Another cool little lab showcasing a new/redesigned functionality that apparently was introduced in IOS15.x that was new to me. The lab has the following requirements:
- Configure EIGRP Classic on R4, R5 & R8 with AS100, only enable it on the .45.0 & .58.0 segments
- Configure loopbacks on R4 & R5 with subnet 160.1.x.x/24 (x - router id) and advertise them
- Advertise a summary default-route from R4
- Advertise a summary route of the two 16.0.1.x.x/24 routes without overlap on R5 to R8
- Modify the summary route of R5 so a null0 route isn’t installed
All basic steps until the last one so no explanation needed I feel. Keeping up with writing the full config in notepad only is starting to pay of as well, improving both speed and overall knowledge of syntax. Highly recommended! :)
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! R4
interface Lo1
ip add 160.1.4.4 255.255.255.0
router eigrp 100
network 155.1.45.0 0.0.0.255
network 160.1.4.0 0.0.0.255
interface Gi1.45
ip summary-address eigrp 100 0.0.0.0 0.0.0.0
! R5
interface Lo1
ip add 160.1.5.5 255.255.255.0
router eigrp 100
network 155.1.45.0 0.0.0.255
network 155.1.58.0 0.0.0.255
network 160.1.5.0 0.0.0.255
interface Gi1.58
ip summary-address eigrp 100 160.1.4.0 255.255.254.0
! R8
router eigrp 100
network 155.1.58.0 0.0.0.255
Checking the routes in R8 confirms that we “should” have connectivity to the two loopbacks.
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R8#sh ip route eigrp | beg Gate
Gateway of last resort is 155.1.58.5 to network 0.0.0.0
**D* 0.0.0.0/0 [90/3328] via 155.1.58.5, 00:18:58, GigabitEthernet1.58**
155.1.0.0/16 is variably subnetted, 7 subnets, 2 masks
D 155.1.45.0/24 [90/3072] via 155.1.58.5, 00:18:58, GigabitEthernet1.58
160.1.0.0/23 is subnetted, 1 subnets
**D 160.1.4.0 [90/130816] via 155.1.58.5, 00:00:43, GigabitEthernet1.58**
But ping is still failing to R4:
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R8#ping 160.1.4.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 160.1.4.4, timeout is 2 seconds:
U.U.U
Success rate is 0 percent (0/5)
R8#ping 160.1.5.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 160.1.5.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/5/6 ms
How come? All summary-routes created in ex. EIGRP & OSPF will also add a Null0-route to avoid routing-loops! As we only have a default-route to R4 longest match will hit our null0-route instead for 160.1.4.4.
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R5#sh ip route eigrp | beg Gate
Gateway of last resort is 155.1.45.4 to network 0.0.0.0
D* 0.0.0.0/0 [90/3072] via 155.1.45.4, 00:22:19, GigabitEthernet1.45
160.1.0.0/16 is variably subnetted, 3 subnets, 3 masks
**D 160.1.4.0/23 is a summary, 00:04:05, Null0**
R5#sh ip cef 160.1.4.0
160.1.4.0/23
attached to Null0
So how do we fix this? As mentioned a new feature was added to EIGRP in 15.x (I think in previous versions something similar was done on the interface instead). We can adjust the AD of our summary-route, and by setting it to 255 it will be deemed invalid (poisoned) and not installed in our routing table.
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! R5
router eigrp 100
summary 160.1.4.0/23 distance 255
The summary-route is still active in a way as our router will keep suppressing the advertisements of 160.1.5.0/24 and just forward the default-route instead.
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R5#sh ip route eigrp | beg Gate
Gateway of last resort is 155.1.45.4 to network 0.0.0.0
D* 0.0.0.0/0 [90/3072] via 155.1.45.4, 00:26:02, GigabitEthernet1.45
We can now reach both R4 & R5’s loopback from R8.
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R8#ping 160.1.4.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 160.1.4.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 5/6/7 ms
R8#ping 160.1.5.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 160.1.5.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 5/5/6 ms