System information

Troubleshooting TCP/IP
Book Title
7-132
BGP: Routes Missing from Routing Table
Symptom: BGP routers and networks are not advertised to other routers. Routers do not receive
routing information from other routers. Some hosts cannot communicate with hosts in other areas,
and routing table information is incomplete.
Table 7-16 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 7-16 BGP: Routes Missing from Routing Table
Possible Problem Solution
BGP routers not
advertising routes
If BGP routers are not advertising routes properly, routing information might not
appear in the routing table. For information on troubleshooting this problem, see
the section “BGP: Routers Not Advertising Routes,” later in this chapter.
Missing neighbor remote-as
command
The neighbor remote-as router configuration command is used to add entries to
the BGP neighbor table.
Step 1 Check local and remote routers and make sure the specified autonomous
system numbers and neighbors are correct.
The following example specifies that a router at the address 131.108.1.2
is a neighbor in autonomous system number 109:
router bgp 110
network 131.108.0.0
neighbor 131.108.1.2 remote-as 109
Step 2
Make sure any route filters that are enabled are not misconfigured.
Access list is misconfigured
Step 1 Use the show access-list privileged exec command on suspect routers to
see whether there are access lists configured on the router.
Step 2 If there are access lists enabled on the router, disable them using the
appropriate commands. For example, to disable input access list 10, use
the following command:
C4000(config)#no ip access-group 10 in
Step 3
After disabling all access lists on the router, determine whether the
missing routing information is now appearing in routing tables.
Step 4 If the information is now appearing, it is likely that an access list is
filtering traffic. To isolate the problem access list, enable access lists one
at a time until the routing information is no longer appearing in the
routing table.
Step 5 Check the access list to see whether it is filtering traffic from specific TCP
ports. If an access list denies specific TCP ports, make sure that it does
not deny TCP port 179, the port used by BGP.
Enter an explicit permit statement for port 179 to ensure that BGP traffic
is forwarded normally.
Example:
access-list 101 permit tcp 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255 eq 179
Step 6 If you altered an access list, enable the list to see whether routing
information can still pass normally.
Step 7 If routing information is no longer missing, perform the preceding steps
on any other routers in the path until all access lists are enabled and
routing information appears in the appropriate routing tables.
For more information on configuring access lists, see the Cisco IOS configuration
guides.