-Out. This allows the
BGP router to reapply the inbound policy.
Example 3-12 shows the verification of route refresh support for a particular BGP session.
Example 3-12
show
Command Output for Route Refresh Capability Verification
Router#show ip bgp neighbor peer address | include [Rr]oute [Rr]efresh
....
Neighbor capabilities:
Route refresh: advertised and received(old & new)
....
If BGP soft reconfiguration is enabled for a particular neighbor, route refresh is operational
for that neighbor. These features are mutually exclusive. Route refresh is handled on a per-
address family basis.
Tr ansmit Side Loop Detection
Transmit (TX) loop detection must be implemented manually. A BGP router advertises pre-
fixes to a peer with that peer's autonomous system number (ASN) in the AS_PATH, relying
on the peer to perform loop detection by checking for its own ASN in the AS_PATH. The
idea is that preventing the prefixes from being advertised in the first place reduces the size
of the BGP update, providing an optimized set of prefixes for the receiving peer to process.
The TX loop-detection configuration is applicable only to external peers. The configuration
is minimal and can be applied using a route map or a filter list. A configuration example is
provided in Example 3-13.
Example 3-13
TX Side Loop-Detection Configuration
router bgp 100
...
neighbor 10.1.1.1 remote-as 1
neighbor 10.1.1.1 filter-list 1 out
...
!
ip as-path 1 deny _1_
ip as-path 1 permit any
!
The as-path list matches on the existence of the remote ASN in the AS_PATHs of the
advertised prefixes and denies them. All other prefixes are explicitly permitted.
