Cisco Configuring BGP: Comprehensive Guide and Best Practices PDF (EN)

Information About BGP

Configuring BGP Restrictions for BGP, on page 1 Information About BGP, on page 1 How to Configure BGP, on page 13 Configuration Examples for BGP, on page 54 Monitoring and Maintaining BGP, on page 56 Feature History for Border Gateway Protocol, on page 57 Restrictions for BGP • When you configure the bgp graceful-restart command , ensure that the BGP hold time is higher than the minimum graceful restart time for the Extended Fast Software Upgrade (xFSU) capable devices. The minimum graceful restart time is specific to a device. A lower hold time value is configurable, however BGP is not supported in an xFSU event.

If a BGP peer sends an unsupported hold timer in the BGP open message, the proposed hold timer is accepted, but BGP is not supported on an xFSU event. • Layer 3 forwarding is delayed until routing tables are populated on a device when you switch on the device or execute the clear ip bgp command. Note The routing tables require around 80 seconds for population. You can use the show ip bgp ip-address command, in privileged EXEC mode, to check whether the routing tables are populated or not.

Information About BGP The Border Gateway Protocol (BGP) is an exterior gateway protocol that is used to set up an interdomain routing system that guarantees the loop-free exchange of routing information between autonomous systems. Autonomous systems are made up of routers that operate under the same administration and that run Interior Gateway Protocols (IGPs), such as RIP or OSPF, within their boundaries and that interconnect by using an Exterior Gateway Protocol (EGP). BGP Version 4 is the standard EGP for interdomain routing in the Internet. The protocol is defined in RFCs 1163, 1267, and 1771. Configuring BGP 1

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BGP Network Topology

BGP Network Topology Routers that belong to the same autonomous system (AS) and that exchange BGP updates run internal BGP (IBGP), and routers that belong to different autonomous systems and that exchange BGP updates run external BGP (EBGP). Most configuration commands are the same for configuring EBGP and IBGP. The difference is that the routing updates are exchanged either between autonomous systems (EBGP) or within an AS (IBGP). The figure given below shows a network that is running both EBGP and IBGP. Figure 1:

EBGP, IBGP, and Multiple Autonomous Systems Before exchanging information with an external AS, BGP ensures that networks within the AS can be reached by defining internal BGP peering among routers within the AS and by redistributing BGP routing information to IGPs that run within the AS, such as IGRP and OSPF. Routers that run a BGP routing process are often referred to as BGP speakers. BGP uses the Transmission Control Protocol (TCP) as its transport protocol (specifically port 179). Two BGP speakers that have a TCP connection to each other for exchanging routing information are known as peers or neighbors.

In the above figure, Routers A and B are BGP peers, as are Routers B and C and Routers C and D. The routing information is a series of AS numbers that describe the full path to the destination network. BGP uses this information to construct a loop-free map of autonomous systems. The network has these characteristics: • Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP peers are directly connected and that the IBGP peers are not. As long as there is an IGP running that allows the two neighbors to reach one another, IBGP peers do not have to be directly connected. • All BGP speakers within an AS must establish a peer relationship with each other.

That is, the BGP speakers within an AS must be fully meshed logically. BGP4 provides two techniques that reduce the requirement for a logical full mesh: confederations and route reflectors. • AS 200 is a transit AS for AS 100 and AS 300—that is, AS 200 is used to transfer packets between AS 100 and AS 300. BGP peers initially exchange their full BGP routing tables and then send only incremental updates. BGP peers also exchange keepalive messages (to ensure that the connection is up) and notification messages (in response to errors or special conditions).

In BGP, each route consists of a network number, a list of autonomous systems that information has passed through (the autonomous system path), and a list of other path attributes. The primary function of a BGP Configuring BGP BGP Network Topology Configuring BGP 2

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Routing Policy Changes

system is to exchange network reachability information, including information about the list of AS paths, with other BGP systems. This information can be used to determine AS connectivity, to prune routing loops, and to enforce AS-level policy decisions. A router or device running Cisco IOS does not select or use an IBGP route unless it has a route available to the next-hop router and it has received synchronization from an IGP (unless IGP synchronization is disabled). When multiple routes are available, BGP bases its path selection on attribute values. See the “Configuring BGP Decision Attributes” section for information about BGP attributes.

BGP Version 4 supports classless interdomain routing (CIDR) so you can reduce the size of your routing tables by creating aggregate routes, resulting in supernets. CIDR eliminates the concept of network classes within BGP and supports the advertising of IP prefixes. Nonstop Forwarding Awareness To enable this feature with BGP routing, you need to enable Graceful Restart.

When the neighboring router is NSF-capable, and this feature is enabled, the Layer 3 device continues to forward packets from the neighboring router, during the interval when the primary Route Processor (RP) in a device is failing and the backup RP is taking over, or while the primary RP is manually reloaded for a nondisruptive software upgrade. Information About BGP Routing To enable BGP routing, you establish a BGP routing process and define the local network. Because BGP must completely recognize the relationships with its neighbors, you must also specify a BGP neighbor. BGP supports two kinds of neighbors: internal and external.

Internal neighbors are in the same AS; external neighbors are in different autonomous systems. External neighbors are usually adjacent to each other and share a subnet, but internal neighbors can be anywhere in the same AS. The switch supports the use of private AS numbers, usually assigned by service providers and given to systems whose routes are not advertised to external neighbors. The private AS numbers are from 64512 to 65535. You can configure external neighbors to remove private AS numbers from the AS path by using the neighbor remove-private-as router configuration command.

Then when an update is passed to an external neighbor, if the AS path includes private AS numbers, these numbers are dropped. If your AS will be passing traffic through it from another AS to a third AS, it is important to be consistent about the routes it advertises. If BGP advertised a route before all routers in the network had learned about the route through the IGP, the AS might receive traffic that some routers could not yet route. To prevent this from happening, BGP must wait until the IGP has propagated information across the AS so that BGP is synchronized with the IGP. Synchronization is enabled by default.

If your AS does not pass traffic from one AS to another AS, or if all routers in your autonomous systems are running BGP, you can disable synchronization, which allows your network to carry fewer routes in the IGP and allows BGP to converge more quickly. Routing Policy Changes Routing policies for a peer include all the configurations that might affect inbound or outbound routing table updates. When you have defined two routers as BGP neighbors, they form a BGP connection and exchange routing information. If you later change a BGP filter, weight, distance, version, or timer, or make a similar configuration change, you must reset the BGP sessions so that the configuration changes take effect.

Configuring BGP Nonstop Forwarding Awareness Configuring BGP 3

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BGP Decision Attributes

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

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