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Revision Topics

  • Congestion Management: Queuing Algorithms

    There are many different types of queuing algorithms, though not all may be suitable in modern networks that can carry many different media types. FIFO – First In, First Out The first packet to be placed on an output interface is the first packet that will leave that interface. All traffic belongs to the same…

  • Policing and Shaping: Types of Policers

    Single-Rate Two-Colour Markers and Policers The first policers implemented use a single rate two colour model based on a single token bucket algorithm. For this type of policer, traffic can be either conforming to the committed information rate or exceeding it. Marking down traffic or dropping it can be performed to either of these two…

  • Policing and Shaping: Single Token Bucket Algorithm Example

    An example of how a single token bucket algorithm works: There is an interface of 1Gbps, with a policer defined with a committed information rate of 120Mbps and committed burst size of 12Mbps. A committed time interval can not be configured in Cisco IOS but can be calculated. Committed time interval = ( Committed Burst…

  • Reverse Path Forwarding (RPF)

    Reverse Path Forwarding is an algorithm that helps prevent loops and ensures that multicast traffic arrives on the correct interface. The RPF algorithm has several features: When a router receives a multicast packet on an interface that it uses to send unicast packets towards the source, that interface is a RPF interface. When a packet…

  • PIM Sparse Mode – Designated Routers

    Where there are multiple PIM-SM routers on a subnet, an election is called to determine a designated router (DR). The designated router helps prevent duplicates of multicast traffic from being sent to to the rendezvous point. In an election between PIM sparse mode routers, the highest priority wins the election. By default the priority of…

  • PIM Shortest Path Tree Switchover

    PIM sparse mode allows the last hop router to switch from the shared tree to a shortest path tree for a specific source. This is a default behaviour in Cisco routers and happens after the first multicast packet is received from the rendezvous point from the shared tree, even if the shortest path is through…

  • PIM Terminlogy

    Reverse Path Forwarding (RPF) Interface The reverse path forwarding interface is the interface with the lowest cost path to the IP address of the root of the shortest path tree (the source of the multicast stream). The lowest cost is based on the factors of administrative distance and metric. If there are multiple interfaces with…

  • PIM Distribution Trees

    A multicast router will create a distribution tree that will define the path multicast traffic follows to reach receiver devices. There are two types of multicast distribution trees known as source trees. They are also known as shortest path trees, SPTs and shared trees. Source Tree A source tree is a multicast distribution tree where…

  • Multicast Addressing

    The Internet Assigned Number Authority (IANA) assigned IP Class D space for multicast addressing – 224.0.0.0/4 This includes the range 224.0.0.0 to 239.255.255.255 Address Range Description 224.0.0.0 – 224.0.0.255 Local Network Control 224.0.1.0 – 224.0.1.255 Internetwork Control 224.0.2.0 – 224.0.255.255 Ad-hoc 1 224.1.0.0 – 224.1.255.255 Reserved 224.2.0.0 – 224.2.255.255 SDP/SAP 224.3.0.0 – 224.4.255.255 Ad-hoc 2…

  • BGP Path Attributes: Minimum Cluster List Length

    With a tie on the Router ID, it falls to the minimum cluster list length to try break the deadlock. The step uses the cluster list to locate the path that has travelled the least number of iBGP advertisement hops. The cluster list is a non-transitive BGP attribute that is appended by a router reflector…