Cisco® CCNP Routing Exam Notes : Multi Homing, Bgp Load Balancing, Is-is Lsps:, Features Common To Ospf And Is-is

(Exam: 642-801)

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51. The three multi-homing classifications are:

  • Basic: Here the ISP will offer only the default route to the AS. This kind of connection is least processor intensive and recommended for simple networks with only one ISP connection.
  • Medium: This uses default routes and BGP. Internal AS can select the best ISP to use depending on the preferences.
  • Full: Full multi-homing uses only BGP. Here the routes are learned using the AS_PATH attribute information to make routing decisions.

52. BGP can load balance up to six links. You can have up to six links to ISPs and use those links for Internet traffic. This arrangement provides redundancy as well as load balancing.

53. BGP version 4 supports CIDR (Classless InterDomain Routing).

54. In BGP, the term Multihoming is used when one AS is connected to two or more ASP. The purpose of multihoming is:

  • To improve the reliability of connectivity to the Internet, that even if one connection fails, the other connection will be available.
  • To share the traffic load, resulting in the performance improvement.

55. While selecting best route in BGP, the order of preferences are as below:

  • Weight - If multiple routes exist, the route with the highest weight is preferred.
  • Local preference - If multiple routes have the same weight, the route with the highest local preference is preferred.
  • Local router - If multiple routes have same local preference, prefer the route originated by the local router.
  • AS path - If multiple routes have the same local preference, prefer the route with shortest AS path.

56. On an OSPF network, when a packet need to traverse from one area to another area to reach its destination, it is routed as below:

Source Area -> Source ABR -> Backbone Area -> Destination ABR -> Destination Area Routers

57. IS-IS:

1. Some of the OSI terms that are used in OSI routing environment are given below:

ES: End System, refers to any node that does not take part in the routing process, such as a work station.
IS: Intermediate System, refers to any network node that takes part in the routing process.
IS-IS: Intermediate System-to-Intermediate System, a routing protocol defined for OSI environment.
CSNP: ConnectionLess Network Protocol, protocol used by IS-IS for routing in OSI environment.

2. The following are the notable features of IS-IS ( Intermediate System to Intermediate System) routing protocol:

1. IS-IS routing protocol is a link state protocol.

2. IS-IS uses different types of Hello packets to form adjacencies.

3. IS-IS protocol can be used in dual (IP and OSI) environment. The term given for IS-IS implementation for mixed environment is Integrated IS-IS or Dual IS-IS.

4. IS-IS has the following metrics:

Cost metric - This is the only required metric.
The optional metrics are Delay, Expense, and Error.

IS-IS uses a single default metric with a maximum path value of 1024. The metric is typically assigned by a network administrator. Any single link value can be up to a maximum of 64.

Cisco IOS running IS-IS supports load balancing up to six equal-cost paths.

58. IS-IS LSPs:

Most routers participating in IS-IS flood LSPs (Link State Packets) to adjacent neighbors, except for the interface on which the LSP was received.

Note that in a shared media like LAN, a DIS (Designated Intermediate System) is elected, and DIS floods the media with LSPs.

The LSPs are used for constructing link state database. The LSPs have a life time of 20 minutes. The LSPs are refreshed by the originator periodically. LSPs maintain a checksum and a sequence number.

59. The following are the features common to both OSPF and IS-IS:

1. SPF (Shortest Path First) algorithm is used by both OSPF and IS-IS for computing shortest route to the destination.

2. They both elect designated router in multi access environment. The term used for designated router in IS-IS environment is Designated Intermediate System (DIS). However, the DIS election in IS-IS is pre-emptive. If a new router boots on the LAN with a higher priority, it becomes the DIS replacing the old DIS.

3. Both OSPF and IS-IS have authentication capability.

4. Both use Hello packets to establish adjacencies.

5. Both use SPF (Shortest Path First) algorithm to compute the shortest path to the destination.

6. Both use Areas, however there is no ABR (Area Border Router) in IS-IS. The L2 router is analogous to ABR.

Some differences between OSPF and IS-IS are:

OSPF uses IP whereas IS-IS uses CLNS in pure OSI environment.

IS-IS uses hierarchical architecture with two level (L1,L2) hierarchy, where as OSPF uses Area 0 with ABRs. Contiguous L2 or L1/L2 routers (ISes) form the backbone in IS-IS environment.

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