BGP Route Not Getting Originated-Cause: BGP Is Autosummarizing to ClassfulNetwork Boundary

BGP Route Not Getting Originated ‚ Cause: BGP Is Autosummarizing to Classful/Network Boundary

Sometimes, classful networks are advertised in BGP when other routing protocols are redistributed in BGP. For example, BGP might be trying to redistribute 100.100.100.0/24, but only 100.0.0.0/8 gets advertised. Another example could be that 131.108.0.0/16 is advertised where 131.108.5.0/24 was redistributed.

BGP autosummarizes subnetted routes to their network boundaries when redistributed into BGP from any other routing protocol. For example, subnetted Class A routes automatically are summarized to the Class A mask /8 when redistributed in BGP from any other protocol.

Figure 15-11 shows the flowchart to follow to fix this problem.

Figure 15-11. Problem-Resolution Flowchart

Debugs and Verification

Example 15-30 shows an example in which R1 has a static route for 100.100.100.0/24 and 131.108.5.0/24. Notice that these are subnetted Class A and B routes, respectively.

When these static routes are redistributed in BGP, BGP autosummarizes them to their natural class masks, which are /8 and /16 respectively.

Example 15-30 shows the relative configuration in R1 to redistribute these static routes in BGP; it also displays the BGP table output for these advertisements.

Example 15-30 Configuring Redistribution of Static Routes in BGP

 R1#  router bgp 109   no synchronization   redistribute static   neighbor 131.108.1.2 remote-as 109   ip route 100.100.100.0 255.255.255.0 Null0   ip route 131.108.5.0 255.255.255.0 Null0   R1#show ip bgp 100.100.100.0  BGP routing table entry for  100.0.0.0/8  , version 2 Paths: (1 available, best #1, table Default-IP-Routing-Table)   Advertised to non peer-group peers:   131.108.1.2   Local     0.0.0.0 from 0.0.0.0 (1.1.1.1)     Origin incomplete, metric 0, localpref 100, weight 32768, valid, sourced, best R1-2503#  show ip bgp 131.108.5.0  BGP routing table entry for  131.108.0.0 /16  , version 3 Paths: (1 available, best #1, table Default-IP-Routing-Table)   Advertised to non peer-group peers:   131.108.1.2   Local     0.0.0.0 from 0.0.0.0 (1.1.1.1)   Origin incomplete, metric 0, localpref 100, weight 32768, valid, sourced, best 

Note the mask in the output. /24 static routes are summarized to the network boundary of Class A and Class B, respectively.

Solution

IP prefixes need to be advertised with their original masks. One exception is when manual summarization is done. The problem of BGP advertising classful networks after redistribution can be overcome by disabling the autosummarization feature in BGP.

Example 15-31 shows the necessary configuration to achieve this; it also shows the BGP advertisement in the BGP table after this change.

Example 15-31 Disabling Autosummarization in BGP

 R1#  router bgp 109   no synchronization   redistribute static    no auto-summary   R1#  show ip bgp 100.100.100.0  BGP routing table entry for  100.100.100.0/24  , version 4 Paths: (1 available, best #1, table Default-IP-Routing-Table) Flag: 0x208   Advertised to non peer-group peers:   131.108.1.2   Local     0.0.0.0 from 0.0.0.0 (1.1.1.1)   Origin incomplete, metric 0, localpref 100, weight 32768, valid, sourced, best R1#  show ip bgp 131.108.5.0  BGP routing table entry for  131.108.5.0/24  , version 6 Paths: (1 available, best #1, table Default-IP-Routing-Table) Flag: 0x208   Advertised to non peer-group peers:   131.108.1.2   Local     0.0.0.0 from 0.0.0.0 (1.1.1.1)   Origin incomplete, metric 0, localpref 100, weight 32768, valid, sourced, best 

Note the masks in the output ‚ they are exactly what there were configured originally.

In most cases, it would be desirable to turn off autosummarization so that proper mask routes can be advertised to BGP neighbors. Autosummarization plays a role only when any other protocol routes are redistributed into BGP. Because it is not common practice to redistribute other protocols into BGP, the autosummarization feature is not used much.