There are two types of reset, hard reset and soft reset. Cisco IOS Releases 12.1 and later support a soft reset without any prior configuration. To use a soft reset without preconfiguration, both BGP peers must support the soft route refresh capability, which is advertised in the OPEN message sent when the peers establish a TCP session. A soft reset allows the dynamic exchange of route refresh requests and routing information between BGP routers and the subsequent re-advertisement of the respective outbound routing table. • When soft reset generates inbound updates from a neighbor, it is called dynamic inbound soft reset.
• When soft reset sends a set of updates to a neighbor, it is called outbound soft reset. A soft inbound reset causes the new inbound policy to take effect. A soft outbound reset causes the new local outbound policy to take effect without resetting the BGP session. As a new set of updates is sent during outbound policy reset, a new inbound policy can also take effect. The table that is given below lists the advantages and disadvantages hard reset and soft reset. Table 1: Advantages and Disadvantages of Hard and Soft Resets Disadvantages The prefixes in the BGP, IP, and FIB t provided by the neighbor are lost. Not recommended.
Does not reset inbound routing table u Both BGP routers must support the rou capability (in Cisco IOS Release 12.1 BGP Decision Attributes When a BGP speaker receives updates from multiple autonomous systems that describe different paths to the same destination, it must choose the single best path for reaching that destination. When chosen, the selected path is entered into the BGP routing table and propagated to its neighbors. The decision is based on the value of attributes that the update contains and other BGP-configurable factors. When a BGP peer learns two EBGP paths for a prefix from a neighboring AS, it chooses the best path and inserts that path in the IP routing table.
If BGP multipath support is enabled and the EBGP paths are learned from the same neighboring autonomous systems, instead of a single best path, multiple paths are installed in the IP routing table. Then, during packet switching, per-packet or per-destination load-balancing is performed among the multiple paths. The maximum-paths router configuration command controls the number of paths allowed. These factors summarize the order in which BGP evaluates the attributes for choosing the best path: 1. If the path specifies a next hop that is inaccessible, drop the update.
The BGP next-hop attribute, automatically determined by the software, is the IP address of the next hop that is going to be used to reach a destination. For EBGP, this is usually the IP address of the neighbor that is specified by the neighbor remote-as router configuration command. You can disable next-hop processing by using route maps or the neighbor next-hop-self router configuration command.
Configuring BGP BGP Decision Attributes Type of Reset Advantages Hard reset No memory overhead Outbound soft reset No configuration, no storing of routing table updates Dynamic inbound soft reset Does not clear the BGP session and cache Does not require storing of routing table updates and has no memory overhead Configuring BGP 4