Fundamental BGP Concepts

The Basics of Internet Routing and Addressing

  • What are the private IPv4 addresses?
  • Which IP range is used when DHCP fails?

Introduction to BGP

  • Which port does BGP uses?
    • TCP 179
  • Could BGP form neighborship with a router outside of it’s subnets?
    • yes
  • What does BGP use to specify the best route?
    • instead of only metric, it uses Path Attributes which contains a variaty of information
  • What logic does BGP use (EIGRP – distance-vector, OSPF – link state) ?
    • path vector logic (similar to distance vector)
    • very scaleable but slow convergence and not always the best path is used
  • What is the two key functions of AS_PATH  attribute?
    • BGP chooses the best route for a prefix based on the shortest AS_PATH
    • preventing routing loops
  • What happens when a router receives an update with it’s own ASN in it?
    • as-path.PNGIt will ignore that advertisement.
  • When does the router changes the AS_PATH PA when sending an advertisement to its neighbor?
    • When it sends to an external neighbor. In this case the router adds its own AS to the AS_PATH.
  • What kind of BGP peers exist?
    • iBGP – internal BGP = neighbor is in the same AS
    • eBGP – external BGP = neighbor is in a different AS
  • Who manages the public AS numbers?
    • IANA
  • What are the public and private ASNs?
    • Public: 1 through 64,495
    • Private: 64,512 through 65,534

Outbound Routing Toward the Internet

  • When should you use default route at WAN edge and when BGP?
    • Use BGP when you have multiple internet connection or when you want to choose one outbound path over another for a particular destination.
  • What is Single-homed internet design?
    • There is a single link between the ISP and the customer.
      Default route recommended.
  • What is Dual-homed internet design?
    • There is 2 (or more ) links to the internet but with all links connecting to a single ISP.
      Default route recommended.
      You can configure EIGRP on E1 and E2 to advertise the default route with different metrics so you can choose where to leave the AS.
  • What kind of route updates do the ISP send (usually)?
    • default route only
    • full updates (the entire BGP table)
    • partial update ( The ISP sends only some prefixes that might be better reached through that a ISP plus default route)
  • What is a Single-multihomed topology?
    • You have a single connectino to the ISP but you have multiple ISPs.
  • What is a Dual-Multihomed topology?

External BGP for Enterprises

  • What are the requirements for forming BGP neighborship?
    • the local ASN must match the neighbor’s “neighbor remote-as asn-number”
    • the RID must be different
    • authentication must pass (if configured)
    • the local router must have a TCP connection to the neighbor
  • How does BGP choose RID?
    • same way as in EIGRP/OSPF
      • configured
      • highest loopback interface
      • highest other interface
  • How do you configure authentication in BGP?
    • neighbor IP  password key
  • Which source IP is used when creating TCP session to build BGP neighborship?
    • outgoing interface
  • What are the dangers of redundant L3 connection to the neighbor? Solution?
    • BGP uses the outgoing interface as source and when the interface goes down the TCP session (socket) goes down and BGP fails. Even if the neighbor is reachable via another interface.
    • neighbor IP update-source loopback-interface
      neighbor IP ebgp-multihop hops-number
  • What is eBGP multihop and why is it needed?
    • By design the BGP sends it’s packets with TTL=1 . When a router receives it and but need to forward the packet (ie.: from the incoming interface to the loopback interface) it decreases the TTL. When the router realizes that the descreased TTL=0 it drops the packet.
    • The solution is: neighbor IP ebgp-multihop hops-number
      This command sets the TTL number for the outgoing BGP packets.
  • What are the steps of neighbor forming?
    • BGP establish TCP connection (port 179)
    • BGP neighbors send BGP Open message  which contains BGP parameters
    • If the parameters match the neighbor relationship is formed and the neighbors switch to Established state
  • What kind of neighbor states exist?
    • Idle: BGP process is either admin down or waiting the next retry attempt
    • Connect: BGP process is waiting for the TCP connection to build
    • Active: TCP connection is build but no BGP messages have yet been sent to the peer
    • Opensent: The BGP Open message has been sent to the neighbor but we did not receive Open message yet
    • Openconfirm: BGP Open messages exchanged and now waiting for BGP Keepalive message (to confirm that all neighbor-related parameters match) or a BGP Notification message (to learn that there is some mismatch in neighbor parameters)
    • Established: The neighbor relationship works and the peers can exchange Update messages.
  • How do you confirm a TCP connection?
    • show tcp brief
  • What type of BGP messages exist?
    • Open: basic parameters for neighbor establishment
    • Keepalive
    • Update
    • Notification: Signals BGP error. Typcially results in a reset to the neighbor relationship.

Verifying the BGP Table


  • What does the “>” symbol means in show ip bgp?
    • the chosen best route
  • How can you see wheter a route learned iBGP or eBGP in the show ip bgp output?
    • iBGP: there is an “i” after the “*>” column
    • eBGP: there is no “i” after the “*>” column
  • What command do you know to check BGP table?
    • sho ip bgp  – default routes
    • sho ip bgp neighbors IP received– learned routes before filters applied
    • sho ip bgp neighbors IP routes – learned routes after filters applied
    • sho ip bgp summary – number of learned prefixes

Injecting Routes into BGP for Advertisement to the ISPs

  • What does the network command do in BGP? What is the difference between IGP network command?
    • BGP: The network command look for a route in the router’s current routing table that exactly matches the parameters of the network command (prefix/length). If there is a match it will put the route into the local BGP table.
    • OSPF/EIGRP: It will do the same as BGP network command but it also compares to all its interface IP addresses. If matched the router enables IGP on those interfaces.
  • What happens if you dont use mask in BGP network command?
    • The IOS assumes a classful network mask.
      network   = network mask
  • How does the default summarizon in BGP work?
    • by default auto-summary is disabled
  • What happens when auto-summary is enabled?
    • It changes the logic of the network command if mask parameter is omitted.
      The router will add a route for that classful network to the BGP table using these conditions:

      • If the exact classful route is in the IP routing table
      • If any subset routes of that classful network are in the routing table

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