Course:
CCNP ENARSI Labs
Summarizing prefixes conserves router resources and accelerates best-path calculation by reducing the size of the table. Summarization can be configured either for prefixes originated by the AS or prefixes received from downstream providers. Summarization also provides the benefits of stability by hiding flapping routes or having to install new prefixes when they are contained within a summary.
Although AS 1000 only has two prefixes 192.168.1.0/27 and 192.168.1.64/26, this customer has been allocated the entire 192.168.1.0/24 prefix. R3 in AS 300 has two prefixes 192.168.3.0/27 and 192.168.3.64/26 but has been allocated the entire 192.168.3.0/24 prefix.
Configure R1 and R3 to advertise a summary or aggregate route using the aggregate-address command. The summary-only option suppresses the specific prefixes that are summarized from also being advertised. Notice that this command is configured in address-family ipv4 configuration mode on R3.
R1(config)# router bgp 1000 R1(config-router)# aggregate-address 192.168.1.0 255.255.255.0 summary-only |
R3(config)# router bgp 300 R3(config-router)# address-family ipv4 R3(config-router-af)# aggregate-address 192.168.3.0 255.255.255.0 summary-only |
a. Examine the routing tables on each router to verify the route summarization for the two prefixes. Verify that R1 and R3 are each receiving the summary route from the other router. Verify that R2 is receiving aggregate routes from both R1 and R3.
R1# show ip route bgp | begin Gateway Gateway of last resort is not set
192.168.1.0/24 is variably subnetted, 5 subnets, 4 masks B 192.168.1.0/24 [200/0], 00:27:47, Null0 192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks B 192.168.2.0/27 [20/0] via 10.1.2.2, 13:34:31 B 192.168.2.64/26 [20/0] via 10.1.2.2, 13:34:31 B 192.168.3.0/24 [20/0] via 10.1.3.3, 00:26:01
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R2# show ip route bgp | begin Gateway Gateway of last resort is not set
B 192.168.1.0/24 [20/0] via 10.1.2.1, 00:33:53 B 192.168.3.0/24 [20/0] via 10.2.3.3, 00:32:08
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R3# show ip route bgp | begin Gateway Gateway of last resort is not set
B 192.168.1.0/24 [20/0] via 10.1.3.1, 00:36:52 192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks B 192.168.2.0/27 [20/0] via 10.2.3.2, 02:10:48 B 192.168.2.64/26 [20/0] via 10.2.3.2, 02:10:48 192.168.3.0/24 is variably subnetted, 5 subnets, 4 masks B 192.168.3.0/24 [200/0], 00:35:07, Null0
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Why do R1 and R3 contain an entry with a next hop address of Null0? What is the result of having this Null0 route in the routing table?
This is the same prefix as the aggregate route advertised by this router. The router’s routing table contains more specific routes. The Null0 route is a discard route for loop prevention. Any packets with a destination IPv4 address matching the aggregated prefix with the Null0 egress interface, but do not match a more specific route, will be dropped.
b. Examine the BGP table on router R2 to verify the route summarization. When a prefix has the default classful mask, the subnet mask is not displayed. Both 192.168.1.0 and 192.168.3.0 prefixes have a /24 prefix length which would be the default mask for a Class C address.
R2# show ip bgp BGP table version is 69, local router ID is 1.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i – internal, <output omitted>
Network Next Hop Metric LocPrf Weight Path * 192.168.1.0 10.2.3.3 0 300 1000 i *> 10.1.2.1 0 0 1000 i *> 192.168.2.0/27 0.0.0.0 0 32768 i *> 192.168.2.64/26 0.0.0.0 0 32768 i * 192.168.3.0 10.1.2.1 0 1000 300 i *> 10.2.3.3 0 0 300 i |
c. Examine the BGP table on routers R2 and R3 and verify that each router is receiving the summary route from the other router.
R1# show ip bgp BGP table version is 69, local router ID is 1.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i – internal, <output omitted>
Network Next Hop Metric LocPrf Weight Path s> 192.168.1.0/27 0.0.0.0 0 32768 i *> 192.168.1.0 0.0.0.0 32768 i s> 192.168.1.64/26 0.0.0.0 0 32768 i * 192.168.2.0/27 10.1.3.130 0 300 500 i * 10.1.3.3 0 300 500 i *> 10.1.2.2 0 0 500 i * 192.168.2.64/26 10.1.3.130 0 300 500 i * 10.1.3.3 0 300 500 i *> 10.1.2.2 0 0 500 i * 192.168.3.0 10.1.2.2 0 500 300 i * 10.1.3.130 0 0 300 i *> 10.1.3.3 0 0 300 i |
R3# show ip bgp BGP table version is 22, local router ID is 3.3.3.3 Status codes: s suppressed, d damped, h history, * valid, > best, i – internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, t secondary path, L long-lived-stale, Origin codes: i – IGP, e – EGP, ? – incomplete RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path * 192.168.1.0 10.2.3.2 0 500 1000 i *> 10.1.3.1 0 0 1000 i * 10.1.3.129 0 0 1000 i * 192.168.2.0/27 10.1.3.1 0 1000 500 i * 10.1.3.129 0 1000 500 i *> 10.2.3.2 0 0 500 i * 192.168.2.64/26 10.1.3.1 0 1000 500 i * 10.1.3.129 0 1000 500 i *> 10.2.3.2 0 0 500 i s> 192.168.3.0/27 0.0.0.0 0 32768 i *> 192.168.3.0 0.0.0.0 32768 i s> 192.168.3.64/26 0.0.0.0 0 32768 i |
Why do two of the entries have the status code of “s”? Specifically, this is the result of what command or option that was configured on these two routers?
This indicates that the route was suppressed. These entries were suppressed because a summary route (“*” valid) was advertised instead. Prefixes with a next hop of 0.0.0.0 are directly connected networks on this router. The summary-only option in the aggregate-address command suppressed these more specific prefixes from being advertised.
d. Examine the explicit 192.168.1.0 prefix entry in R2’s BGP table. The route’s NLRI information indicates that the route was aggregated in AS 1000 by router with the RID 1.1.1.1.
R2# show ip bgp 192.168.1.0 BGP routing table entry for 192.168.1.0/24, version 45 Paths: (2 available, best #2, table default) Advertised to update-groups: 1 Refresh Epoch 1 300 1000, (aggregated by 1000 1.1.1.1) 10.2.3.3 from 10.2.3.3 (3.3.3.3) Origin IGP, localpref 100, valid, external, atomic-aggregate rx pathid: 0, tx pathid: 0 Refresh Epoch 2 1000, (aggregated by 1000 1.1.1.1) 10.1.2.1 from 10.1.2.1 (1.1.1.1) Origin IGP, metric 0, localpref 100, valid, external, atomic-aggregate, best rx pathid: 0, tx pathid: 0x0 |
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