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2 IPv4

ARP 101

1. ARP ARP is a protocol used on broadcast networks such as Ethernet, Token Ring or FDDI that is used to map L3 Addresses (like IP) to layer 2 Addresses (like Ethernet MAC). When a host needs to send traffic to another host, it knows its Layer 3 address, but …

DNS 101

1. Defining hosts and domains locally To define a static host name to address mapping, use the following command: For hosts that are accessed without a domain name at the end, (hostname.domain-name), you can define a default domain name or a list of domain names to be used, using one …

Route Redistribution

1. Route Redistribution You can redistribute routes from one routing process to another using the redistribute command inside the destination routing process: When a routing protocol starts, it automatically redistributes connected routes that are matched by the network command. This also happens for static routes that point to an interface …

BGP Attributes

1. BGP Data Structures 1.1 Neighbor Table The address in the bgp summary table shows the IP used in the peering, not the Router ID. 1.2 BGP Table Lists all prefixes learned from all peers If no routes towards a destination show the “>” code, you should investigate why no …

BGP 101

1. Starting the Routing Process 1.1 Define the routing process Only one BGP process can run on a router and it can be started using: The AS Number used to be a 16 bits number ranging from 0 to 65535. According to RFC 4893, the AS number can have 32 …

DHCP 101

1. DHCP Server 1.1 DHCP Pools On a router, you have to create one or more pools of DHCP addresses available for lease. When a DHCP server receives a DHCP request, it will know what pool to use based on the IP address of interface that received it. If the …

FHRP 101

1. HSRP HSRP provides a virtual MAC address and a virtual IP address that is shared among a group of routers in order to have a HA infrastructure for the default gateway in a subnet. 1.1 Starting HSRP 1.2 Timers Timers are usually learned from the active router. Millisecond timers …

OSPF Mechanics

1. OSPF Router ID Each router selects an OSPF Router ID when the OSPF process starts. The Router ID is a 32 bit number, usually written in dotted decimal format. The selection process is: Manually configured Router ID Highest Loopback IP address Highest non-Loopback “up/up” IP address The interface used …

OSPF LSAs

1. LSA components Sequence number InitialSequenceNumber = 0x80000001 MaxSequenceNumber= 0x7FFFFFFF When MaxSequnceNumber is reached, and a new update must be sent, the LSA is flushed by setting the Age to MaxAge and reflooding it over all adjacencies. Then, a new version of the LSA is sent with sequence number set …

OSPF Areas

Areas are identified by a 32 bit Area ID. This can be represented as a number in decimal or in dotted decimal format. Area 0 (0.0.0.0) is reserved for backbone. The backbone is responsible for summarizing the topologies of each area to every other area => all inter-area traffic must …