Core Issue
Border Gateway Protocol (BGP) is an interdomain routing protocol used to exchange network reachability information with configured peers known as BGP neighbors. Unlike other Interior Gateway Protocols (IGPs) used for routing, BGP sends packets to the unicast address of the neighbor using Transmission Control Protocol (TCP) port 179.
To successfully establish a peering relationship, the neighbors should be directly connected (typically the case with exterior BGP [eBGP]), or reachable through static routes or an IGP (mostly the case with interior BGP [iBGP]).
In contrast to the IGPs, BGP is not enabled under a specific interface on the router, but globally. It uses the information available in the routing table to choose the outgoing interface and send packets to the neighbor.
When redundancy is desired for networks using a primary link (like a leased line or a Frame Relay link), a dialup link like ISDN is generally used for back up. Under normal circumstances, the primary link is used for exchanging routing information and transmitting data packets. When the primary link goes down, the ISDN link is used. This is done by triggering a call based on the definition of interesting traffic. This is the traffic that is defined as valid for triggering a call and then disconnecting the call when there is no more such traffic to be sent. In general, routing protocol traffic should not be defined as interesting traffic, since they exchange periodic messages that always keep the ISDN link up, increasing the cost incurred.
While using ISDN to provide redundancy for networks running BGP, the call may be triggered by BGP packets. This can occur if the ISDN interface is selected for reaching the configured neighbor (based on the information in the IP routing table) and BGP is defined as part of interesting traffic. Since BGP exchanges periodic keepalive messages between the neighbors, this triggers and keeps the ISDN link up until the primary link comes up.
Resolution
If all IP traffic is configured as interesting, using a broad definition with the dialer-list dialer-group> protocol ip permit command, then any IP packet (including BGP) brings up the ISDN link.
To resolve this issue, perform these steps:
- Remove BGP from the definition of interesting traffic by configuring an extended IP Access Control List (ACL) that denies BGP packets. This is done by issuing the access-list access-list-number> deny tcp any any eq bgp and access-list access-list-number> deny tcp any eq bgp any commands from global configuration mode.
- Since TCP port 179 (used by BGP on the router) can either be a source or destination port, depending on the peer addresses, configure both the commands and deny BGP. Add other traffic definitions that you want to consider interesting and trigger the ISDN link by permitting them in the configured ACL.
- Enable the definition of interesting traffic by attaching the configured extended ACL to the dialer. This is done by issuing the dialer-list dialer-group> protocol ip list access-list-number> command from global configuration mode.
- Configure the interface that is used for the ISDN call to use the new definition of interesting traffic. This is done by issuing the dialer-group dialer-group> command from interface configuration mode.
How are Neighbors Connected
Connected via redundant link
