Category Archives: 3.7.h Implement and troubleshoot other features

3.7.h Implement and troubleshoot other features

 3.7.h [ii] BGP synchronization

 

Synchronization in BGP is a tough concept at first. That’s why they love to teach it even though it’s off by default.

The rule of Synchronization:

The BGP synchronization rule states that if an AS provides transit service to another AS, BGP should not advertise a route until all of the routers within the AS have learned about the route via an IGP.

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3.7.h Implement and troubleshoot other features

3.7.h [i] Multipath

The BGP Multipath Load Sharing for eBGP and iBGP feature allows you to configure multipath load balancing with both external BGP (eBGP) and internal BGP (iBGP) paths in Border Gateway Protocol (BGP) networks that are configured to use Multiprotocol Label Switching (MPLS) Virtual Private Networks (VPNs). This feature provides improved load balancing deployment and service offering capabilities and is useful for multi-homed autonomous systems and Provider Edge (PE) routers that import both eBGP and iBGP paths from multi-homed and stub networks.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4115-4118).  . Kindle Edition.

 http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3s/irg-xe-3s-book/irg-multi-load.html

 

3.7.h Implement and troubleshoot other features

3.7.h [ii] BGP synchronization

If your AS passes traffic from another AS to a third AS, BGP should not advertise a route before all routers in your AS learn about the route via IGP. BGP waits until IGP propagates the route within the AS and then advertises it to external peers. A BGP router with synchronization enabled does not install iBGP learned routes into its routing table if it is not able to validate those routes in its IGP. Issue the no synchronization command under router bgp in order to disable synchronization . This prevents BGP from validating iBGP routes in IGP.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4120-4124).  . Kindle Edition.

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/5816-bgpfaq-5816.html

 

 

3.7.h Implement and troubleshoot other features

3.7.h [iii] Soft reconfiguration, route refresh

When the routing policy of a BGP neighbor changes, the session must be reset (cleared) for the changes to take effect. Because resetting a BGP session can be disruptive to networks, a soft reset method is recommended for reconfiguring the routing table.

Previously, in order to reconfigure the inbound routing table, both the local BGP router and the BGP peer first needed to be configured to store incoming routing policy updates using the neighbor soft-reconfiguration command. Additional resources, particularly memory, were required to store the inbound routing table updates. The clear ip bgp command could then initiate the soft reset, which generated a new set of inbound routing table updates using the stored information.

This feature provides an additional method for soft reset that allows the dynamic exchange of route refresh requests and routing information between BGP routers, and the subsequent re-advertisement of the respective outbound routing table. Soft reset using the route refresh capability does not require pre-configuration and consumes no additional memory resources.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4133-4135).  . Kindle Edition.

http://ccieblog.co.uk/bgp/route-refresh-capability-vs-soft-reconfiguration

3.7.h Implement and troubleshoot other features

3.7.h [ii] BGP synchronization

this is arguably the best definition i have yet read concerning sync in bgp, until the very end..

ccie quick ref guide pg 83

For an iBGP route to be added to the BGP table, the exact prefix must be in the routing table from an IGP.
The synchronization rule is a method that guarantees that a route is known to all routers within the AS even if they are not running BGP. If a route is advertised via iBGP and a non-BGP router sits logically between the BGP peers, the non-BGP router will black hole the traffic because the destination is not known via IGP first. The synchronization check can be turned off (and is by default as of IOS version 12.2(8)T) with the router configuration command:

Router(config-router)# no synchronization
If disabled, it must be guaranteed that a routing black hole exists within the AS by creating a full-mesh iBGP network or using a BGP tool such as route reflectors or confederations.

read that last sentence again… the opposite, i believe is true… if synchronization is to be turned off, it must be guaranteed that a routing black hole DOESN’T exist…