Download Cisco.Realtests.642-885.v26-5.2015-04-08.1e.80q.vcex

http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.7/routing/configuration/guide/rc37bgp.html
As the iBGP learned routes are reflected, routing information may loop. The route reflector model has the following mechanisms to avoid routing loops:
·Originator ID is an optional, nontransitive BGP attribute. It is a 4-byte attributed created by a route reflector. 
The attribute carries the router ID of the originator of the route in the local autonomous system. Therefore, if a misconfiguration causes routing information to come back to the originator, the information is ignored. 
·Cluster-list is an optional, nontransitive BGP attribute. It is a sequence of cluster IDs that the route has passed. When a route reflector reflects a route from its clients to nonclient peers, and vice versa, it appends the local cluster ID to the cluster-list. If the cluster-list is empty, a new cluster-list is created. Using this attribute, a route reflector can identify if routing information is looped back to the same cluster due to misconfiguration. If the local cluster ID is found in the cluster-list, the advertisement is ignored. 

http://packetlife.net/blog/2009/nov/23/understanding-bgp-ttl-security/

Source-Based RTBH Filtering 
With destination-based black holing, all traffic to a specific destination is dropped after the black hole has been activated, regardless of where it is coming from. Obviously, this could include legitimate traffic destined for the target. Source-based black holes provide the ability to drop traffic at the network edge based on a specific source address or range of source addresses. 
If the source address (or range of addresses) of the attack can be identified (spoofed or not), it would be better to drop all traffic at the edge based on the source address, regardless of the destination address. This would permit legitimate traffic from other sources to reach the target. 
Implementation of source-based black hole filtering depends on Unicast Reverse Path Forwarding (uRPF), most often loose mode uRPF. 
Loose mode uRPF checks the packet and forwards it if there is a route entry for the source IP of the incoming packet in the router forwarding information base (FIB). If the router does not have an FIB entry for the source IP address, or if the entry points to a null interface, the Reverse Path Forwarding (RPF) check fails and the packet is dropped, as shown in Figure 2. Because uRPF validates a source IP address against its FIB entry, dropping traffic from specific source addresses is accomplished by configuring loose mode uRPF on the external interface and ensuring the RPF check fails by inserting a route to the source with a next hop of Null0. This can be done by using a trigger device to send IBGP updates. These updates set the next hop for the source IP to an unused IP address that has a static entry at the edge, setting it to null as shown in Figure 2. 
  

Source-Based RTBH Filtering 
With destination-based black holing, all traffic to a specific destination is dropped after the black hole has been activated, regardless of where it is coming from. Obviously, this could include legitimate traffic destined for the target. Source-based black holes provide the ability to drop traffic at the network edge based on a specific source address or range of source addresses. If the source address (or range of addresses) of the attack can be identified (spoofed or not), it would be better to drop all traffic at the edge based on the source address, regardless of the destination address. This would permit legitimate traffic from other sources to reach the target. Implementation of source-based black hole filtering depends on Unicast Reverse Path Forwarding (uRPF), most often loose mode uRPF. Loose mode uRPF checks the packet and forwards it if 
there is a route entry for the source IP of the incoming packet in the router forwarding information base (FIB). If the router does not have an FIB entry for the source IP address, or if the entry points to a null interface, the Reverse Path Forwarding (RPF) check fails and the packet is dropped, as shown in Figure 2. Because uRPF validates a source IP address against its FIB entry, dropping traffic from specific source addresses is accomplished by configuring loose mode uRPF on the external interface and ensuring the RPF check fails by inserting a route to the source with a next hop of Null0. This can be done by using a trigger device to send IBGP updates. These updates set the next hop for the source IP to an unused IP address that has a static entry at the edge, setting it to null as shown in Figure 2. 
  

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a008010a28a.s html

http://packetlife.net/blog/2009/nov/23/understanding-bgp-ttl-security/