Download Cisco.NewQuestions.642-885.2015-07-21.1e.86q.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/

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

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

http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.7/routing/configuration/guide/rc37bgp.html# wp1191371
BGP Routing Domain Confederation 
One way to reduce the iBGP mesh is to divide an autonomous system into multiple subautonomous systems and group them into a single confederation. To the outside world, the confederation looks like a single autonomous system. Each autonomous system is fully meshed within itself and has a few connections to other autonomous systems in the same confederation. Although the peers in different autonomous systems have eBGP sessions, they exchange routing information as if they were iBGP peers. Specifically, the next hop, MED, and local preference information is preserved. This feature allows you to retain a single IGP for all of the autonomous systems.

Cisco IOS XR allows you to control the configuration of the number of distributed speakers and enables you to selectively assign neighbors to specific speakers. 
On the CRS-1 platform, multiple speaker processes up to 15 may be configured. However, configuring all the different speakers on the primary route processor simply adds to the load on the single RP. 
Distributed speaker functionality is useful if Distributed Route Processor (DRP) hardware is available to take advantage of process placement. Later sections in this chapter depict distributed BGP and placement of BGP process speakers on DRPs on a CRS-1 router. 
In addition to the speaker process, BPM starts the bRIB process once BGP is configured. bRIB process is responsible for performing the best-path calculation based on partial best paths received from the speaker processes. The best route is installed into the bRIB and is advertised back to all speakers. The bRIB process is also responsible for installing routes

Reverse Path Forwarding (RPF) 
RPF is a fundamental concept in multicast routing that enables routers to correctly forward multicast traffic down the distribution tree. RPF makes use of the existing unicast routing table to determine the upstream and downstream neighbors. A router will only forward a multicast packet if it is received on the upstream interface. 
This RPF check helps to guarantee that the distribution tree will be loop free. 
RPF Check 
When a multicast packet arrives at a router, the router will perform an RPF check on the packet. If the RPF check is successful, the packet will be forwarded. 
Otherwise it will be dropped. 
For traffic flowing down a source tree, the RPF check procedure works as follows:
Step 1. Router looks up the source address in the unicast routing table to determine if it has arrived on the interface that is on the reverse path back to the source. 
Step 2. If packet has arrived on the interface leading back to the source, the RPF check is successful and the packet will be forwarded. 
Step 3. If the RPF check in 2 fails, the packet is dropped.

http://prakashkalsaria.wordpress.com/2010/08/11/mbgp-msdp/
MSDP In the PIM-SM model, multicast sources and receivers must register with their local RP. Actually, the router closest to the sources or receivers registers with the RP, but the key point to note is that the RP knows about all the sources and receivers for any particular group. RPs in other domains have no way of knowing about sources located in other domains. MSDP is an elegant way to solve this problem. 
MSDP is a mechanism that allows RPs to share information about active sources. RPs know about the receivers in their local domain. When RPs in remote domains hear about the active sources, they can pass on that information to their local receivers and multicast data can then be forwarded between the domains. A useful feature of MSDP is that it allows each domain to maintain an independent RP that does not rely on other domains, but it does enable RPs to forward traffic between domains. PIM-SM is used to forward the traffic between the multicast domains. 
The RP in each domain establishes an MSDP peering session using a TCP connection with the RPs in other domains or with border routers leading to the other domains. When the RP learns about a new multicast source within its own domain (through the normal PIM register mechanism), the RP encapsulates the first data packet in a Source-Active (SA) message and sends the SA to all MSDP peers. The SA is forwarded by each receiving peer using a modified RPF check, until the SA reaches every MSDP router in the interconnected networks--theoretically the entire multicast internet. If the receiving MSDP peer is an RP, and the RP has a (*, G) entry for the group in the SA (there is an interested receiver), the RP creates (S, G) state for the source and joins to the shortest path tree for the source. 
The encapsulated data is decapsulated and forwarded down the shared tree of that RP. When the packet is received by the last hop router of the receiver, the last hop router also may join the shortest path tree to the source. The MSDP speaker periodically sends SAs that include all sources within the own domain of the RP http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.2/routing/configuration/guide/rc32bgp.html
Multiprotocol BGP 
Multiprotocol BGP is an enhanced BGP that carries routing information for multiple network layer protocols and IP multicast routes. BGP carries two sets of routes, one set for unicast routing and one set for multicast routing. 
The routes associated with multicast routing are used by the Protocol Independent Multicast (PIM) feature to build data distribution trees. 
Multiprotocol BGP is useful when you want a link dedicated to multicast traffic, perhaps to limit which resources are used for which traffic. Multiprotocol BGP allows you to have a unicast routing topology different from a multicast routing topology providing more control over your network and resources. 
In BGP, the only way to perform interdomain multicast routing was to use the BGP infrastructure that was in place for unicast routing. Perhaps you want all multicast traffic exchanged at one network access point (NAP). 
If those routers were not multicast capable, or there were differing policies for which you wanted multicast traffic to flow, multicast routing could not be supported without multiprotocol BGP. Note It is possible to configure BGP peers that exchange both unicast and multicast network layer reachability information (NLRI), but you cannot connect multiprotocol BGP clouds with a BGP cloud. That is, you cannot redistribute multiprotocol BGP routes into BGP.