Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 196 Limitations of IGRP (continued) Router B’s routing table will then look like: RouterB# show ip route Gateway of last resort is not set C C I 192.168.123.0 is directly connected, Serial0 192.168.111.0 is directly connected, Serial1 10.0.0.0 [120/1] via 192.168.123.1, 00:00:00, Serial0 [120/1] via 192.168.111.2, 00:00:00, Serial1 That’s right, Router B now has two equal metric routes to get to the summarized 10.0.0.0 network, one through Router A and the other through Router C. Router B will now load balance all traffic to any 10.x.x.x network between routers A and C. Suffice to say, this is not a good thing. ☺ It gets better. Router B then tries to send routing updates to Router A and Router C, including the summary route of 10.0.0.0/8. Router A’s routing table looks like: RouterA# show ip route Gateway of last resort is not set C C 192.168.123.0 is directly connected, Serial0 10.0.0.0/16 is subnetted, 1 subnet 10.1.0.0 is directly connected, Ethernet0 Router A will receive the summarized 10.0.0.0/8 route from Router B, and will reject it. This is because it already has the summary network of 10.0.0.0 in its routing table, and it’s directly connected. Router C will respond exactly the same, and the 10.1.0.0/16 and 10.2.0.0/16 networks will never be able to communicate. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 197 IGRP Timers IGRP has four basic timers: Update Timer (default 90 seconds) – indicates how often the router will send out a routing table update. Invalid Timer (default 270 seconds) – indicates how long a route will remain in a routing table before being marked as invalid, if no new updates are heard about this route. The invalid timer will be reset if an update is received for that particular route before the timer expires. A route marked as invalid is not immediately removed from the routing table. Instead, the route is marked (and advertised) with a metric of 101 (remember, 100 maximum hops is default), indicating it is unreachable, and placed in a hold-down state. Hold-down Timer (default 280 seconds) – indicates how long IGRP will “suppress” a route that it has placed in a hold-down state. IGRP will not accept any new updates for routes in a hold-down state, until the hold-down timer expires. A route will enter a hold-down state for one of three reasons: • The invalid timer has expired. • An update has been received from another router, marking that route with a metric of 101 (unreachable). • An update has been received from another router, marking that route with a higher metric than what is currently in the routing table (this is to prevent loops). Flush Timer (default 630 seconds) – indicates how long a route can remain in a routing table before being flushed, if no new updates are heard about this route. The flush timer runs concurrently with the invalid timer, and thus will flush out a route 360 seconds after it has been marked invalid. IGRP timers must be identical on all routers on the IGRP network, otherwise massive instability will occur. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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Cisco CCNA Study Guide - Router Alley

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