BGP

• Border Gateway Protocolv4
• Dynamic routing protocol
• De facto Internet routing protocol

How it works

• Neighbors peer in that one neighbor, or peer, opens a TCP connection to the other
• Routing information is transferred across that connection

Common confusion

• BGP transfers routing information
• BGP does not in itself “route”
• A routes takes in BGP information forms a BGP
• A router also knows its connected and static routes
• A router compares all these tables and forms a Routing Information Base (RIB) and possibly a Forwarding Information Base (FIB)
• The RIB or FIB (if it exists) is what a router uses to route packets

How is BGP used in the PTD network

• Load-sharing to and from our providers by implementaiton of policies
  • Any change to the default behavior of BGP is accomplished with routing policies
  • Policies applied to OUTBOUND advertisements to peers affect INBOUND traffic from our peers
  • Policies applied to inbound advertisements from our peers affect OUTBOUND traffic to our peers
• Spoken internally to distribute traffic laod
• Spoken with multihomed customers

Basic Config

See Introduction to BGP slides 8-13

Basic Troubleshooting

• Needed in two scenarios
  • A BGP session is down
  • Routing to the internet is broken
• Commands
  • show bgp summary - gives list of neighbors, their ASNs, states of the sessions, and number of prefixes received
  • State of active means the session is actually down
  • show bgp neighbor gives details on a particular session
• Looking for misconfigs
  • Is the peer IP address correct?
  • Is the peer ASN correct?
  • Is the peer IP reachable? Can you ping it?
  • If EGBP, is the IP more than one hop away?
• If routing to the internet is broken
  • see if there is a route from our netowrk to the destination
    • show ip bgp
    • show ip route
  • see if there is a route from somewhere on the Internet back to the source that
    • there are many route-servers availabe
    • show ip bgp
• Traceroutes from our network and from a route-server will help pinpoint a problem
• Work with the problem AS for possible routing problems or route filter problems