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
Class Address Range Natural Mask
A 1.0.0.0 to 126.0.0.0 255.0.0.0 or /8
B 128.1.0.0 to 191.254.0.0 255.255.0.0 or /16
C 192.0.1.0 to 223.255.254.0 255.255.255.0 or /24
Reserved addresses include 128.0.0.0, 191.255.0.0, 192.0.0.0, and 223.255.255.0

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 Summarization

This 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

graphics/02fig01.gif

 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   
Verification

Verify 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
Step 1. Verify that the BGP neighbors are in the Established state using the show ip bgp neighbors command.

If the neighbor relationship is not in the Established state, see section 8-23.

Step 2. If routes are being redistributed into BGP, use the no auto-summary command. Verify that redistributed routes are not summarized to a classful boundary using the show ip bgp command.

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Cisco[r] BGP-4 Command and Configuration Handbook
Cisco BGP-4 Command and Configuration Handbook
ISBN: 1587055732
EAN: 2147483647
Year: 2001
Pages: 300

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