How to Read the BGP Table (2024)

Lesson Contents

All prefixes that BGP learns are stored in the BGP table. In this lesson we’ll take a look at this table and you will learn how to read it. We’ll start with a simple topology and finish with a quick peek at a full Internet routing table.

Configuration

Here’s the topology we will use. 4 routers, each in a different autonomous system:

Configurations

Want to take a look for yourself? Here you will find the startup configuration of each device.

hostname R1!interface fastEthernet0/0 ip address 192.168.12.1 255.255.255.0!interface fastEthernet0/1 ip address 192.168.13.1 255.255.255.0!router bgp 1 neighbor 192.168.12.2 remote-as 2 neighbor 192.168.13.3 remote-as 3!end

hostname R2!interface fastEthernet0/0 ip address 192.168.12.2 255.255.255.0!interface fastEthernet0/1 ip address 192.168.24.2 255.255.255.0!router bgp 2 neighbor 192.168.12.1 remote-as 1 neighbor 192.168.24.4 remote-as 4!end

Reading the BGP Table

Let’s take a look at the BGP tables. We’ll start with R4:

R4#show ip bgpBGP table version is 2, local router ID is 192.168.34.4Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*> 4.4.4.4/32 0.0.0.0 0 32768 i

Ok so what do we see here? Let’s start with the items I highlighted in red first. This router has network 4.4.4.4/32 in its BGP table and in front of the network there’s the *> symbol:

The * means that this is a valid route and that BGP is able to use it.
The > means that this entry has been selected as the best path.

The next hop is 0.0.0.0. The next hop of 0.0.0.0 means that this network originated on this router, that makes sense since I used the network command on R4 to advertise this network into BGP.

Further to the right you see metric, local preference and weight. These are the BGP attributes that are used to select the best path.

Path will show the AS path, there’s nothing there since this network was advertised in BGP on this router. On the other routers you’ll see something here.

The ‘i’ is the origin code and indicates that this network was advertised into BGP using the network command, the table says it refers to IGP but it doesn’t have anything to do with “interior gateway protocols”. When you redistribute something into BGP it will show up with the ? symbol. You will never see the ‘e’ symbol, this refers to EGP (Exterior Gateway Protocol) which is the predecessor of BGP.

Some of the other things you see here is the BGP table version, every time the best path changes this number will increase. You can see the BGP router ID of this router and there are some other status codes:

supressed: BGP knows the network but won’t advertise it, this can occur when the network is part of a summary.
damped: BGP doesn’t advertise this network because it was flapping too often (network appears, disapears, appears, etc.) so it got a penalty.
history: BGP learned this network but doesn’t have a valid route at the moment.
RIB-failure: BGP learned this network but didn’t install it in the routing table. This occurs when another routing protocol with a lower administrative distance also learned it.
stale: this is used for non-stop forwarding, this entry has to be refreshed when the remote BGP neighbor has returned.

Let’s look at the BGP tables of the other routers, we’ll continue with R2:

R2#show ip bgpBGP table version is 2, local router ID is 192.168.24.2Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*> 4.4.4.4/32 192.168.24.4 0 0 4 i

The output of R2 is similar to what we have seen on R4 but there are two important differences. The first one is the next hop, R2 learned about this network from 192.168.24.4. The second thing is the AS path, it’s showing AS 4.

Let’s check R1:

FAQs

What is a full BGP table? ›

A full routing table is a table which contains all the routes the BGP neighbor is aware of. If you are connected to the Internet through a single ISP, a default route is enough, since there's only one possible path, so there's no point in having 771056 routes in memory that all point to the same next-hop.

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What is the command to check BGP table? ›

The show ip bgp command is used to display the contents of the BGP routing table. The output can be filtered to display entries for a specific prefix, prefix length, and prefixes injected through a prefix list, route map, or conditional advertisem*nt.

What does the BGP table contain? ›

Essentially, NLRI is a prefix and its length. BGP table contains all the routes from all the neighbors, several routes to the same network with different attributes.

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How much memory is needed for a full BGP table? ›

Full BGP Table. Full feeds include all routes in the K-20 Customer and HP offerings, as well as commodity Internet routes. Caution: many routers don't have enough memory to carry a full table. A full BGP table requires at least 512 MB of router memory.

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What is 0.0 0.0 in BGP? ›

The 0.0. 0.0 means the route was locally originated through the network command or aggregate-address command. For example if you have: R1#show ip bgp 10.1.1.0/24. BGP routing table entry for 10.1.1.0/24, version 2.

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What is BGP commands? ›

bgp redistribute-internal

	bgp redistribute-internal no bgp redistribute-internal Enables iBGP redistribution into an interior gateway protocol (IGP). The no form of the command disables iBGP redistribution into an interior gateway protocol (IGP).
Syntax Description	remote-as <as-number>	Remote peer's number
Default	Disabled

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Nov 15, 2023

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What is the I in the BGP table? ›

The 'i' is the origin code and indicates that this network was advertised into BGP using the network command, the table says it refers to IGP but it doesn't have anything to do with “interior gateway protocols”. When you redistribute something into BGP it will show up with the ? symbol.

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Can you explain BGP in detail? ›

Border Gateway Protocol (BGP) is a set of rules that determine the best network routes for data transmission on the internet. The internet consists of thousands of private, public, corporate, and government networks linked together through standardized protocols, devices, and communication technologies.

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How do I clear my BGP table? ›

The most straightforward way to reset a BGP session is with the clear ip bgp command. clear ip bgp is the original version of the command. There's also clear bgp ipv6 unicast for IPv6 BGP sessions and the corresponding clear bgp ipv4 unicast for IPv4 BGP sessions.

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How to check BGP peer status? ›

The BGP Live Status Check window displays the current BGP peering's operational status in real time via SNMP collection. It is accessed by right-clicking on the BGP Map and selecting Live Status Check. Select the desired Node Peers using the checkboxes and press Start to begin the SNMP collection.

What is the difference between default route and full table in BGP? ›

A default route is the route that will be used if there's no other route that matches the destination in the router's forwarding table. A full routing table is a table which contain all the routes the BGP neighbor is aware of.

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What is BGP full for? ›

Border Gateway Protocol (BGP) is classified as internal and external, depending on where the data is being routed. External BGP routers connect an autonomous system to the global internet. However, large autonomous systems are themselves made up of smaller autonomous systems within them.

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How many routes are in the BGP table? ›

There is not a single plot in this figure where each BGP speak sees essentially the same network. There is a variance across the various peers of these route collectors, which is around 50,000 routes.

What is BGP full mesh? ›

For the correct operation of Internal BGP (iBGP) , a full mesh of iBGP peerings must be established within the autonomous system. That means that each iBGP router must be peered with every other iBGP router within the AS. Unlike eBGP, iBGP peerings do not need to be directly connected.