CCNP Exam Prep Tips and Must Knows

students765003

Things You’ll Need to Know about BGP & Path Control Tools:

Offset List

  • Offset-list command adds an offset to incoming and outgoing metrics to routes learned via EIGRP or RIP
  • The offset value is added to the routing metric
  • R2(config-route-map)# offset-list 21 in 2 serial 0/0
  • The router applies an offset of 2 to routes learned from the S0/0 interface with the match on ACL 21

IP SLAs (Service Level Agreements)

  • Perform network performance measurement
  • Configuration: 
    • Define one or more probes
    • Define one or more tracking objects
    • Define the action on the tracking object

Policy Based Routing (PBR)

  • Allows policies to be implemented that selectively cause packets to take different paths
    • IP routing is destination-based
    • PBR avoids destination-based routing
  • Applied to incoming packets
  • Requires a route map to implement the policy

Enterprise to ISP Connection

  • Single –Homed ISP Connection – Option 1
    • Customer uses default route to ISP (with single ISP)
    • Service Provider uses static route(s) for customer public network
    • No automatic adjustment to any changes in network
  • Single-Homed ISP Connection – Option 2
    • Customer deploys BGP to announce its public network
    • ISP announces a default route, a subset of Internet routes or a complete Internet routing table
  • Dual-Homed ISP Connections
    • Connected with two links to the same ISP
    • Can use a single router or two edge routers
    • Can use static routes or BGP
  • Multihomed ISP Connectivity
    • Connected to two or more different ISPs
    • Can use single router or multiple edge routers
    • Default route from all providers
    • Default route and partial internet routing from providers
    • Dynamic routing with BGP
    • Dual Multihomed ISP Connections
    • Connected to two or more different ISPs with two links per ISP
    • Typically uses multiple edge routers (one per ISP)
    • Dynamic routing with BGP

BGP

  • IBGP – Internal BGP –  intra-AS peering
  • EBGP – External BGP – inter-AS peering
  • BGP does not consider speed to determine the best path
  • BGP is policy-based routing protocol
  • BGP allows an AS to control traffic flow using multiple BGP path attributes
  • BGP allows a provider to use all paths by manipulating path attributes
  • BGP uses TCP port 179 to establish a session

When to Use BGP

  • If AS is a transit AS
  • An AS is Multihomed
  • Inter-AS routing policy must be manipulated
  • Sufficient memory and processor resources are necessary to handle BGP routing

BGP Message Types

  • Open Message – contains the following information:
    • BGP version number
    • AS number
    • Hold time
    • BGP router ID
    • Optional parameters (ex: authentication)                      
  • Keepalive Message
    • Exchanged between BGP peers often enough to keep the hold timer from expiring 
    • Keepalive message consists of only a message header
  • Update Message                       
    • A BGP update message has information on one path only
    • Update message can include the following fields:
      • Withdrawn Route
      • Path attributes
      • Network layer reachability information
  • Notification Message
    • Sent when an error condition is detected
    • BGP session is closed immediately after this is sent

Requirements for EBGP Neighbors

  • Peers must be in different AS
  • Neighbors must be defined on both sides of the peering
  • Neighbor must be directly connected and IP addresses must be reachable

Requirements for IBGP Neighbors

  • Peers must be in the same AS
  • Neighbors must be defined on both sides of the peering
  • Neighbor need not be directly connected but IP address of peer must be reachable

BGP States

  • Idle: Router is searching the routing table to see whether a route exists to reach the neighbor
  • Connected: Router found a route to the neighbor and has completed the 3-way TCP handshake
  • Open Sent: Open message sent, with the parameters for the BGP session
  • Open Confirm: Router received an agreement on the parameters for establishing a session
  • Established: Peering is established; routing begins

BGP Path Selection

  1. Prefer highest weight (local) to router (Cisco proprietary)
  2. Prefer highest local preference (global within AS)
  3. Prefer route originated by the local router (next hop = 0.0.0.0)
  4. Prefer shortest AS path
  5. Prefer lowest origin code
  6. Prefer lowest MED
  7. Prefer an EBGP path over an IBGP path     
  8. Prefer the path through the closes IGP neighbor
  9. Prefer the oldest route for EBGP paths
  10. Prefer the path with the lowest neighbor BGP router ID
  11. Prefer the path with the lowest neighbor IP address

Some BGP Attributes

  • Well Known Mandatory Attributes – must be recognized by all BGP routers and must be present in any BGP update
    • AS path
    • Next hop
    • Origin code
  • Well Known Discretionary Attributes – must be recognized by all BGP routers but need not be present in BGP advertisements
    • Local Preference – only used in updates between IBGP peers
  • Optional non-transitive Attributes – Do not have to be recognized by all BGP routers and BGP peers can ignore the update and not advertise the path to its other peers    
    • Multi Exit Discriminator (MED) – To influence incoming traffic, the MED is used to inform EBGP peers of its preferred ingress point. 
In this article

Join the Conversation