Quick Guide Ip Routing


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Quick Guide Ip Routing

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Quick Guide Ip Routing

  1. 1. CCNA4.com Quick Guide - Ip Routing - Typically, in a large network, a combination of both static and dynamic routing is used. - A packet can also be dropped on the RETURN trip. Example -- if you ping a host and it doesn't answer, that doesn't necessarily mean the forward path is broken. - Static Routing -- an entry is added to the routing table of each router, for each remote network. - Static Routing Advantages -- no overhead on the router CPU, no bandwidth usage between routers, security. Primarily used in small networks (< id="fullpost"> - Default Routing -- can only be used on stub networks (only one exit port out of the network). - Before setting up a default route, remove any static routing entries -- "no ip route". - Set up a default route -- "ip route". Verify the changes -- "sh ip route" -- the default route shows as "S*". - ALWAYS remember to use "ip subnet-zero" and "ip classless" -- both are enabled by default in Cisco IOS 12.x. - Dynamic Routing -- happens automatically, but uses CPU and network resources. - Dynamic Routing Protocols within a network: - RIP -- Routing Information Protocol. - IGRP -- Interior Gateway Routing Protocol -- Cisco proprietary. - EIGRP -- Enhanced Interior Gateway Routing Protocol -- Cisco proprietary. - OSPF -- Open Shortest Path First -- non-proprietary. - Dynamic Routing Protocols across networks: - IGP -- Interior Gateway Protocol -- routing between routers in the same Autonomous System (AS). - EGP -- Exterior Gateway Protocol -- routing between different Autonomous Systems. BGP (Border Gateway Protocol) is an example of an EGP. - Administrative Distance -- a number from 0 to 255, where 0 is most reliable, 255 is blocked. - If two advertised routes for the same network have the same ADs, the router makes a decision by looking at hop count or bandwidth. - AD values: - 0 -- Connected interface. - 1 -- Static route. - 90 -- EIGRP. - 100 -- IGRP. - 110 -- OSPF. - 120 -- RIP. - 170 -- External EIGRP. - 255 -- Unknown -- this one will never be used. - Routing Protocol Types: - Distance Vector -- distance is measured in hops. Examples -- RIP, IGRP. - Link State -- also called Shortest Path First -- 3 tables per router -- 1 for directly attached neighbors, 1 for network topology, and 1 for routing. Examples -- OSPF. - Hybrid -- these use a combination of both methods -- EIGRP. - Distance Vector Routing Protocols -- they use "routing by rumor" -- exchange of routing tables. RIP looks at ADs first, then at hop count. If everything is equal, it performs round- robin load balancing for up to 6 equal cost links. - Pinhole Congestion -- with RIP routing, if a 56K link has less hops than a T1 link, the 56K link will be used -- this is bad, and happens because hop count is the only metric used with RIP routing. - Slow Convergence is another problem of RIP. While the routers are converging CCNA4.com
  2. 2. CCNA4.com (synchronizing their routing tables), no data is passed. - RIP Routing table fields -- network number, exit interface, and hop count. - Routing loops are also a common problem in RIP. They result from the slow convergence of RIP. - Maximum Hop Count -- RIP has this set to 15. After that, a packet is dropped. Maximum Hop Count is a good feature to decrease the severe effects of routing loops. - Split Horizon -- information cannot be sent back in the direction from which it was received. This method is able to prevent routing loops. - Route Poisoning -- when a network becomes unreachable, the first directly attached router places a "16" entry (unreachable) for this network in its routing table, and then advertises it to all other routers. They reply with a "poison reverse" (acknowledgement). - Holddowns -- these prevent regular update messages from a flapping network, router, or interface. Thus, the flow of information continues. - Holddown behavior: - Holddowns have a timer. When it expires, the link is reinstated. - If another update is received, with a better metric, the link is reinstated. If the metric is the same, nothing happens. - If a flush timer removes the bad route from the routing table (if it happens to expire -- coincidence), the link is reinstated. - RIP is a true distance-vector routing protocol. It sends the complete routing table to all active interfaces every 30 seconds. - RIP Version 1 uses only classful routing. RIP Version 2 provides prefix routing (classless routing) -- no subnet mask is sent with the updates. - RIP Timers: - Route update timer -- how often to send out updates -- default is 30 seconds. - Route invalid timer -- when there are no updates for a specific route over a time period (default is 90 seconds), the route is advertised as invalid. - Route flush timer -- how long after a route becomes invalid before it is removed from the routing table -- default is 240 seconds. - Configure RIP routing -- Make sure there are no static routes, as they take precedence. Then -- "config t", "router rip", "network" -- "network" tells the router which network to advertise. - RIP is configured with classful routing network addresses -- ALL subnet masks must be the same on all devices on the network. - "sh ip route" displays something like this -- "R [120/3] via, FastEthernet0/0" -- "[120/3]" is the AD and the hop count. - Blocking RIP advertisements after a certain point of the network -- "config t", "router rip", "network", "passive-interface serial 0" -- serial 0 will stop advertising, but will still receive updates. - IGRP -- Cisco proprietary, maximum hop count of 255 with default of 100, helpful in larger networks. IGRP uses bandwidth and delay of the line as metrics -- this combination is called a composite metric. - IGRP can also use other metrics, but they are not used by default -- reliability, load, and MTU. - IGRP Timers: - Update timer -- how frequently routing-update messages should be sent -- default is 90 seconds. - Invalid timer -- how long a router should wait before declaring a route invalid -- default is 3 x update timer. - Holddown timer -- specifies the holddown period -- default is 3 x update timer + 10 seconds. - Flush timer -- how long before a route is flushed from the routing table -- default is 7 x update timer. CCNA4.com
  3. 3. CCNA4.com - Configure IGRP -- "config t", "router igrp 10", "network" -- "10" is the Autonomous System (AS). All routers must be in the same AS in order to communicate. - You must ALWAYS use a classful network number when configuring IGRP. Example -- if you type "", the router will change it to "". Still, DO NOT type anything like this. - IGRP can load balance up to 6 unequal links (while with RIP, they must be equal). The "variance" command controls the load balancing between the best and the worst metric. - If both RIP and IGRP are enabled on a router, it will always use IGRP, as IGRP has higher precedence. Therefore, when using IGRP, disable RIP in order to spare resources. - Commands to troubleshoot routing: - "show ip route" -- displays the routing table. - "show protocols" -- displays hardware information and link status. - "show ip protocols" -- lots of routing information, including various parameters. - "debug ip rip" -- sends debugging messages to the console. Can be redirected to the terminal via "terminal monitor". Disable with "undebug all". - "debug ip igrp events" -- debug summary of IGRP. Disable with "undebug" or "undebug all". - "debug ip igrp transactions" -- full debug of IGRP. Again, disable with "undebug all". By badboy CCNA4.com