CCIE Pursuit Blog

April 14, 2008

Question Of The Day: 11 April, 2008 Extended

Filed under: Cisco,IOS — cciepursuit @ 8:31 am
Tags: , , , , , ,

I’m extending this question because I forgot to mention that auto-summary is disabled for both the RIP (version 2) and EIGRP domains.  Thank you to Ron for pointing this out.  Also, major props to the folks who develop CCxx questions –  this is a lot more difficult to do than I thought that it would be.  🙂

In the scenario below r1 is receiving the 100.1.4.0/24 route from both protocols:

From RIP (int f0/0 shutdown):
r1#sh ip route 100.1.4.0
Routing entry for 100.1.4.0/24
  Known via “rip”, distance 90, metric 2
  Redistributing via rip
  Last update from 10.1.13.3 on FastEthernet1/0, 00:00:03 ago
  Routing Descriptor Blocks:
  * 10.1.13.3, from 10.1.13.3, 00:00:03 ago, via FastEthernet1/0
      Route metric is 2, traffic share count is 1

From EIGRP (int f1/0 shutdown):
r1#sh ip route 100.1.4.0
Routing entry for 100.1.4.0/24
  Known via “eigrp 100”, distance 90, metric 158720, type internal
  Redistributing via eigrp 100
  Last update from 10.1.12.2 on FastEthernet0/0, 00:00:16 ago
  Routing Descriptor Blocks:
  * 10.1.12.2, from 10.1.12.2, 00:00:16 ago, via FastEthernet0/0
      Route metric is 158720, traffic share count is 1
      Total delay is 5200 microseconds, minimum bandwidth is 100000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2

 

Topic: IP Routing

Refer to the network diagram below (click for larger version).

Question of The Day Network Diagram

The network engineer wants to load balance traffic from r1 to r4’s user network (100.1.4.0/24). He has altered the Administrative Distance of the RIP routes on r1 to 90 so that they match the Administrative Distance of the EIGRP routes.

r1#sh ip route | e 100.
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 4 subnets
C 10.1.13.0 is directly connected, FastEthernet1/0
C 10.1.12.0 is directly connected, FastEthernet0/0
D 10.1.24.0 [90/30720] via 10.1.12.2, 06:48:57, FastEthernet0/0
R 10.1.34.0 [90/1] via 10.1.13.3, 00:00:16, FastEthernet1/0

r1#sh run | sec router
router eigrp 100
  network 10.1.12.1 0.0.0.0
  no auto-summary
router rip
  version 2
  passive-interface default
  no passive-interface FastEthernet1/0
  network 10.0.0.0
  distance 90
  no auto-summary

r4#sh run | sec router
router eigrp 100
  network 10.1.24.4 0.0.0.0
  network 100.1.4.4 0.0.0.0
  no auto-summary
router rip
  version 2
  passive-interface default
  no passive-interface FastEthernet1/0
  network 10.0.0.0
  network 100.0.0.0
  no auto-summary

*The user network is being emulated by lo0 on r4.  It is being advertised by both EIGRP and RIP.  Auto-summary is disabled on all routers in both the RIP (version 2) and EIGRP domains.

Will packets from r1 to 100.1.4.0/24 be load-balanced between r2 and r3? Why?

Click Here For The Answer

Advertisements

3 Comments »

  1. I am still going to say no. The reason being is that the router will make several decisions on deciding which learned route should be placed in the routing table:

    1) The router looks at the prefix of the routes. Since both processes are receiving prefix 100.1.4.0/24, the router needs to use a different criteria to determine the best route.

    2) The router will then look at the administrative distance. In this case, the router is receiving 2 advertisements for 100.1.4.0/24 with an AD of 90. So we go to the last criteria:

    3) The router then looks at the metric. In this case the protocols themselves are using their default metrics. From the output above, the metric for the RIP route is 2 and the metric for the EIGRP learned route is 158720. Therefore, only the RIP route would be installed in the routing table. I do not believe the routers use “the routing protocol legend from the show ip route” command to determine which route is best. Therefore, it relies on the AD and since they are both the same, it has to use the metric even though they are from different routing processes.

    4) If the metric for both the learned routes were the same then the router would install both prefixes in the routing table and load balance between them.

    So final answer. No, because only the prefix for this destination received from RIP would be installed in the routing table due to the metric being smaller than EIGRP’s metric.

    Comment by Ron — April 14, 2008 @ 1:32 pm | Reply

  2. No. With all things being equal (prefix length, AD, etc.) the IOS will make its final decision to put a route into the RIB based on the default AD of each protocol which the route is being advertised. I nthis case, the IOS will install the EIGRP route and follow the path over f0/0.

    Comment by kevhat — April 15, 2008 @ 4:54 am | Reply

  3. […] Question Of The Day: 11 April, 2008  […]

    Pingback by Question Of The Day: 15 April, 2008 « CCIE Pursuit — April 15, 2008 @ 7:43 am | Reply


RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Blog at WordPress.com.

%d bloggers like this: