2-1 auto-summary
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| Syntax Description: This command has no arguments. auto-summary is enabled by default. Purpose: When auto-summary is enabled, routes injected into BGP via redistribution are summarized on a classful boundary. A 32-bit IP address consists of a network address and a host address. The subnet mask determines the number of bits used for the network address and the number of bits used for the host address. The IP address classes have a natural or standard subnet mask, as shown in Table 2-1. Table 2-1. IP Address Classes
When using the standard subnet mask, Class A addresses have one octet for the network, Class B addresses have two octets for the network, and Class C addresses have three octets for the network. As an example, consider the Class B address 156.26.32.1 with a 24-bit subnet mask. The 24-bit subnet mask selects three octets, 156.26.32, for the network. The last octet is the host address. If the network 156.26.32.1/24 is learned via an IGP and is then redistributed into BGP, the network is automatically summarized to the natural mask for a Class B network. The network that BGP advertises is 156.26.0.0/16. BGP is advertising that it can reach the entire Class B address space from 156.26.0.0 to 156.26.255.255. If the only network that can be reached via the BGP router is 156.26.32.0/24, BGP is advertising 254 networks that cannot be reached via this router. auto-summary does not apply to routes injected into BGP via the network command or through IBGP or EBGP. Cisco IOS Software Release: 10.0 Configuration Example: Automatic Route SummarizationThis example demonstrates the effect of automatic route summarization. In Figure 2-1, static routes, connected routes, and routes learned via OSPF are being redistributed into BGP as shown in the following configurations. The redistributed routes are summarized to a classful boundary by BGP because auto-summary is enabled by default. Figure 2-1. Automatic BGP Route Summarization Applies Only to Redistributed Routes Router A interface loopback 0 ip address 144.223.1.1 255.255.255.0 ! router bgp 1 network 144.223.1.0 mask 255.255.255.0 neighbor 172.17.1.1 remote-as 2 ___________________________________________________________________________ Router B router ospf 1 network 10.0.0.0 0.255.255.255 area 0 ! router bgp 2 redistribute ospf 1 redistribute static redistribute connected neighbor 172.17.1.2 remote-as 1 ! ip route 198.8.4.128 255.255.255.128 Ethernet 0 ___________________________________________________________________________ Router C interface loopback 0 ip address 205.40.30.129 255.255.255.192 ! router ospf 1 network 10.0.0.0 0.255.255.255 area 0 network 205.0.0.0 0.255.255.255 area 51 On Router A, prefix 144.223.1.0/24 is being injected into BGP via the network command. If this prefix were automatically summarized to a classful boundary, the prefix in the BGP table would be 144.223.0.0/16. Because the network command was used to inject this route into BGP, summarization should not have occurred. rtrA# show ip bgp BGP table version is 14, local router ID is 144.223.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 10.0.0.0 172.17.1.1 0 0 2 ? *> 144.223.1.0/24 0.0.0.0 0 32768 i *> 172.16.0.0 172.17.1.1 0 0 2 ? *> 198.8.4.0 172.17.1.1 0 0 2 ? *> 205.40.30.0 172.17.1.1 0 0 2 ? Router B is learning about network 205.40.30.128 from router C via OSPF. Router B also has a route to network 198.8.4.128 via a static route, as shown in the IP routing table. rtrB# show ip route Codes: C - connected, S - static, I - IGRP, 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, E - EGP i - IS-IS, 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 172.17.0.0/24 is subnetted, 1 subnets C 172.17.1.0 is directly connected, Ethernet0/0 172.16.0.0/24 is subnetted, 1 subnets C 172.16.2.0 is directly connected, Loopback0 144.223.0.0/24 is subnetted, 1 subnets B 144.223.1.0 [20/0] via 172.17.1.2, 00:27:30 10.0.0.0/30 is subnetted, 1 subnets C 10.1.1.0 is directly connected, Serial2/0 205.40.30.0/25 is subnetted, 1 subnets O IA 205.40.30.128 [110/49] via 10.1.1.2, 00:00:12, Serial2/0 198.8.4.0/25 is subnetted, 1 subnets S 198.8.4.128 is directly connected, Ethernet0/0 The OSPF and static routes on Router B are being redistributed into BGP. auto-summary will summarize these prefixes to a classful boundary: rtrB# show ip bgp BGP table version is 12, local router ID is 172.16.2.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 10.0.0.0 0.0.0.0 0 32768 ? *> 144.223.1.0/24 172.17.1.2 0 0 1 i *> 172.16.0.0 0.0.0.0 0 32768 ? *> 198.8.4.0 0.0.0.0 0 32768 ? *> 205.40.30.0 0.0.0.0 0 32768 ? Router B advertises the 198.8.4.0 and 205.40.30.0 prefixes to Router A via BGP: rtrA# show ip bgp BGP table version is 14, local router ID is 144.223.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 10.0.0.0 172.17.1.1 0 0 2 ? *> 144.223.1.0/24 0.0.0.0 0 32768 i *> 172.16.0.0 172.17.1.1 0 0 2 ? *> 198.8.4.0 172.17.1.1 0 0 2 ? *> 205.40.30.0 172.17.1.1 0 0 2 ? auto-summary can cause routing problems if the advertising router does not own the entire summarized prefix. For example, if you redistribute only one subnet of a Class B address into BGP, BGP advertises the entire Class B address space if auto-summary is enabled. Typically, you want to disable auto-summary on your BGP routers. Modify the configuration on Router B so that auto-summary is disabled. Router B router bgp 2 redistribute ospf 1 redistribute static redistribute connected neighbor 172.17.1.2 remote-as 1 no auto-summary VerificationVerify that Router B is no longer creating summaries for the redistributed routes. router B# show ip bgp BGP table version is 20, local router ID is 172.16.2.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 10.1.1.0/30 0.0.0.0 0 32768 ? *> 144.223.1.0/24 172.17.1.2 0 0 1 i *> 172.16.2.0/24 0.0.0.0 0 32768 ? *> 198.8.4.128/25 0.0.0.0 0 32768 ? *> 205.40.30.128/25 10.1.1.2 49 32768 ? Troubleshooting
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