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Greetings GS, A question about BGP aggregation referring to IE Lab14. A full reachability was there after IGP section. SW2 got default OSPF default route from R5 that directly connected and able to reach all IP addresses. Now they are peering in BGP in next section and SW2 is asked to configure with aggregate-address 150.1.0.0 255.255.240.0 summary-only, SW2 then lost reachability to loopback IP address of routers in EIGRP. I am copying a few snapshots below. It appeared to me that I had to use distance command to push the BGP aggregated route out in order to restore full reachability while keep BGP part remain working. My thought is aggregated route is still more specific than default route, therefore preferred. It must be a elegant way to do that and I may have missed something terribly here, since nobody asked this question. Please let me know, Would you? Before: Rack1SW2#sh ip ro 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via “ospf 1″, distance 110, metric 1, candidate default path Tag 1, type extern 2, forward metric 10 Last update from 167.1.58.5 on FastEthernet0/13, 00:34:53 ago Routing Descriptor Blocks: * 167.1.58.5, from 150.1.5.5, 00:34:53 ago, via FastEthernet0/13 Route metric is 1, traffic share count is 1 Route tag 1 Now they are peering in BGP in next section and SW2 is configured with aggregate-address 150.1.0.0 255.255.240.0 summary-only Rack1SW2#sh ip bgp | i 150.1.0.0 *> 150.1.0.0/20 0.0.0.0 32768 i Rack1SW2#sh ip bgp 150.1.1.0 BGP routing table entry for 150.1.0.0/20, version 19 Paths: (1 available, best #1, table Default-IP-Routing-Table) Advertised to update-groups: 1 2 Local, (aggregated by 65078 150.1.8.8) 0.0.0.0 from 0.0.0.0 (150.1.8.8) Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate, best Rack1SW2#sh ip cef 150.1.0.0 255.255.240.0 150.1.0.0/20 attached to Null0 Rack1SW2#sh ip cef 150.1.1.0 150.1.0.0/20 attached to Null0 Because BGP aggregated route is more specific than default route, when ping 150.1.1.1 for example, it went to Null0, thus failed My workaround is to issue a distance command in BGP with ACL 8 distance 255 0.0.0.0 255.255.255.255 8 access-list 8 deny 150.1.7.0 0.0.0.255 access-list 8 permit 150.1.0.0 0.0.15.255 After: Rack1SW2#sh ip bgp | i 150.1.0.0 r> 150.1.0.0/20 0.0.0.0 32768 i Rack1SW2# Rack1SW2#sh ip bgp 150.1.1.0 BGP routing table entry for 150.1.0.0/20, version 20 Paths: (1 available, best #1, table Default-IP-Routing-Table, RIB-failure(2)) Advertised to update-groups: 1 2 Local, (aggregated by 65078 150.1.8.8) 0.0.0.0 from 0.0.0.0 (150.1.8.8) Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate, best Rack1SW2#sh ip cef 150.1.0.0 255.255.240.0 %Prefix not found Rack1SW2# Rack1SW2#sh ip cef 150.1.1.0 0.0.0.0/0 nexthop 167.1.58.5 FastEthernet0/13 Rack1SW2# Now the full reachability is restored
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