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BGP Basics

BGP Attributes

Route Reflector

IGMP Multipath Load Shating

Cisco Q&A





• BGP route selection algorithm

•Weight - Cisco Specific - Not advertised to neighbor routers - Two links to one router both advertising the same network - Higher weight is applied to one and installed into the IP routing table

•Local preference - Two routers receiving updates from a neighbor for the same network - the higher local preference value wins

•Multi-exit discriminator - (MED) suggestion to an external AS for the preferred route - Lower value metric wins - MED's are advertised throughout the local AS

•Origin - How did BGP learn about the route - Three Possible ways -

1.IGP Interior to the originating AS (IGP) Value set when the command was used to inject the route into BGP
2. EGP The route was learned via an Exterior Border Gateway Protocol (EGBP)
3. Incomplete The origin of the route is unknown or learned in some other way. Redistributed into BGP

•AS_path - Shorter AS path wins - updates with the router's own AS are rejected

•Next hop - The IP address that is used to reach the advertising router. For EBGP peers the IP is the connection between the peers - in IBGP the EBGP next hop address is preserved in the local AS.

•Community - Route maps are used to group destinations into a community - Predefined communities are:
no-export - do not advertise this route to EBGP peers
no-advertise - do not advertise this route to any peer
internet - advertise this route to the internet all routers in the network belong to is

• BGP Valid AS Numbers
1 to 64,511
64,512 to 65,535 are reserved fr private use

• In order for two BGP peers to form a BGP session they must first have an active TCP session and be able to reach one another over TCP port 179.

• Alias Commands:
alias exec cib clear ip bgp *
alias sb show ip bgp summary


• Show Commands:
show proc cpu | include BGP
show ip bgp (*> noting best reachable route)
show ip bgp | exclude [AS#]
show run | begin router bgp
show ip bgp neighbors